| 0/0 |
2026/04/05 00:03 |
flow |
moderation |
32m
Results: map[Actionable:true Confident:true Explanation:The bug report is self-consistent and actionable. It describes a
`slab-use-after-free` on a `dentry` object in a `PREEMPT_RT` kernel.
### Analysis of the Stacks:
1. **Bad Access Stack**: The UAF occurs in `rt_mutex_slowunlock+0xbf/0x8b0`,
which is called by `end_dirop` (inlined in `filename_mknodat`). In a
`PREEMPT_RT` kernel, `spin_lock` is implemented using `rt_mutex`. The stack
trace indicates that `dput(de)` was called in `end_dirop`, which then called
`fast_dput`. Inside `fast_dput`, a `spin_unlock(&dentry->d_lock)` was performed.
The access at offset 208 of the dentry corresponds exactly to
`dentry->d_lockref.lock.lock.wait_lock.dep_map` (where `d_lockref` is at offset
184, and the `dep_map` of the `raw_spinlock` inside the `rt_mutex` is at offset
24).
2. **Allocation Stack**: The dentry was allocated by task 20778 in
`lookup_one_qstr_excl` via `d_alloc`. This is the "original" dentry created
during the lookup process in `filename_create`.
3. **Free Stack**: The dentry was freed by an RCU callback (`rcu_do_batch`).
The "Last potentially related work creation" stack trace shows that `call_rcu`
was triggered by `ocfs2_dentry_attach_lock+0x748/0xe20` at
`fs/ocfs2/dcache.c:328`, which calls `dput(alias)`.
### Root Cause:
The bug is a race condition in the `ocfs2` filesystem's dentry locking logic. In
`ocfs2_dentry_attach_lock`, the code searches for an existing alias using
`ocfs2_find_local_alias`. If another thread has already attached a lock to the
same dentry (setting `dentry->d_fsdata`), `ocfs2_find_local_alias` may return
the dentry itself as an `alias`. The function then calls `dput(alias)` at line
328. If this `dput` drops the reference count to zero (e.g., because another
concurrent thread also dropped its reference), the dentry is queued for freeing
via RCU. However, the `ocfs2_lookup` function continues and returns this dentry
(or the caller `lookup_one_qstr_excl` continues to use it), leading to a
Use-After-Free when the dentry is later accessed in `end_dirop`.
### Conclusion:
The report contains all three required stack traces, they are consistent with
each other and the memory layout of the involved structures, and they point to a
specific logical error in the `ocfs2` subsystem.]
|
| 1/1 |
2026/04/05 00:03 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/48e76c04594d7dd375d53ba61a6cd0b9e945d4d6]
|
| 2/1 |
2026/04/05 00:03 |
action |
kernel-builder |
12m
Results: map[KernelObj:/app/workdir/cache/build/9d74cd2efcaeda014a11362f152cdbdc2eca0005]
|
| 3/1 |
2026/04/05 00:16 |
action |
codesearch-prepare |
9m
Results: map[Index:codesearch-index]
|
| 4/1 |
2026/04/05 00:25 |
agent |
expert |
10m
Model:
Results: map[Actionable:true Confident:true]
Instruction:
You are an experienced Linux kernel developer tasked with determining if the given kernel bug
report is actionable or not. Actionable means that it contains enough info to root cause
the underlying bug, and that the report is self-consistent and makes sense, rather than
e.g. a one-off nonsensical crash induced by a previous memory corruption.
The bug report is about a use-after-free bug generated by KASAN tool.
It should contain 3 stack traces: the bad memory access stack, the heap block allocation stack,
and the heap block free stack. If the report does not contain 3 stacks, it's not actionable.
All 3 stack traces should be related to the same object type,
and usually be in the same kernel subsystem (at least leaf stack frames).
An example of an actionable and consistent report would be: first access stack relates
to an access to a field of struct Foo, allocation/free stacks relate to allocation/free
of the struct Foo.
In inconsistent/nonsensical reports an access may be to a struct Foo, but allocation
stack allocates a different structure in a different subsystem.
Look for other suspicious signals/inconsistencies that can make this report hard to
debug/understand.
In the final reply explain why you think the report is self-consistent and actionable,
or why it's inconsistent and/or not actionable.
Use the provided tools to confirm any assumptions, variables/fields being accessed, etc.
In particular, don't make assumptions about the kernel source code,
use codesearch tools to read the actual source code.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The bug report is:
==================================================================
BUG: KASAN: slab-use-after-free in __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:132 [inline]
BUG: KASAN: slab-use-after-free in _raw_spin_lock_irqsave+0x40/0x60 kernel/locking/spinlock.c:162
Read of size 1 at addr ffff888028a58968 by task syz.6.2811/20778
CPU: 1 UID: 0 PID: 20778 Comm: syz.6.2811 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xba/0x230 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
__kasan_check_byte+0x2a/0x40 mm/kasan/common.c:574
kasan_check_byte include/linux/kasan.h:402 [inline]
lock_acquire+0x79/0x2e0 kernel/locking/lockdep.c:5842
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:132 [inline]
_raw_spin_lock_irqsave+0x40/0x60 kernel/locking/spinlock.c:162
rt_mutex_slowunlock+0xbf/0x8b0 kernel/locking/rtmutex.c:1417
end_dirop fs/namei.c:2954 [inline]
end_creating include/linux/namei.h:126 [inline]
end_creating_path fs/namei.c:4968 [inline]
filename_mknodat+0x464/0x670 fs/namei.c:5162
__do_sys_mknod fs/namei.c:5185 [inline]
__se_sys_mknod+0x3a/0x150 fs/namei.c:5182
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fd8003ec819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fd7fe63e028 EFLAGS: 00000246 ORIG_RAX: 0000000000000085
RAX: ffffffffffffffda RBX: 00007fd800665fa0 RCX: 00007fd8003ec819
RDX: 0000000000000000 RSI: 0000000008001420 RDI: 0000200000000040
RBP: 00007fd800482c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fd800666038 R14: 00007fd800665fa0 R15: 00007fff9b1495d8
</TASK>
Allocated by task 20778:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
unpoison_slab_object mm/kasan/common.c:340 [inline]
__kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:366
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4538 [inline]
slab_alloc_node mm/slub.c:4866 [inline]
kmem_cache_alloc_lru_noprof+0x33c/0x680 mm/slub.c:4885
__d_alloc+0x37/0x6f0 fs/dcache.c:1740
d_alloc+0x4b/0x190 fs/dcache.c:1819
lookup_one_qstr_excl+0xdc/0x360 fs/namei.c:1801
__start_dirop fs/namei.c:2931 [inline]
start_dirop fs/namei.c:2940 [inline]
filename_create+0x20e/0x370 fs/namei.c:4928
filename_mknodat+0x18c/0x670 fs/namei.c:5136
__do_sys_mknod fs/namei.c:5185 [inline]
__se_sys_mknod+0x3a/0x150 fs/namei.c:5182
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task 20:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2685 [inline]
slab_free mm/slub.c:6165 [inline]
kmem_cache_free+0x185/0x6b0 mm/slub.c:6295
rcu_do_batch kernel/rcu/tree.c:2617 [inline]
rcu_core kernel/rcu/tree.c:2869 [inline]
rcu_cpu_kthread+0x99e/0x1470 kernel/rcu/tree.c:2957
smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
Last potentially related work creation:
kasan_save_stack+0x3e/0x60 mm/kasan/common.c:57
kasan_record_aux_stack+0xbd/0xd0 mm/kasan/generic.c:556
__call_rcu_common kernel/rcu/tree.c:3131 [inline]
call_rcu+0xee/0x890 kernel/rcu/tree.c:3251
__dentry_kill+0x469/0x5e0 fs/dcache.c:689
finish_dput+0xc9/0x480 fs/dcache.c:879
ocfs2_dentry_attach_lock+0x748/0xe20 fs/ocfs2/dcache.c:328
ocfs2_lookup+0x3fe/0xa50 fs/ocfs2/namei.c:163
lookup_open fs/namei.c:4462 [inline]
open_last_lookups fs/namei.c:4589 [inline]
path_openat+0x11cb/0x38a0 fs/namei.c:4833
do_file_open+0x23e/0x4a0 fs/namei.c:4865
do_sys_openat2+0x113/0x200 fs/open.c:1366
do_sys_open fs/open.c:1372 [inline]
__do_sys_openat fs/open.c:1388 [inline]
__se_sys_openat fs/open.c:1383 [inline]
__x64_sys_openat+0x138/0x170 fs/open.c:1383
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
The buggy address belongs to the object at ffff888028a58898
which belongs to the cache dentry of size 376
The buggy address is located 208 bytes inside of
freed 376-byte region [ffff888028a58898, ffff888028a58a10)
The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x28a58
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff8880358df301
flags: 0x80000000000040(head|node=0|zone=1)
page_type: f5(slab)
raw: 0080000000000040 ffff88801b2a18c0 dead000000000100 dead000000000122
raw: 0000000000000000 0000000800120012 00000000f5000000 ffff8880358df301
head: 0080000000000040 ffff88801b2a18c0 dead000000000100 dead000000000122
head: 0000000000000000 0000000800120012 00000000f5000000 ffff8880358df301
head: 0080000000000001 ffffea0000a29601 00000000ffffffff 00000000ffffffff
head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000002
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 1, migratetype Reclaimable, gfp_mask 0xd20d0(__GFP_RECLAIMABLE|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5181, tgid 5181 (udevadm), ts 36033904550, free_ts 35987001422
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x231/0x280 mm/page_alloc.c:1889
prep_new_page mm/page_alloc.c:1897 [inline]
get_page_from_freelist+0x28bb/0x2950 mm/page_alloc.c:3962
__alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5250
alloc_slab_page mm/slub.c:3292 [inline]
allocate_slab+0x77/0x660 mm/slub.c:3481
new_slab mm/slub.c:3539 [inline]
refill_objects+0x334/0x3c0 mm/slub.c:7175
refill_sheaf mm/slub.c:2812 [inline]
__pcs_replace_empty_main+0x35c/0x710 mm/slub.c:4615
alloc_from_pcs mm/slub.c:4717 [inline]
slab_alloc_node mm/slub.c:4851 [inline]
kmem_cache_alloc_lru_noprof+0x433/0x680 mm/slub.c:4885
__d_alloc+0x37/0x6f0 fs/dcache.c:1740
d_alloc_parallel+0xe6/0x1610 fs/dcache.c:2604
__lookup_slow+0x152/0x440 fs/namei.c:1901
lookup_slow+0x53/0x70 fs/namei.c:1933
walk_component fs/namei.c:2279 [inline]
link_path_walk+0xd1e/0x18d0 fs/namei.c:2653
path_lookupat+0xe4/0x8c0 fs/namei.c:2809
filename_lookup+0x256/0x5d0 fs/namei.c:2839
vfs_statx+0xfc/0x200 fs/stat.c:353
vfs_fstatat+0x11b/0x170 fs/stat.c:373
page last free pid 5174 tgid 5174 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
__free_pages_prepare mm/page_alloc.c:1433 [inline]
__free_frozen_pages+0xfe3/0x1170 mm/page_alloc.c:2978
__slab_free+0x24f/0x2a0 mm/slub.c:5573
qlink_free mm/kasan/quarantine.c:163 [inline]
qlist_free_all+0x97/0x100 mm/kasan/quarantine.c:179
kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
__kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:350
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4538 [inline]
slab_alloc_node mm/slub.c:4866 [inline]
kmem_cache_alloc_noprof+0x33b/0x680 mm/slub.c:4873
alloc_filename fs/namei.c:142 [inline]
do_getname+0x2e/0x250 fs/namei.c:182
getname include/linux/fs.h:2512 [inline]
class_filename_constructor include/linux/fs.h:2539 [inline]
do_sys_openat2+0xca/0x200 fs/open.c:1365
do_sys_open fs/open.c:1372 [inline]
__do_sys_openat fs/open.c:1388 [inline]
__se_sys_openat fs/open.c:1383 [inline]
__x64_sys_openat+0x138/0x170 fs/open.c:1383
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Memory state around the buggy address:
ffff888028a58800: fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc fc
ffff888028a58880: fc fc fc fa fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888028a58900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888028a58980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888028a58a00: fb fb fc fc fc fc fc fc fc fc 00 00 00 00 00 00
==================================================================
Reply: The bug report is self-consistent and actionable. It describes a `slab-use-after-free` on a `dentry` object in a `PREEMPT_RT` kernel.
### Analysis of the Stacks:
1. **Bad Access Stack**: The UAF occurs in `rt_mutex_slowunlock+0xbf/0x8b0`, which is called by `end_dirop` (inlined in `filename_mknodat`). In a `PREEMPT_RT` kernel, `spin_lock` is implemented using `rt_mutex`. The stack trace indicates that `dput(de)` was called in `end_dirop`, which then called `fast_dput`. Inside `fast_dput`, a `spin_unlock(&dentry->d_lock)` was performed. The access at offset 208 of the dentry corresponds exactly to `dentry->d_lockref.lock.lock.wait_lock.dep_map` (where `d_lockref` is at offset 184, and the `dep_map` of the `raw_spinlock` inside the `rt_mutex` is at offset 24).
2. **Allocation Stack**: The dentry was allocated by task 20778 in `lookup_one_qstr_excl` via `d_alloc`. This is the "original" dentry created during the lookup process in `filename_create`.
3. **Free Stack**: The dentry was freed by an RCU callback (`rcu_do_batch`). The "Last potentially related work creation" stack trace shows that `call_rcu` was triggered by `ocfs2_dentry_attach_lock+0x748/0xe20` at `fs/ocfs2/dcache.c:328`, which calls `dput(alias)`.
### Root Cause:
The bug is a race condition in the `ocfs2` filesystem's dentry locking logic. In `ocfs2_dentry_attach_lock`, the code searches for an existing alias using `ocfs2_find_local_alias`. If another thread has already attached a lock to the same dentry (setting `dentry->d_fsdata`), `ocfs2_find_local_alias` may return the dentry itself as an `alias`. The function then calls `dput(alias)` at line 328. If this `dput` drops the reference count to zero (e.g., because another concurrent thread also dropped its reference), the dentry is queued for freeing via RCU. However, the `ocfs2_lookup` function continues and returns this dentry (or the caller `lookup_one_qstr_excl` continues to use it), leading to a Use-After-Free when the dentry is later accessed in `end_dirop`.
### Conclusion:
The report contains all three required stack traces, they are consistent with each other and the memory layout of the involved structures, and they point to a specific logical error in the `ocfs2` subsystem.
|
| 5/2 |
2026/04/05 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 7858
output: 82
thoughts: 449
Thoughts: **Analyzing a Nasty Slab-Use-After-Free Bug**
Okay, so I'm looking at a pretty nasty bug report here: a `slab-use-after-free` in `__raw_spin_lock_irqsave`. The crash involves a read, size one, at a specific address. The object in question is a `dentry`, which is 376 bytes in size. The interesting part is that the access happens within `rt_mutex_slowunlock`, specifically at `kernel/locking/rtmutex.c:1417`. That function is calling `_raw_spin_lock_irqsave`, which is the key here.
Let me take a look at the full stack trace: it starts with the `_raw_spin_lock_irqsave` call within `rt_mutex_slowunlock`, and then goes up to `end_dirop` and `filename_mknodat`. It looks like this is all happening during file operations.
From what I can gather, the `dentry` was allocated by `__d_alloc`, so standard dentry allocation. And it seems to have been freed by `rcu_do_batch` which eventually calls `kmem_cache_free`. The crucial "last potentially related work creation" indicates a `call_rcu` from `__dentry_kill`. That's likely the key: it's being freed with RCU, and something is still trying to access it after the fact.
The stack points right to `rt_mutex_slowunlock`, so I need to check `kernel/locking/rtmutex.c:1417` closely. I'll need to look at that function, and the code around it, to figure out what's going on. I'll need to trace the dentry's lifecycle, specifically how it's handled within `rt_mutex_slowunlock` and related file system operations like `end_dirop`.
|
| 6/2 |
2026/04/05 00:25 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/rtmutex.c IncludeLines:true Name:rt_mutex_slowunlock]
Results: map[SourceCode:1408: /*
1409: * Slow path to release a rt-mutex.
1410: */
1411: static void __sched rt_mutex_slowunlock(struct rt_mutex_base *lock)
1412: {
1413: DEFINE_RT_WAKE_Q(wqh);
1414: unsigned long flags;
1415:
1416: /* irqsave required to support early boot calls */
1417: raw_spin_lock_irqsave(&lock->wait_lock, flags);
1418:
1419: debug_rt_mutex_unlock(lock);
1420:
1421: /*
1422: * We must be careful here if the fast path is enabled. If we
1423: * have no waiters queued we cannot set owner to NULL here
1424: * because of:
1425: *
1426: * foo->lock->owner = NULL;
1427: * rtmutex_lock(foo->lock); <- fast path
1428: * free = atomic_dec_and_test(foo->refcnt);
1429: * rtmutex_unlock(foo->lock); <- fast path
1430: * if (free)
1431: * kfree(foo);
1432: * raw_spin_unlock(foo->lock->wait_lock);
1433: *
1434: * So for the fastpath enabled kernel:
1435: *
1436: * Nothing can set the waiters bit as long as we hold
1437: * lock->wait_lock. So we do the following sequence:
1438: *
1439: * owner = rt_mutex_owner(lock);
1440: * clear_rt_mutex_waiters(lock);
1441: * raw_spin_unlock(&lock->wait_lock);
1442: * if (cmpxchg(&lock->owner, owner, 0) == owner)
1443: * return;
1444: * goto retry;
1445: *
1446: * The fastpath disabled variant is simple as all access to
1447: * lock->owner is serialized by lock->wait_lock:
1448: *
1449: * lock->owner = NULL;
1450: * raw_spin_unlock(&lock->wait_lock);
1451: */
1452: while (!rt_mutex_has_waiters(lock)) {
1453: /* Drops lock->wait_lock ! */
1454: if (unlock_rt_mutex_safe(lock, flags) == true)
1455: return;
1456: /* Relock the rtmutex and try again */
1457: raw_spin_lock_irqsave(&lock->wait_lock, flags);
1458: }
1459:
1460: /*
1461: * The wakeup next waiter path does not suffer from the above
1462: * race. See the comments there.
1463: *
1464: * Queue the next waiter for wakeup once we release the wait_lock.
1465: */
1466: mark_wakeup_next_waiter(&wqh, lock);
1467: raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1468:
1469: rt_mutex_wake_up_q(&wqh);
1470: }
SourceFile:kernel/locking/rtmutex.c]
|
| 7/2 |
2026/04/05 00:25 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:end_dirop]
Results: map[SourceCode:2943: /**
2944: * end_dirop - signal completion of a dirop
2945: * @de: the dentry which was returned by start_dirop or similar.
2946: *
2947: * If the de is an error, nothing happens. Otherwise any lock taken to
2948: * protect the dentry is dropped and the dentry itself is release (dput()).
2949: */
2950: void end_dirop(struct dentry *de)
2951: {
2952: if (!IS_ERR(de)) {
2953: inode_unlock(de->d_parent->d_inode);
2954: dput(de);
2955: }
2956: }
SourceFile:fs/namei.c]
|
| 8/2 |
2026/04/05 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 10033
output: 20
thoughts: 777
Thoughts: **Analyzing a Potential Use-After-Free in the Filesystem**
Okay, so I'm looking at this stack trace and something's immediately jumping out at me. It's showing `rt_mutex_slowunlock` being called, and the trace points to `end_dirop` in `fs/namei.c` at line 2954. But then, isn't `dput(de)` on line 2954? That doesn't seem right. `dput` shouldn't be calling a real-time mutex unlock, unless something is inlined.
Let me double-check `fs/namei.c`. Line 2953 is `inode_unlock(de->d_parent->d_inode)`, which could potentially call `rt_mutex_slowunlock` under the right circumstances. And the report does mention a `PREEMPT_RT` kernel: `syz.6.2811/20778 Not tainted syzkaller #0 PREEMPT_{RT,(full)}`. Bingo. In a `PREEMPT_RT` kernel, the `rwsem` used by `inode_lock`/`unlock` is implemented using `rt_mutex`. The stack trace seems to be consistent with this, since the `end_dirop` function is inlined and showing the unlock in the same line as `dput`. The call to `rt_mutex_slowunlock` must be coming from `inode_unlock` when the `PREEMPT_RT` is enabled.
Let me refresh my memory of the `end_dirop` function itself:
```c
2950: void end_dirop(struct dentry *de)
2951: {
2952: if (!IS_ERR(de)) {
2953: inode_unlock(de->d_parent->d_inode);
2954: dput(de);
2955: }
2956: }
```
Okay, the code confirms my suspicion. `inode_unlock` is at line 2953, and `dput` is at line 2954. So when it's inlined, the `inode_unlock` is going to look like line 2953 when the stack trace reports it. The `dput` on line 2954 would be directly after `inode_unlock`. In a `PREEMPT_RT` kernel, `inode_unlock` translates into something like `up_write(&inode->i_rwsem)`, which then calls `rt_mutex_unlock`.
The UAF is reported to be on a `dentry`. Time to check the layout of `struct dentry` and get the address information so I can reconstruct what's going on. The UAF is at 208 bytes inside a 376-byte region, which is a good clue. I need to understand what's happening to that dentry after the unlock and `dput`.
|
| 9/2 |
2026/04/05 00:25 |
tool |
codesearch-struct-layout |
0m
Args:
Results: map[Fields:[map[Name:d_flags OffsetBits:0 SizeBits:32] map[Name:d_seq OffsetBits:64 SizeBits:448] map[Name:d_hash OffsetBits:512 SizeBits:128] map[Name:d_parent OffsetBits:640 SizeBits:64] map[Name: OffsetBits:704 SizeBits:128] map[Name:d_inode OffsetBits:832 SizeBits:64] map[Name:d_shortname OffsetBits:896 SizeBits:320] map[Name:d_op OffsetBits:1216 SizeBits:64] map[Name:d_sb OffsetBits:1280 SizeBits:64] map[Name:d_time OffsetBits:1344 SizeBits:64] map[Name:d_fsdata OffsetBits:1408 SizeBits:64] map[Name:d_lockref OffsetBits:1472 SizeBits:1088] map[Name: OffsetBits:2560 SizeBits:128] map[Name:d_sib OffsetBits:2688 SizeBits:128] map[Name:d_children OffsetBits:2816 SizeBits:64] map[Name:d_u OffsetBits:2880 SizeBits:128]]]
|
| 10/2 |
2026/04/05 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 11854
output: 20
thoughts: 92
Thoughts: Okay, let me break this down.
**Analyzing the Memory Layout**
Right, so I'm looking at this memory structure, and the critical variable, `d_lockref`, is starting at an offset of 1472 bits, which, after the conversion, is 184 bytes into the structure. Now, the Use-After-Free vulnerability, or UAF, is happening at 208 bytes.
Hmm, let's see... If `d_lockref` begins at 184 bytes, and the UAF is at 208 bytes, then the distance between the start of `d_lockref` and where the UAF manifests itself is just a simple calculation: 208 bytes - 184 bytes = 24 bytes. So, the bug is 24 bytes *into* `d_lockref`.
Now, I need to understand what's happening within those 24 bytes, and that means I need to dig into the definition of `struct lockref`. Time to consult the structure definition to figure out the relevant data members.
|
| 11/2 |
2026/04/05 00:25 |
tool |
codesearch-struct-layout |
0m
Args:
Results: map[Fields:[map[Name: OffsetBits:0 SizeBits:1088]]]
|
| 12/2 |
2026/04/05 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 12234
output: 23
thoughts: 27
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving Deeper into `lockref`**
Okay, so I was initially thinking about `lockref` in a fairly straightforward manner, but something's telling me it's more nuanced than my initial assessment. I need to take a closer look. Let's not make any assumptions here. The best place to refresh my understanding and dig into the actual implementation is going to be `include/linux/lockref.h`. That header file is where the core definitions and structures are. Time to see what's really going on under the hood before I make any definitive pronouncements about it. I have a feeling the interaction with the locking primitives will be more intricate than I first imagined.
|
| 13/2 |
2026/04/05 00:25 |
tool |
read-file |
0m
Args: map[File:include/linux/lockref.h]
Results: map[Contents:/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_LOCKREF_H
#define __LINUX_LOCKREF_H
/*
* Locked reference counts.
*
* These are different from just plain atomic refcounts in that they
* are atomic with respect to the spinlock that goes with them. In
* particular, there can be implementations that don't actually get
* the spinlock for the common decrement/increment operations, but they
* still have to check that the operation is done semantically as if
* the spinlock had been taken (using a cmpxchg operation that covers
* both the lock and the count word, or using memory transactions, for
* example).
*/
#include <linux/spinlock.h>
#include <generated/bounds.h>
#define USE_CMPXCHG_LOCKREF \
(IS_ENABLED(CONFIG_ARCH_USE_CMPXCHG_LOCKREF) && \
IS_ENABLED(CONFIG_SMP) && SPINLOCK_SIZE <= 4)
struct lockref {
union {
#if USE_CMPXCHG_LOCKREF
aligned_u64 lock_count;
#endif
struct {
spinlock_t lock;
int count;
};
};
};
/**
* lockref_init - Initialize a lockref
* @lockref: pointer to lockref structure
*
* Initializes @lockref->count to 1.
*/
static inline void lockref_init(struct lockref *lockref)
{
spin_lock_init(&lockref->lock);
lockref->count = 1;
}
void lockref_get(struct lockref *lockref);
int lockref_put_return(struct lockref *lockref);
bool lockref_get_not_zero(struct lockref *lockref);
bool lockref_put_or_lock(struct lockref *lockref) __cond_acquires(false, &lockref->lock);
void lockref_mark_dead(struct lockref *lockref);
bool lockref_get_not_dead(struct lockref *lockref);
/* Must be called under spinlock for reliable results */
static inline bool __lockref_is_dead(const struct lockref *l)
{
return ((int)l->count < 0);
}
#endif /* __LINUX_LOCKREF_H */
]
|
| 14/2 |
2026/04/05 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 13012
output: 24
thoughts: 289
Thoughts: **Dissecting the `struct lockref` Implementation**
Okay, so I'm looking at this `struct lockref` and it's hitting me – wait, it includes a `spinlock_t`. That rings a bell, particularly with how `PREEMPT_RT` handles spinlocks. Right, in a real-time context like `PREEMPT_RT`, a `spinlock_t` isn't your garden-variety spinlock; it's implemented as an `rt_mutex`. Now, let's refresh our memory on `struct lockref` itself. We have:
```c
struct lockref {
union {
#if USE_CMPXCHG_LOCKREF
aligned_u64 lock_count;
#endif
struct {
spinlock_t lock;
int count;
};
};
};
```
So, if `USE_CMPXCHG_LOCKREF` is *false* – which it absolutely is on `PREEMPT_RT` because `spinlock_t` is larger than our atomic compare-and-swap type – we're dealing with a `spinlock_t` and an `int`. But, that `spinlock_t`... on `PREEMPT_RT`, that's effectively a `struct rt_mutex_base`. Let's confirm that. Time to check how `spinlock_t` is defined in the `PREEMPT_RT` context. Ah, here it is:
```c
typedef struct spinlock {
union {
struct raw_spinlock rlock;
#ifdef CONFIG_PREEMPT_RT
struct rt_mutex_base lock;
#endif
};
} spinlock_t;
```
Excellent, just as expected. Now, I need to double-check my mental map here and go verify the definition of `spinlock_t` in `include/linux/spinlock_types.h`.
|
| 15/2 |
2026/04/05 00:25 |
tool |
read-file |
0m
Args: map[File:include/linux/spinlock_types.h]
Results: map[Contents:#ifndef __LINUX_SPINLOCK_TYPES_H
#define __LINUX_SPINLOCK_TYPES_H
/*
* include/linux/spinlock_types.h - generic spinlock type definitions
* and initializers
*
* portions Copyright 2005, Red Hat, Inc., Ingo Molnar
* Released under the General Public License (GPL).
*/
#include <linux/spinlock_types_raw.h>
#ifndef CONFIG_PREEMPT_RT
/* Non PREEMPT_RT kernels map spinlock to raw_spinlock */
context_lock_struct(spinlock) {
union {
struct raw_spinlock rlock;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
# define LOCK_PADSIZE (offsetof(struct raw_spinlock, dep_map))
struct {
u8 __padding[LOCK_PADSIZE];
struct lockdep_map dep_map;
};
#endif
};
};
typedef struct spinlock spinlock_t;
#define ___SPIN_LOCK_INITIALIZER(lockname) \
{ \
.raw_lock = __ARCH_SPIN_LOCK_UNLOCKED, \
SPIN_DEBUG_INIT(lockname) \
SPIN_DEP_MAP_INIT(lockname) }
#define __SPIN_LOCK_INITIALIZER(lockname) \
{ { .rlock = ___SPIN_LOCK_INITIALIZER(lockname) } }
#define __SPIN_LOCK_UNLOCKED(lockname) \
(spinlock_t) __SPIN_LOCK_INITIALIZER(lockname)
#define DEFINE_SPINLOCK(x) spinlock_t x = __SPIN_LOCK_UNLOCKED(x)
#else /* !CONFIG_PREEMPT_RT */
/* PREEMPT_RT kernels map spinlock to rt_mutex */
#include <linux/rtmutex.h>
context_lock_struct(spinlock) {
struct rt_mutex_base lock;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
};
typedef struct spinlock spinlock_t;
#define __SPIN_LOCK_UNLOCKED(name) \
{ \
.lock = __RT_MUTEX_BASE_INITIALIZER(name.lock), \
SPIN_DEP_MAP_INIT(name) \
}
#define __LOCAL_SPIN_LOCK_UNLOCKED(name) \
{ \
.lock = __RT_MUTEX_BASE_INITIALIZER(name.lock), \
LOCAL_SPIN_DEP_MAP_INIT(name) \
}
#define DEFINE_SPINLOCK(name) \
spinlock_t name = __SPIN_LOCK_UNLOCKED(name)
#endif /* CONFIG_PREEMPT_RT */
#include <linux/rwlock_types.h>
#endif /* __LINUX_SPINLOCK_TYPES_H */
]
|
| 16/2 |
2026/04/05 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 14452
output: 23
thoughts: 146
Thoughts: **Analyzing the RT Preempt Spinlock and Dentry Structure**
Alright, let's break this down. With `PREEMPT_RT` enabled, a standard `spinlock_t` actually holds a `struct rt_mutex_base lock` and a `struct lockdep_map dep_map`. The vulnerability, this UAF, is happening at offset 208 within the dentry. Now, `d_lockref` sits at offset 184. That places the UAF 24 bytes into `d_lockref`. Crucially, `d_lockref` itself begins with a `spinlock_t lock`, so we're talking about 24 bytes within that `spinlock_t` member. Because `spinlock_t` starts with `struct rt_mutex_base lock` I want to check the layout of `struct rt_mutex_base`. Let's see what we're dealing with there.
|
| 17/2 |
2026/04/05 00:25 |
tool |
codesearch-struct-layout |
0m
Args:
Results: map[Fields:[map[Name:wait_lock OffsetBits:0 SizeBits:512] map[Name:waiters OffsetBits:512 SizeBits:128] map[Name:owner OffsetBits:640 SizeBits:64]]]
|
| 18/2 |
2026/04/05 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 14893
output: 21
thoughts: 48
Thoughts: Here's my take on the thought process:
**Diving into `rt_mutex_base` and Spinlocks**
Okay, so I'm looking at this code and need to clarify the structure. First things first, `wait_lock` is at offset zero within the `rt_mutex_base` structure. That's a good starting point. Now, if I remember correctly, `wait_lock` is a `raw_spinlock_t`. Time to confirm the layout of `struct raw_spinlock` to be sure I understand the data it contains. That'll help me figure out how this mutex is really working at the lowest level.
|
| 19/2 |
2026/04/05 00:25 |
tool |
codesearch-struct-layout |
0m
Args:
Results: map[Fields:[map[Name:raw_lock OffsetBits:0 SizeBits:32] map[Name:magic OffsetBits:32 SizeBits:32] map[Name:owner_cpu OffsetBits:64 SizeBits:32] map[Name:owner OffsetBits:128 SizeBits:64] map[Name:dep_map OffsetBits:192 SizeBits:320]]]
|
| 20/2 |
2026/04/05 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 15212
output: 40
thoughts: 1651
Thoughts: **Analyzing a Use-After-Free Vulnerability in the dentry Cache**
Okay, let's break this down. First, I'm tracing a Use-After-Free (UAF) bug. The crash report pinpoints an access at a specific offset within a `dentry` structure. I need to verify that offset and what it corresponds to. I'm calculating that `dentry->d_lockref.lock.lock.wait_lock.dep_map` is at offset 208, and it matches the UAF address in the crash report. So the code is trying to access the lock dependency map, which is expected during lock operations. The stack trace confirms the call chain: `rt_mutex_slowunlock` calls `raw_spin_lock_irqsave`, which calls `lock_acquire`, which is the function that accesses the `dep_map`.
Now the core issue: Why is `rt_mutex_slowunlock`, which should be acting on the parent inode's `i_rwsem`, apparently accessing a `dentry`'s `lockref`? That's the anomaly. `rt_mutex_slowunlock` is invoked from `inode_unlock`, where it is called with the parent's inode. So the access *should* be through `de->d_parent->d_inode`. But KASAN (Kernel Address Sanitizer) is flagging the dentry object itself. It says the buggy access is 208 bytes inside a freed dentry object of size 376 bytes.
My working hypothesis is that somehow `de->d_parent` has become the freed dentry. Looking at the `end_dirop` function, this looks like the function in which the access is occurring. If `de` is valid, so *should* `de->d_parent` be. The question is, is `de` valid?
Looking at the allocation and free stacks gives a clearer picture. The allocation involves `filename_create`, calling `start_dirop` and `d_alloc` at some point. The free path includes `kmem_cache_free` and `__dentry_kill`. The dentry `de` that was passed to `end_dirop` was created during a `filename_create` operation. The free operation is happening in a separate call path: the RCU infrastructure is being used to deallocate the dentry. The free is happening inside `ocfs2_dentry_attach_lock`, suggesting some interesting interactions within this specific filesystem.
The fact that `ocfs2_lookup` is in the free path is a significant clue. The allocation was in `lookup_one_qstr_excl`, which calls `d_alloc`. This is likely a filesystem-specific issue, and my focus should shift to understanding how `ocfs2` interacts with the dentry cache. I should look specifically at `filename_mknodat` to understand how the dentry is being used.
|
| 21/2 |
2026/04/05 00:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:filename_mknodat]
Results: map[SourceCode:5122: int filename_mknodat(int dfd, struct filename *name, umode_t mode,
5123: unsigned int dev)
5124: {
5125: struct delegated_inode di = { };
5126: struct mnt_idmap *idmap;
5127: struct dentry *dentry;
5128: struct path path;
5129: int error;
5130: unsigned int lookup_flags = 0;
5131:
5132: error = may_mknod(mode);
5133: if (error)
5134: return error;
5135: retry:
5136: dentry = filename_create(dfd, name, &path, lookup_flags);
5137: if (IS_ERR(dentry))
5138: return PTR_ERR(dentry);
5139:
5140: error = security_path_mknod(&path, dentry,
5141: mode_strip_umask(path.dentry->d_inode, mode), dev);
5142: if (error)
5143: goto out2;
5144:
5145: idmap = mnt_idmap(path.mnt);
5146: switch (mode & S_IFMT) {
5147: case 0: case S_IFREG:
5148: error = vfs_create(idmap, dentry, mode, &di);
5149: if (!error)
5150: security_path_post_mknod(idmap, dentry);
5151: break;
5152: case S_IFCHR: case S_IFBLK:
5153: error = vfs_mknod(idmap, path.dentry->d_inode,
5154: dentry, mode, new_decode_dev(dev), &di);
5155: break;
5156: case S_IFIFO: case S_IFSOCK:
5157: error = vfs_mknod(idmap, path.dentry->d_inode,
5158: dentry, mode, 0, &di);
5159: break;
5160: }
5161: out2:
5162: end_creating_path(&path, dentry);
5163: if (is_delegated(&di)) {
5164: error = break_deleg_wait(&di);
5165: if (!error)
5166: goto retry;
5167: }
5168: if (retry_estale(error, lookup_flags)) {
5169: lookup_flags |= LOOKUP_REVAL;
5170: goto retry;
5171: }
5172: return error;
5173: }
SourceFile:fs/namei.c]
|
| 22/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 18348
output: 40
thoughts: 156
Thoughts: **Debugging a Filesystem Operation**
Okay, so the debugger just pointed me to `end_creating_path(&path, dentry)` at line 5162. Hmm, that's where the problem's hitting. The stack trace is pretty clear: it's showing `end_dirop`, then `end_creating` (which is inlined), then `end_creating_path` (also inlined) and finally, the call in question, `filename_mknodat` at line 5162 of `fs/namei.c`. Right, so `end_creating_path` is calling `end_creating`, I need to keep that in mind. Looks like the `end_creating_path` function in `fs/namei.c` is the next logical place to look. I'll get that opened up... let's see what's happening there.
|
| 23/2 |
2026/04/05 00:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:end_creating_path]
Results: map[SourceCode:4955: /**
4956: * end_creating_path - finish a code section started by start_creating_path()
4957: * @path: the path instantiated by start_creating_path()
4958: * @dentry: the dentry returned by start_creating_path()
4959: *
4960: * end_creating_path() will unlock and locks taken by start_creating_path()
4961: * and drop an references that were taken. It should only be called
4962: * if start_creating_path() returned a non-error.
4963: * If vfs_mkdir() was called and it returned an error, that error *should*
4964: * be passed to end_creating_path() together with the path.
4965: */
4966: void end_creating_path(const struct path *path, struct dentry *dentry)
4967: {
4968: end_creating(dentry);
4969: mnt_drop_write(path->mnt);
4970: path_put(path);
4971: }
SourceFile:fs/namei.c]
|
| 24/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 19066
output: 23
thoughts: 17
Thoughts: Okay, here's my interpretation of that thought, expressed as if it's my own internal monologue, keeping in mind I'm an expert in my field:
**Internal Musings on File System Implementation**
Right, so I'm thinking about file system internals again. Specifically, my mind is drifting towards `end_creating`... specifically in `include/linux/namei.h`. That's a critical spot. I'm almost certain I've reviewed that function many times, but let's just make sure I have the full picture. My immediate recall is that `end_creating` is part of the namei lookup process and is responsible for finalizing a new file or directory creation, after the initial path resolution and allocation have occurred. It would handle tasks like potentially setting initial attributes, maybe linking the new inode into its parent directory, and calling any necessary file system-specific callbacks to complete the new object construction. I need to make sure I am totally familiar with its interaction with the inode itself, its implications on concurrency if multiple processes are creating files simultaneously in the same directory, and whether there are any potential race conditions I should be on the lookout for. It's a key function for understanding how the kernel deals with file creation and I need to solidify my understanding of how it interacts with the underlying file system, whether it is ext4 or XFS or any other virtual file system. I should probably review the surrounding code to ensure I'm not missing anything. Yes, focusing on `end_creating` is the right angle.
|
| 25/2 |
2026/04/05 00:26 |
tool |
read-file |
0m
Args: map[File:include/linux/namei.h]
Results: map[Contents:/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_NAMEI_H
#define _LINUX_NAMEI_H
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/path.h>
#include <linux/fcntl.h>
#include <linux/errno.h>
#include <linux/fs_struct.h>
enum { MAX_NESTED_LINKS = 8 };
#define MAXSYMLINKS 40
/*
* Type of the last component on LOOKUP_PARENT
*/
enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};
/* pathwalk mode */
#define LOOKUP_FOLLOW BIT(0) /* follow links at the end */
#define LOOKUP_DIRECTORY BIT(1) /* require a directory */
#define LOOKUP_AUTOMOUNT BIT(2) /* force terminal automount */
#define LOOKUP_EMPTY BIT(3) /* accept empty path [user_... only] */
#define LOOKUP_LINKAT_EMPTY BIT(4) /* Linkat request with empty path. */
#define LOOKUP_DOWN BIT(5) /* follow mounts in the starting point */
#define LOOKUP_MOUNTPOINT BIT(6) /* follow mounts in the end */
#define LOOKUP_REVAL BIT(7) /* tell ->d_revalidate() to trust no cache */
#define LOOKUP_RCU BIT(8) /* RCU pathwalk mode; semi-internal */
#define LOOKUP_CACHED BIT(9) /* Only do cached lookup */
#define LOOKUP_PARENT BIT(10) /* Looking up final parent in path */
/* 5 spare bits for pathwalk */
/* These tell filesystem methods that we are dealing with the final component... */
#define LOOKUP_OPEN BIT(16) /* ... in open */
#define LOOKUP_CREATE BIT(17) /* ... in object creation */
#define LOOKUP_EXCL BIT(18) /* ... in target must not exist */
#define LOOKUP_RENAME_TARGET BIT(19) /* ... in destination of rename() */
/* 4 spare bits for intent */
/* Scoping flags for lookup. */
#define LOOKUP_NO_SYMLINKS BIT(24) /* No symlink crossing. */
#define LOOKUP_NO_MAGICLINKS BIT(25) /* No nd_jump_link() crossing. */
#define LOOKUP_NO_XDEV BIT(26) /* No mountpoint crossing. */
#define LOOKUP_BENEATH BIT(27) /* No escaping from starting point. */
#define LOOKUP_IN_ROOT BIT(28) /* Treat dirfd as fs root. */
/* LOOKUP_* flags which do scope-related checks based on the dirfd. */
#define LOOKUP_IS_SCOPED (LOOKUP_BENEATH | LOOKUP_IN_ROOT)
/* 3 spare bits for scoping */
extern int path_pts(struct path *path);
extern int user_path_at(int, const char __user *, unsigned, struct path *);
struct dentry *lookup_one_qstr_excl(const struct qstr *name,
struct dentry *base,
unsigned int flags);
extern int kern_path(const char *, unsigned, struct path *);
struct dentry *kern_path_parent(const char *name, struct path *parent);
extern struct dentry *start_creating_path(int, const char *, struct path *, unsigned int);
extern struct dentry *start_creating_user_path(int, const char __user *, struct path *, unsigned int);
extern void end_creating_path(const struct path *, struct dentry *);
extern struct dentry *start_removing_path(const char *, struct path *);
extern struct dentry *start_removing_user_path_at(int , const char __user *, struct path *);
static inline void end_removing_path(const struct path *path , struct dentry *dentry)
{
end_creating_path(path, dentry);
}
int vfs_path_parent_lookup(struct filename *filename, unsigned int flags,
struct path *parent, struct qstr *last, int *type,
const struct path *root);
int vfs_path_lookup(struct dentry *, struct vfsmount *, const char *,
unsigned int, struct path *);
extern struct dentry *try_lookup_noperm(struct qstr *, struct dentry *);
extern struct dentry *lookup_noperm(struct qstr *, struct dentry *);
extern struct dentry *lookup_noperm_unlocked(struct qstr *, struct dentry *);
extern struct dentry *lookup_noperm_positive_unlocked(struct qstr *, struct dentry *);
struct dentry *lookup_one(struct mnt_idmap *, struct qstr *, struct dentry *);
struct dentry *lookup_one_unlocked(struct mnt_idmap *idmap,
struct qstr *name, struct dentry *base);
struct dentry *lookup_one_positive_unlocked(struct mnt_idmap *idmap,
struct qstr *name,
struct dentry *base);
struct dentry *lookup_one_positive_killable(struct mnt_idmap *idmap,
struct qstr *name,
struct dentry *base);
struct dentry *start_creating(struct mnt_idmap *idmap, struct dentry *parent,
struct qstr *name);
struct dentry *start_removing(struct mnt_idmap *idmap, struct dentry *parent,
struct qstr *name);
struct dentry *start_creating_killable(struct mnt_idmap *idmap,
struct dentry *parent,
struct qstr *name);
struct dentry *start_removing_killable(struct mnt_idmap *idmap,
struct dentry *parent,
struct qstr *name);
struct dentry *start_creating_noperm(struct dentry *parent, struct qstr *name);
struct dentry *start_removing_noperm(struct dentry *parent, struct qstr *name);
struct dentry *start_creating_dentry(struct dentry *parent,
struct dentry *child);
struct dentry *start_removing_dentry(struct dentry *parent,
struct dentry *child);
/* end_creating - finish action started with start_creating
* @child: dentry returned by start_creating() or vfs_mkdir()
*
* Unlock and release the child. This can be called after
* start_creating() whether that function succeeded or not,
* but it is not needed on failure.
*
* If vfs_mkdir() was called then the value returned from that function
* should be given for @child rather than the original dentry, as vfs_mkdir()
* may have provided a new dentry.
*
*
* If vfs_mkdir() was not called, then @child will be a valid dentry and
* @parent will be ignored.
*/
static inline void end_creating(struct dentry *child)
{
end_dirop(child);
}
/* end_creating_keep - finish action started with start_creating() and return result
* @child: dentry returned by start_creating() or vfs_mkdir()
*
* Unlock and return the child. This can be called after
* start_creating() whether that function succeeded or not,
* but it is not needed on failure.
*
* If vfs_mkdir() was called then the value returned from that function
* should be given for @child rather than the original dentry, as vfs_mkdir()
* may have provided a new dentry.
*
* Returns: @child, which may be a dentry or an error.
*
*/
static inline struct dentry *end_creating_keep(struct dentry *child)
{
if (!IS_ERR(child))
dget(child);
end_dirop(child);
return child;
}
/**
* end_removing - finish action started with start_removing
* @child: dentry returned by start_removing()
* @parent: dentry given to start_removing()
*
* Unlock and release the child.
*
* This is identical to end_dirop(). It can be passed the result of
* start_removing() whether that was successful or not, but it not needed
* if start_removing() failed.
*/
static inline void end_removing(struct dentry *child)
{
end_dirop(child);
}
extern int follow_down_one(struct path *);
extern int follow_down(struct path *path, unsigned int flags);
extern int follow_up(struct path *);
extern struct dentry *lock_rename(struct dentry *, struct dentry *);
extern struct dentry *lock_rename_child(struct dentry *, struct dentry *);
extern void unlock_rename(struct dentry *, struct dentry *);
int start_renaming(struct renamedata *rd, int lookup_flags,
struct qstr *old_last, struct qstr *new_last);
int start_renaming_dentry(struct renamedata *rd, int lookup_flags,
struct dentry *old_dentry, struct qstr *new_last);
int start_renaming_two_dentries(struct renamedata *rd,
struct dentry *old_dentry, struct dentry *new_dentry);
void end_renaming(struct renamedata *rd);
/**
* mode_strip_umask - handle vfs umask stripping
* @dir: parent directory of the new inode
* @mode: mode of the new inode to be created in @dir
*
* In most filesystems, umask stripping depends on whether or not the
* filesystem supports POSIX ACLs. If the filesystem doesn't support it umask
* stripping is done directly in here. If the filesystem does support POSIX
* ACLs umask stripping is deferred until the filesystem calls
* posix_acl_create().
*
* Some filesystems (like NFSv4) also want to avoid umask stripping by the
* VFS, but don't support POSIX ACLs. Those filesystems can set SB_I_NOUMASK
* to get this effect without declaring that they support POSIX ACLs.
*
* Returns: mode
*/
static inline umode_t __must_check mode_strip_umask(const struct inode *dir, umode_t mode)
{
if (!IS_POSIXACL(dir) && !(dir->i_sb->s_iflags & SB_I_NOUMASK))
mode &= ~current_umask();
return mode;
}
extern int __must_check nd_jump_link(const struct path *path);
static inline void nd_terminate_link(void *name, size_t len, size_t maxlen)
{
((char *) name)[min(len, maxlen)] = '\0';
}
/**
* retry_estale - determine whether the caller should retry an operation
* @error: the error that would currently be returned
* @flags: flags being used for next lookup attempt
*
* Check to see if the error code was -ESTALE, and then determine whether
* to retry the call based on whether "flags" already has LOOKUP_REVAL set.
*
* Returns true if the caller should try the operation again.
*/
static inline bool
retry_estale(const long error, const unsigned int flags)
{
return unlikely(error == -ESTALE && !(flags & LOOKUP_REVAL));
}
#endif /* _LINUX_NAMEI_H */
]
|
| 26/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 22202
output: 37
thoughts: 334
Thoughts: **Analyzing a Potential Use-After-Free Vulnerability**
Okay, let's break this down. It looks like `end_creating` calls `end_dirop`. I know `end_dirop` lives in `fs/namei.c`, so I'll check that. Right, here it is:
```c
void end_dirop(struct dentry *de)
{
if (!IS_ERR(de)) {
inode_unlock(de->d_parent->d_inode);
dput(de);
}
}
```
Wait a second… `de->d_parent->d_inode` is accessed within this function. If `de` represents a freed dentry, then accessing `de->d_parent` becomes a potential access to a freed object – a classic Use-After-Free! But here's the rub: KASAN is reporting the UAF at offset 208 of the dentry. `de->d_parent` is at offset 640 bits, which translates to 80 bytes. That's not the same. The reported UAF is at 208 bytes, but this code is dereferencing `de->d_parent` which is at 80 bytes.
Let's drill down: The critical line is `inode_unlock(de->d_parent->d_inode)`. This operation involves three distinct memory accesses:
1. First, it reads `de->d_parent`.
2. Then, it reads `de->d_parent->d_inode`.
3. Finally, it calls `inode_unlock` using that inode.
If `de` has indeed been freed, the read of `de->d_parent` represents a Use-After-Free condition. But KASAN flags the UAF at 208 bytes, which is `d_lockref.lock.lock.wait_lock.dep_map`, and the `de->d_parent` is at a different location. I'm wondering if `dput(de)` might also involve a similar access. Let me check the `dput` implementation now.
|
| 27/2 |
2026/04/05 00:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dput]
Results: map[SourceCode: 909: /*
910: * dput - release a dentry
911: * @dentry: dentry to release
912: *
913: * Release a dentry. This will drop the usage count and if appropriate
914: * call the dentry unlink method as well as removing it from the queues and
915: * releasing its resources. If the parent dentries were scheduled for release
916: * they too may now get deleted.
917: */
918: void dput(struct dentry *dentry)
919: {
920: if (!dentry)
921: return;
922: might_sleep();
923: rcu_read_lock();
924: if (likely(fast_dput(dentry))) {
925: rcu_read_unlock();
926: return;
927: }
928: finish_dput(dentry);
929: }
SourceFile:fs/dcache.c]
|
| 28/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 23356
output: 39
thoughts: 215
Thoughts: **Debugging a Suspected Deadlock**
Okay, so I'm digging into this stack trace. It's indicating a problem deep within the file system, specifically around `rt_mutex_slowunlock`. That's pointing me towards a potential locking issue. The trace leads me to `end_dirop` in `fs/namei.c`, and line 2954 is where things get interesting. From the trace, it looks like `end_dirop` is inlined, and line 2954 is calling `dput(de)`.
Now, I'm trying to mentally parse this. I know `dput` usually calls either `fast_dput` or `finish_dput`, and I need to figure out which of those is the culprit. I need to make sure `inode_unlock` isn't called after `dput`. The line number after `inode_unlock`, 2954, is where `dput` is called.
The trace definitively points to `dput(de)` being the function being executed at `fs/namei.c:2954` if `end_dirop` is inlined, so I need to check the internals of `fast_dput` now, to further investigate this potential deadlock.
|
| 29/2 |
2026/04/05 00:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:fast_dput]
Results: map[SourceCode: 803: /*
804: * Try to do a lockless dput(), and return whether that was successful.
805: *
806: * If unsuccessful, we return false, having already taken the dentry lock.
807: * In that case refcount is guaranteed to be zero and we have already
808: * decided that it's not worth keeping around.
809: *
810: * The caller needs to hold the RCU read lock, so that the dentry is
811: * guaranteed to stay around even if the refcount goes down to zero!
812: */
813: static inline bool fast_dput(struct dentry *dentry)
814: {
815: int ret;
816:
817: /*
818: * try to decrement the lockref optimistically.
819: */
820: ret = lockref_put_return(&dentry->d_lockref);
821:
822: /*
823: * If the lockref_put_return() failed due to the lock being held
824: * by somebody else, the fast path has failed. We will need to
825: * get the lock, and then check the count again.
826: */
827: if (unlikely(ret < 0)) {
828: spin_lock(&dentry->d_lock);
829: if (WARN_ON_ONCE(dentry->d_lockref.count <= 0)) {
830: spin_unlock(&dentry->d_lock);
831: return true;
832: }
833: dentry->d_lockref.count--;
834: goto locked;
835: }
836:
837: /*
838: * If we weren't the last ref, we're done.
839: */
840: if (ret)
841: return true;
842:
843: /*
844: * Can we decide that decrement of refcount is all we needed without
845: * taking the lock? There's a very common case when it's all we need -
846: * dentry looks like it ought to be retained and there's nothing else
847: * to do.
848: */
849: if (retain_dentry(dentry, false))
850: return true;
851:
852: /*
853: * Either not worth retaining or we can't tell without the lock.
854: * Get the lock, then. We've already decremented the refcount to 0,
855: * but we'll need to re-check the situation after getting the lock.
856: */
857: spin_lock(&dentry->d_lock);
858:
859: /*
860: * Did somebody else grab a reference to it in the meantime, and
861: * we're no longer the last user after all? Alternatively, somebody
862: * else could have killed it and marked it dead. Either way, we
863: * don't need to do anything else.
864: */
865: locked:
866: if (dentry->d_lockref.count || retain_dentry(dentry, true)) {
867: spin_unlock(&dentry->d_lock);
868: return true;
869: }
870: return false;
871: }
SourceFile:fs/dcache.c]
|
| 30/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 24924
output: 44
thoughts: 1364
Thoughts: **UAF Deep Dive into `dput` and `ocfs2`**
Okay, so I'm looking at this stack trace, and it's pointing to a use-after-free (UAF) situation involving `dput`. I need to break this down.
First, I see `fast_dput` is called, which then calls `spin_lock(&dentry->d_lock)`. On `PREEMPT_RT`, this translates to `rt_mutex_lock`. Wait, isn't that a locking mechanism? The trace says it's unlocking. Then, the trace says `rt_mutex_slowunlock` which is used for unlocking. Specifically, it's calling `spin_unlock(&dentry->d_lock)`. Okay, so `fast_dput` unlocks the dentry lock.
If `fast_dput` is inlined, that `spin_unlock` *could* become `rt_mutex_slowunlock`. And the stack trace shows `raw_spin_lock_irqsave(&lock->wait_lock, flags)`. Yep, that matches.
The `dput(de)` is called in `end_dirop`. From there, `fast_dput` is called, which calls `spin_lock` and then `spin_unlock`. So the dentry's lock is acquired and released. But... where's the UAF? If the dentry was already freed, the `spin_lock` call would cause the UAF. But the stack trace shows it's unlocking the lock, implying the dentry *was* alive for the lock. So it was freed *before* the unlock. That's strange in a single thread!
Hold on, I see the object was freed by `rcu_do_batch` which means it was freed via `call_rcu`. The "Last potentially related work creation" shows a bunch of `ocfs2` calls: `ocfs2_lookup`, `ocfs2_dentry_attach_lock`, `finish_dput`, and `__dentry_kill`. Wait, is this an `ocfs2` filesystem? `ocfs2_lookup` calls `ocfs2_dentry_attach_lock`. If that fails, it *might* call `dput`. `finish_dput` then calls `__dentry_kill`, which then calls `call_rcu`. Okay, so the dentry is being scheduled for deferred freeing.
Looking at the allocation stack, it was allocated by `__d_alloc` which is called by `lookup_one_qstr_excl`, which is used in the `filename_create` function. So it looks like it was created when a file was being created. `filename_create` then calls `vfs_mknod` or `vfs_create`. `end_creating_path` is then called and that calls `end_creating(dentry)` which calls `end_dirop(dentry)` which calls `dput(dentry)`.
So, *if* the dentry was already freed before `dput` was called, that would be the UAF. How can that happen? Did `vfs_mknod` or `vfs_create` drop the last reference? `filename_create` creates a dentry with a reference count. But `lookup_one_qstr_excl` returns a dentry with refcount 1. Then `vfs_mknod` is called. If `vfs_mknod` calls `dput`, the refcount goes to 0 and the dentry is freed. That would explain the UAF, but that seems wrong.
Wait a second... back to `ocfs2`. The allocation stack *doesn't* have `ocfs2`, but it does show `lookup_one_qstr_excl`. Now I see that `lookup_one_qstr_excl` calls `d_alloc`. If the *parent* dentry is on `ocfs2`, `d_alloc` *could* create an `ocfs2` dentry! I need to check this closely. I know `lookup_one_qstr_excl` calls `lookup_dcache` and `__lookup_slow`. Let me go back and re-examine the code for `lookup_one_qstr_excl`...
|
| 31/2 |
2026/04/05 00:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:lookup_one_qstr_excl]
Results: map[SourceCode:1776: /*
1777: * Parent directory has inode locked exclusive. This is one
1778: * and only case when ->lookup() gets called on non in-lookup
1779: * dentries - as the matter of fact, this only gets called
1780: * when directory is guaranteed to have no in-lookup children
1781: * at all.
1782: * Will return -ENOENT if name isn't found and LOOKUP_CREATE wasn't passed.
1783: * Will return -EEXIST if name is found and LOOKUP_EXCL was passed.
1784: */
1785: struct dentry *lookup_one_qstr_excl(const struct qstr *name,
1786: struct dentry *base, unsigned int flags)
1787: {
1788: struct dentry *dentry;
1789: struct dentry *old;
1790: struct inode *dir;
1791:
1792: dentry = lookup_dcache(name, base, flags);
1793: if (dentry)
1794: goto found;
1795:
1796: /* Don't create child dentry for a dead directory. */
1797: dir = base->d_inode;
1798: if (unlikely(IS_DEADDIR(dir)))
1799: return ERR_PTR(-ENOENT);
1800:
1801: dentry = d_alloc(base, name);
1802: if (unlikely(!dentry))
1803: return ERR_PTR(-ENOMEM);
1804:
1805: old = dir->i_op->lookup(dir, dentry, flags);
1806: if (unlikely(old)) {
1807: dput(dentry);
1808: dentry = old;
1809: }
1810: found:
1811: if (IS_ERR(dentry))
1812: return dentry;
1813: if (d_is_negative(dentry) && !(flags & LOOKUP_CREATE)) {
1814: dput(dentry);
1815: return ERR_PTR(-ENOENT);
1816: }
1817: if (d_is_positive(dentry) && (flags & LOOKUP_EXCL)) {
1818: dput(dentry);
1819: return ERR_PTR(-EEXIST);
1820: }
1821: return dentry;
1822: }
SourceFile:fs/namei.c]
|
| 32/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 28008
output: 44
thoughts: 774
Thoughts: **Analyzing a Potential Use-After-Free Vulnerability**
Okay, let's break this down systematically. I'm seeing a possible use-after-free situation, and I need to understand the control flow to confirm the root cause. My initial thought is that `lookup_one_qstr_excl` is the key function to dissect. It appears to call `dir->i_op->lookup`, which, in the case of `ocfs2`, resolves to `ocfs2_lookup`.
If `ocfs2_lookup` is involved, it might call `ocfs2_dentry_attach_lock`. The stack trace is showing `ocfs2_lookup` calling `ocfs2_dentry_attach_lock` which then calls `finish_dput`. If `ocfs2_dentry_attach_lock` fails, then a `dput(dentry)` might be called. This is a crucial point: if `ocfs2_lookup` *does* call `dput(dentry)`, that `dentry` gets freed.
Now, here's where it gets interesting. `lookup_one_qstr_excl` then returns either `old` or `dentry`. If `old` is returned, a `dput(dentry)` is called at line 1807. This `dentry` is the same one allocated at line 1801, meaning the original allocated dentry. So the original `dentry` gets freed.
The key observation is that the allocation stack for the freed object, according to KASAN, is `__d_alloc` at line 1801. So, the freed object *is* the `dentry` allocated at 1801. If `dir->i_op->lookup` returns `old`, then `dentry` is freed. Specifically, if `dentry` is freed at line 1807, `lookup_one_qstr_excl` returns `old`. Then `filename_create` returns `old`, followed by `filename_mknodat` using `old` and `end_dirop` being called with `old`. Thus, `old` should be a live object, shouldn't it?
The question is, why would `old` be the same as the freed `dentry`? KASAN reports the UAF on the object allocated at `__d_alloc` at line 1801.
If `old` is `NULL`, then `dentry` is returned. If `old` is not `NULL`, `dentry` is freed, and `old` is returned. Thus, the UAF is on `dentry`. It appears, and I need to check my work, that `filename_mknodat` should be using `old`, the one *returned* by `lookup_one_qstr_excl`, but KASAN shows a use-after-free on `dentry`. This means there's a disconnect.
Wait, if `old == dentry`, then `dput(dentry)` is called and then `dentry` is returned? This can't be correct, it would be a bug in `lookup_one_qstr_excl`.
Okay, let me reframe this: `old = dir->i_op->lookup(dir, dentry, flags);`. If `old` is *not* `NULL`, it means `lookup` found an existing dentry, and that one should be used instead of the new one we just created. So, calling `dput(dentry)` on the new one is correct.
Now, let's revisit the stack trace. `ocfs2_lookup` -> `ocfs2_dentry_attach_lock` -> `finish_dput` -> `__dentry_kill` -> `call_rcu`. So `ocfs2_lookup` itself is calling `dput` on *some* dentry. Is it the same dentry as the one allocated? Let's go and check `ocfs2_lookup`.
|
| 33/2 |
2026/04/05 00:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/namei.c IncludeLines:true Name:ocfs2_lookup]
Results: map[SourceCode: 97: static struct dentry *ocfs2_lookup(struct inode *dir, struct dentry *dentry,
98: unsigned int flags)
99: {
100: int status;
101: u64 blkno;
102: struct inode *inode = NULL;
103: struct dentry *ret;
104: struct ocfs2_inode_info *oi;
105:
106: trace_ocfs2_lookup(dir, dentry, dentry->d_name.len,
107: dentry->d_name.name,
108: (unsigned long long)OCFS2_I(dir)->ip_blkno, 0);
109:
110: if (dentry->d_name.len > OCFS2_MAX_FILENAME_LEN) {
111: ret = ERR_PTR(-ENAMETOOLONG);
112: goto bail;
113: }
114:
115: status = ocfs2_inode_lock_nested(dir, NULL, 0, OI_LS_PARENT);
116: if (status < 0) {
117: if (status != -ENOENT)
118: mlog_errno(status);
119: ret = ERR_PTR(status);
120: goto bail;
121: }
122:
123: status = ocfs2_lookup_ino_from_name(dir, dentry->d_name.name,
124: dentry->d_name.len, &blkno);
125: if (status < 0)
126: goto bail_add;
127:
128: inode = ocfs2_iget(OCFS2_SB(dir->i_sb), blkno, 0, 0);
129: if (IS_ERR(inode)) {
130: ret = ERR_PTR(-EACCES);
131: goto bail_unlock;
132: }
133:
134: oi = OCFS2_I(inode);
135: /* Clear any orphaned state... If we were able to look up the
136: * inode from a directory, it certainly can't be orphaned. We
137: * might have the bad state from a node which intended to
138: * orphan this inode but crashed before it could commit the
139: * unlink. */
140: spin_lock(&oi->ip_lock);
141: oi->ip_flags &= ~OCFS2_INODE_MAYBE_ORPHANED;
142: spin_unlock(&oi->ip_lock);
143:
144: bail_add:
145: ret = d_splice_alias(inode, dentry);
146: if (IS_ERR(ret))
147: goto bail_unlock;
148:
149: if (inode) {
150: /*
151: * If d_splice_alias() finds a DCACHE_DISCONNECTED
152: * dentry, it will d_move() it on top of ourse. The
153: * return value will indicate this however, so in
154: * those cases, we switch them around for the locking
155: * code.
156: *
157: * NOTE: This dentry already has ->d_op set from
158: * ocfs2_get_parent() and ocfs2_get_dentry()
159: */
160: if (ret)
161: dentry = ret;
162:
163: status = ocfs2_dentry_attach_lock(dentry, inode,
164: OCFS2_I(dir)->ip_blkno);
165: if (status) {
166: mlog_errno(status);
167: if (ret)
168: dput(ret);
169: ret = ERR_PTR(status);
170: }
171: } else
172: ocfs2_dentry_attach_gen(dentry);
173:
174: bail_unlock:
175: /* Don't drop the cluster lock until *after* the d_add --
176: * unlink on another node will message us to remove that
177: * dentry under this lock so otherwise we can race this with
178: * the downconvert thread and have a stale dentry. */
179: ocfs2_inode_unlock(dir, 0);
180:
181: bail:
182:
183: trace_ocfs2_lookup_ret(ret);
184:
185: return ret;
186: }
SourceFile:fs/ocfs2/namei.c]
|
| 34/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 31103
output: 40
thoughts: 922
Thoughts: **Analyzing a Potential Use-After-Free Vulnerability in `ocfs2_lookup`**
Okay, let me break this down. It seems I've got a potential use-after-free scenario in `ocfs2_lookup`. The sequence is as follows. First, I see that `ocfs2_lookup` calls `ocfs2_dentry_attach_lock`. Critically, within this function, `d_splice_alias` is invoked. If `d_splice_alias` returns a non-NULL `ret` - a potential existing dentry - it means it's "spliced" in, effectively taking the place of the `dentry` passed to `ocfs2_lookup`.
Now, if `ocfs2_dentry_attach_lock` *fails* after this splicing, it calls `dput(ret)`. The returned `ret` from `d_splice_alias` is then handled as the existing dentry. `ocfs2_lookup` then returns `ERR_PTR(status)`.
Here's where it gets interesting, I see that `lookup_one_qstr_excl` sees `old` (which is `ret`) as an error. If `ocfs2_lookup` returns `ERR_PTR(status)`, then `lookup_one_qstr_excl` returns `ERR_PTR(status)`.
But here's the kicker! `d_splice_alias`'s documentation is clear: it returns the existing dentry, *and* the passed-in `dentry` that `ocfs2_lookup` initiated is already `dput` by `d_splice_alias`. This means the `dentry` passed to `ocfs2_lookup` might already be freed.
So, if `ocfs2_dentry_attach_lock` fails, it calls `dput(ret)`. But the original `dentry` passed to `ocfs2_lookup` may already be freed. If `d_splice_alias` returns `ret`, the original `dentry` is unhashed and `dput`. This original `dentry` is then called by the caller of `ocfs2_lookup`.
Crucially, in `lookup_one_qstr_excl`, the code goes through this line:
```c
1805: old = dir->i_op->lookup(dir, dentry, flags);
1806: if (unlikely(old)) {
1807: dput(dentry);
1808: dentry = old;
1809: }
```
If `old` is an error pointer (ERR_PTR), the `if (unlikely(old))` condition will be true. This means `dput(dentry)` gets called, but `dentry` was already `dput` by `d_splice_alias`! This is the double `dput` - the use-after-free. The UAF is happening on the `dentry` allocated at line 1801 of `lookup_one_qstr_excl`.
|
| 35/2 |
2026/04/05 00:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:d_splice_alias]
Results: map[SourceCode:3114: /**
3115: * d_splice_alias - splice a disconnected dentry into the tree if one exists
3116: * @inode: the inode which may have a disconnected dentry
3117: * @dentry: a negative dentry which we want to point to the inode.
3118: *
3119: * If inode is a directory and has an IS_ROOT alias, then d_move that in
3120: * place of the given dentry and return it, else simply d_add the inode
3121: * to the dentry and return NULL.
3122: *
3123: * If a non-IS_ROOT directory is found, the filesystem is corrupt, and
3124: * we should error out: directories can't have multiple aliases.
3125: *
3126: * This is needed in the lookup routine of any filesystem that is exportable
3127: * (via knfsd) so that we can build dcache paths to directories effectively.
3128: *
3129: * If a dentry was found and moved, then it is returned. Otherwise NULL
3130: * is returned. This matches the expected return value of ->lookup.
3131: *
3132: * Cluster filesystems may call this function with a negative, hashed dentry.
3133: * In that case, we know that the inode will be a regular file, and also this
3134: * will only occur during atomic_open. So we need to check for the dentry
3135: * being already hashed only in the final case.
3136: */
3137: struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
3138: {
3139: return d_splice_alias_ops(inode, dentry, NULL);
3140: }
SourceFile:fs/dcache.c]
|
| 36/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 33272
output: 42
thoughts: 33
Thoughts: Okay, let's break this down.
**Diving into the Alias Splice Operations**
Alright, so `d_splice_alias` is clearly the entry point here. My first thought is, "Where does it go from there?" The immediate clue is that it calls `d_splice_alias_ops`. This suggests that the real work, the actual *operations* related to this alias splicing, are delegated to something within that `d_splice_alias_ops` function. Makes sense, that's generally how you organize these kinds of things for clarity and potential flexibility. So, my next logical step is to delve into the implementation of `d_splice_alias_ops`. Let's see what's actually happening under the hood. I need to understand what this `ops` function does to get the full picture.
|
| 37/2 |
2026/04/05 00:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:d_splice_alias_ops]
Results: map[SourceCode:3063: struct dentry *d_splice_alias_ops(struct inode *inode, struct dentry *dentry,
3064: const struct dentry_operations *ops)
3065: {
3066: if (IS_ERR(inode))
3067: return ERR_CAST(inode);
3068:
3069: BUG_ON(!d_unhashed(dentry));
3070:
3071: if (!inode)
3072: goto out;
3073:
3074: security_d_instantiate(dentry, inode);
3075: spin_lock(&inode->i_lock);
3076: if (S_ISDIR(inode->i_mode)) {
3077: struct dentry *new = __d_find_any_alias(inode);
3078: if (unlikely(new)) {
3079: /* The reference to new ensures it remains an alias */
3080: spin_unlock(&inode->i_lock);
3081: write_seqlock(&rename_lock);
3082: if (unlikely(d_ancestor(new, dentry))) {
3083: write_sequnlock(&rename_lock);
3084: dput(new);
3085: new = ERR_PTR(-ELOOP);
3086: pr_warn_ratelimited(
3087: "VFS: Lookup of '%s' in %s %s"
3088: " would have caused loop\n",
3089: dentry->d_name.name,
3090: inode->i_sb->s_type->name,
3091: inode->i_sb->s_id);
3092: } else if (!IS_ROOT(new)) {
3093: struct dentry *old_parent = dget(new->d_parent);
3094: int err = __d_unalias(dentry, new);
3095: write_sequnlock(&rename_lock);
3096: if (err) {
3097: dput(new);
3098: new = ERR_PTR(err);
3099: }
3100: dput(old_parent);
3101: } else {
3102: __d_move(new, dentry, false);
3103: write_sequnlock(&rename_lock);
3104: }
3105: iput(inode);
3106: return new;
3107: }
3108: }
3109: out:
3110: __d_add(dentry, inode, ops);
3111: return NULL;
3112: }
SourceFile:fs/dcache.c]
|
| 38/2 |
2026/04/05 00:26 |
llm |
expert |
0m
Model:
Tokens:
input: 34296
output: 49
thoughts: 770
Thoughts: **Analyzing the dentry lifecycle in `ocfs2_lookup` and potential UAFs**
Okay, let's break this down. My initial understanding is that `d_splice_alias_ops` returns a new dentry (`new`) if it finds an alias, and *doesn't* call `dput(dentry)` in that scenario. Makes sense. Also, `lookup_one_qstr_excl` calls `dput(dentry)` if it finds an existing dentry (`old`) in the directory entry. Specifically:
```c
old = dir->i_op->lookup(dir, dentry, flags);
if (unlikely(old)) {
dput(dentry);
dentry = old;
}
```
This seems correct to me; we're essentially replacing the new dentry we allocated with the existing one found by the `lookup` and freeing the original. The potential use-after-free (UAF) is happening in `end_dirop` when it calls `dput(de)`. If `de` is the dentry returned by `lookup_one_qstr_excl`, then it should be `old`. If `old` was already freed, `dput(old)` is a UAF.
Now, let's focus on `ocfs2_lookup` because that's the key to the problem.
```c
ret = d_splice_alias(inode, dentry);
...
if (ret)
dentry = ret;
status = ocfs2_dentry_attach_lock(dentry, inode, OCFS2_I(dir)->ip_blkno);
if (status) {
mlog_errno(status);
if (ret)
dput(ret);
ret = ERR_PTR(status);
}
```
If `d_splice_alias` returns a non-NULL `ret` (which indicates it found or created an alias), then the code assigns `dentry` to `ret`. After that, `ocfs2_dentry_attach_lock(ret, ...)` is called. If the lock attachment fails, the code calls `dput(ret)` to free it, and returns an error pointer.
Therefore, the following scenarios may occur:
1. `d_splice_alias` returns `ret` (alias exists or is created). `ocfs2_dentry_attach_lock` fails. `dput(ret)` is called, and `ocfs2_lookup` returns an error pointer. In this case, `lookup_one_qstr_excl` would see `old` as an error pointer, `dput(dentry)` would be called (on the original dentry), and it returns an error pointer. This is not vulnerable.
2. `d_splice_alias` returns `ret`. `ocfs2_dentry_attach_lock` succeeds. `ocfs2_lookup` returns `ret`. `lookup_one_qstr_excl` calls `dput(dentry)` (the original), and returns `ret`. This seems fine too.
But now, I'm concerned about the "Last potentially related work creation" mention: it highlights `ocfs2_lookup` -> `ocfs2_dentry_attach_lock` -> `finish_dput` -> `__dentry_kill`. Wait, does `ocfs2_dentry_attach_lock` call `dput`? Let's verify that quickly, because *that* would be the smoking gun.
|
| 39/2 |
2026/04/05 00:27 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/dcache.c IncludeLines:true Name:ocfs2_dentry_attach_lock]
Results: map[SourceCode: 167: /*
168: * Attach this dentry to a cluster lock.
169: *
170: * Dentry locks cover all links in a given directory to a particular
171: * inode. We do this so that ocfs2 can build a lock name which all
172: * nodes in the cluster can agree on at all times. Shoving full names
173: * in the cluster lock won't work due to size restrictions. Covering
174: * links inside of a directory is a good compromise because it still
175: * allows us to use the parent directory lock to synchronize
176: * operations.
177: *
178: * Call this function with the parent dir semaphore and the parent dir
179: * cluster lock held.
180: *
181: * The dir semaphore will protect us from having to worry about
182: * concurrent processes on our node trying to attach a lock at the
183: * same time.
184: *
185: * The dir cluster lock (held at either PR or EX mode) protects us
186: * from unlink and rename on other nodes.
187: *
188: * A dput() can happen asynchronously due to pruning, so we cover
189: * attaching and detaching the dentry lock with a
190: * dentry_attach_lock.
191: *
192: * A node which has done lookup on a name retains a protected read
193: * lock until final dput. If the user requests and unlink or rename,
194: * the protected read is upgraded to an exclusive lock. Other nodes
195: * who have seen the dentry will then be informed that they need to
196: * downgrade their lock, which will involve d_delete on the
197: * dentry. This happens in ocfs2_dentry_convert_worker().
198: */
199: int ocfs2_dentry_attach_lock(struct dentry *dentry,
200: struct inode *inode,
201: u64 parent_blkno)
202: {
203: int ret;
204: struct dentry *alias;
205: struct ocfs2_dentry_lock *dl = dentry->d_fsdata;
206:
207: trace_ocfs2_dentry_attach_lock(dentry->d_name.len, dentry->d_name.name,
208: (unsigned long long)parent_blkno, dl);
209:
210: /*
211: * Negative dentry. We ignore these for now.
212: *
213: * XXX: Could we can improve ocfs2_dentry_revalidate() by
214: * tracking these?
215: */
216: if (!inode)
217: return 0;
218:
219: if (d_really_is_negative(dentry) && dentry->d_fsdata) {
220: /* Converting a negative dentry to positive
221: Clear dentry->d_fsdata */
222: dentry->d_fsdata = dl = NULL;
223: }
224:
225: if (dl) {
226: mlog_bug_on_msg(dl->dl_parent_blkno != parent_blkno,
227: " \"%pd\": old parent: %llu, new: %llu\n",
228: dentry,
229: (unsigned long long)parent_blkno,
230: (unsigned long long)dl->dl_parent_blkno);
231: return 0;
232: }
233:
234: alias = ocfs2_find_local_alias(inode, parent_blkno, 0);
235: if (alias) {
236: /*
237: * Great, an alias exists, which means we must have a
238: * dentry lock already. We can just grab the lock off
239: * the alias and add it to the list.
240: *
241: * We're depending here on the fact that this dentry
242: * was found and exists in the dcache and so must have
243: * a reference to the dentry_lock because we can't
244: * race creates. Final dput() cannot happen on it
245: * since we have it pinned, so our reference is safe.
246: */
247: dl = alias->d_fsdata;
248: mlog_bug_on_msg(!dl, "parent %llu, ino %llu\n",
249: (unsigned long long)parent_blkno,
250: (unsigned long long)OCFS2_I(inode)->ip_blkno);
251:
252: mlog_bug_on_msg(dl->dl_parent_blkno != parent_blkno,
253: " \"%pd\": old parent: %llu, new: %llu\n",
254: dentry,
255: (unsigned long long)parent_blkno,
256: (unsigned long long)dl->dl_parent_blkno);
257:
258: trace_ocfs2_dentry_attach_lock_found(dl->dl_lockres.l_name,
259: (unsigned long long)parent_blkno,
260: (unsigned long long)OCFS2_I(inode)->ip_blkno);
261:
262: goto out_attach;
263: }
264:
265: /*
266: * There are no other aliases
267: */
268: dl = kmalloc_obj(*dl, GFP_NOFS);
269: if (!dl) {
270: ret = -ENOMEM;
271: mlog_errno(ret);
272: return ret;
273: }
274:
275: dl->dl_count = 0;
276: /*
277: * Does this have to happen below, for all attaches, in case
278: * the struct inode gets blown away by the downconvert thread?
279: */
280: dl->dl_inode = igrab(inode);
281: dl->dl_parent_blkno = parent_blkno;
282: ocfs2_dentry_lock_res_init(dl, parent_blkno, inode);
283:
284: out_attach:
285: spin_lock(&dentry_attach_lock);
286: if (unlikely(dentry->d_fsdata && !alias)) {
287: /* d_fsdata is set by a racing thread which is doing
288: * the same thing as this thread is doing. Leave the racing
289: * thread going ahead and we return here.
290: */
291: spin_unlock(&dentry_attach_lock);
292: iput(dl->dl_inode);
293: ocfs2_lock_res_free(&dl->dl_lockres);
294: kfree(dl);
295: return 0;
296: }
297:
298: dentry->d_fsdata = dl;
299: dl->dl_count++;
300: spin_unlock(&dentry_attach_lock);
301:
302: /*
303: * This actually gets us our PRMODE level lock. From now on,
304: * we'll have a notification if one of these names is
305: * destroyed on another node.
306: */
307: ret = ocfs2_dentry_lock(dentry, 0);
308: if (!ret)
309: ocfs2_dentry_unlock(dentry, 0);
310: else
311: mlog_errno(ret);
312:
313: /*
314: * In case of error, manually free the allocation and do the iput().
315: * We need to do this because error here means no d_instantiate(),
316: * which means iput() will not be called during dput(dentry).
317: */
318: if (ret < 0 && !alias) {
319: ocfs2_lock_res_free(&dl->dl_lockres);
320: BUG_ON(dl->dl_count != 1);
321: spin_lock(&dentry_attach_lock);
322: dentry->d_fsdata = NULL;
323: spin_unlock(&dentry_attach_lock);
324: kfree(dl);
325: iput(inode);
326: }
327:
328: dput(alias);
329:
330: return ret;
331: }
SourceFile:fs/ocfs2/dcache.c]
|
| 40/2 |
2026/04/05 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 38491
output: 48
thoughts: 73
Thoughts: **Analyzing Potential Resource Leak in OCFS2**
Okay, hold on a second. I'm focusing in on `ocfs2_dentry_attach_lock` and something's not quite right. Line 328 is calling `dput(alias)`. The thing is, this `alias` pointer came from a call to `ocfs2_find_local_alias(inode, parent_blkno, 0)`. My immediate concern is whether we're potentially over-releasing or double-releasing something here. I need to understand how `ocfs2_find_local_alias` actually behaves, especially its reference counting. I need to make sure this `dput` call is valid and doesn't introduce a resource leak or, even worse, a use-after-free scenario. Let's delve into that function.
|
| 41/2 |
2026/04/05 00:27 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/dcache.c IncludeLines:true Name:ocfs2_find_local_alias]
Results: map[SourceCode: 134: /*
135: * Walk the inode alias list, and find a dentry which has a given
136: * parent. ocfs2_dentry_attach_lock() wants to find _any_ alias as it
137: * is looking for a dentry_lock reference. The downconvert thread is
138: * looking to unhash aliases, so we allow it to skip any that already
139: * have that property.
140: */
141: struct dentry *ocfs2_find_local_alias(struct inode *inode,
142: u64 parent_blkno,
143: int skip_unhashed)
144: {
145: struct dentry *dentry;
146:
147: spin_lock(&inode->i_lock);
148: hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias) {
149: spin_lock(&dentry->d_lock);
150: if (ocfs2_match_dentry(dentry, parent_blkno, skip_unhashed)) {
151: trace_ocfs2_find_local_alias(dentry->d_name.len,
152: dentry->d_name.name);
153:
154: dget_dlock(dentry);
155: spin_unlock(&dentry->d_lock);
156: spin_unlock(&inode->i_lock);
157: return dentry;
158: }
159: spin_unlock(&dentry->d_lock);
160: }
161: spin_unlock(&inode->i_lock);
162: return NULL;
163: }
SourceFile:fs/ocfs2/dcache.c]
|
| 42/2 |
2026/04/05 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 39326
output: 39
thoughts: 360
Thoughts: **Analyzing a Potential Use-After-Free Vulnerability**
Okay, let's break this down. First, I was tracing through the `ocfs2_find_local_alias` function and noted that it correctly calls `dget_dlock(dentry)`, implying the `alias` variable now holds a reference. Then, `ocfs2_dentry_attach_lock` calls `dput(alias)`, which seems right – we're releasing the reference. So far, so good.
But then, I zeroed in on the potential Use-After-Free (UAF) on the dentry allocated in `lookup_one_qstr_excl` at line 1801, specifically, the `dentry` passed to `ocfs2_lookup`. The crucial point seems to be when `d_splice_alias` returns `ret` within `ocfs2_lookup`. My concern is that if this happens, the original `dentry` is still around.
I went back and double-checked the documentation for `d_splice_alias`, which clearly states that "If it finds a dentry, it returns it and the passed dentry is unhashed and dput". But I had to re-examine the `d_splice_alias_ops` code, line by line.
Looking at the code block in the notes, it says:
```c
3102: __d_move(new, dentry, false);
3103: write_sequnlock(&rename_lock);
3104: }
3105: iput(inode);
3106: return new;
```
Here's the problem I see: `__d_move(new, dentry, false)` moves the contents of `new` into `dentry`! What happens to the original `dentry`? It looks like `__d_move` effectively swaps or replaces the dentries. I really need to double-check `__d_move` because this is where the potential UAF could happen. This calls for further investigation into the implications of this 'move' operation.
|
| 43/2 |
2026/04/05 00:27 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:__d_move]
Results: map[SourceCode:2869: /*
2870: * __d_move - move a dentry
2871: * @dentry: entry to move
2872: * @target: new dentry
2873: * @exchange: exchange the two dentries
2874: *
2875: * Update the dcache to reflect the move of a file name. Negative dcache
2876: * entries should not be moved in this way. Caller must hold rename_lock, the
2877: * i_rwsem of the source and target directories (exclusively), and the sb->
2878: * s_vfs_rename_mutex if they differ. See lock_rename().
2879: */
2880: static void __d_move(struct dentry *dentry, struct dentry *target,
2881: bool exchange)
2882: {
2883: struct dentry *old_parent, *p;
2884: wait_queue_head_t *d_wait;
2885: struct inode *dir = NULL;
2886: unsigned n;
2887:
2888: WARN_ON(!dentry->d_inode);
2889: if (WARN_ON(dentry == target))
2890: return;
2891:
2892: BUG_ON(d_ancestor(target, dentry));
2893: old_parent = dentry->d_parent;
2894: p = d_ancestor(old_parent, target);
2895: if (IS_ROOT(dentry)) {
2896: BUG_ON(p);
2897: spin_lock(&target->d_parent->d_lock);
2898: } else if (!p) {
2899: /* target is not a descendent of dentry->d_parent */
2900: spin_lock(&target->d_parent->d_lock);
2901: spin_lock_nested(&old_parent->d_lock, DENTRY_D_LOCK_NESTED);
2902: } else {
2903: BUG_ON(p == dentry);
2904: spin_lock(&old_parent->d_lock);
2905: if (p != target)
2906: spin_lock_nested(&target->d_parent->d_lock,
2907: DENTRY_D_LOCK_NESTED);
2908: }
2909: spin_lock_nested(&dentry->d_lock, 2);
2910: spin_lock_nested(&target->d_lock, 3);
2911:
2912: if (unlikely(d_in_lookup(target))) {
2913: dir = target->d_parent->d_inode;
2914: n = start_dir_add(dir);
2915: d_wait = __d_lookup_unhash(target);
2916: }
2917:
2918: write_seqcount_begin(&dentry->d_seq);
2919: write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);
2920:
2921: /* unhash both */
2922: if (!d_unhashed(dentry))
2923: ___d_drop(dentry);
2924: if (!d_unhashed(target))
2925: ___d_drop(target);
2926:
2927: /* ... and switch them in the tree */
2928: dentry->d_parent = target->d_parent;
2929: if (!exchange) {
2930: copy_name(dentry, target);
2931: target->d_hash.pprev = NULL;
2932: dentry->d_parent->d_lockref.count++;
2933: if (dentry != old_parent) /* wasn't IS_ROOT */
2934: WARN_ON(!--old_parent->d_lockref.count);
2935: } else {
2936: target->d_parent = old_parent;
2937: swap_names(dentry, target);
2938: if (!hlist_unhashed(&target->d_sib))
2939: __hlist_del(&target->d_sib);
2940: hlist_add_head(&target->d_sib, &target->d_parent->d_children);
2941: __d_rehash(target);
2942: fsnotify_update_flags(target);
2943: }
2944: if (!hlist_unhashed(&dentry->d_sib))
2945: __hlist_del(&dentry->d_sib);
2946: hlist_add_head(&dentry->d_sib, &dentry->d_parent->d_children);
2947: __d_rehash(dentry);
2948: fsnotify_update_flags(dentry);
2949: fscrypt_handle_d_move(dentry);
2950:
2951: write_seqcount_end(&target->d_seq);
2952: write_seqcount_end(&dentry->d_seq);
2953:
2954: if (dir)
2955: end_dir_add(dir, n, d_wait);
2956:
2957: if (dentry->d_parent != old_parent)
2958: spin_unlock(&dentry->d_parent->d_lock);
2959: if (dentry != old_parent)
2960: spin_unlock(&old_parent->d_lock);
2961: spin_unlock(&target->d_lock);
2962: spin_unlock(&dentry->d_lock);
2963: }
SourceFile:fs/dcache.c]
|
| 44/2 |
2026/04/05 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 41806
output: 49
thoughts: 2996
Thoughts: **Analyzing a Potential Use-After-Free in OCFS2 Dentry Handling**
Okay, let's break this down. I'm looking at a potential use-after-free vulnerability in the OCFS2 filesystem. The crux seems to revolve around dentry manipulation, particularly with aliasing and reference counts.
The initial hunch is that a dentry allocated in `lookup_one_qstr_excl` might be freed prematurely. The critical path involves `ocfs2_lookup`, and I need to track the dentry's journey. It seems to hinge on what `d_splice_alias_ops` does, and whether the original `dentry` or a new alias is returned. I've got a good handle on how the code flows through `dput(dentry)` and the dentry's release.
Now, this UAF is supposed to be on the dentry from `lookup_one_qstr_excl`. If `ocfs2_lookup` returns a new dentry, `dput(dentry)` in `lookup_one_qstr_excl` is the release point. But the stack trace points to `__dentry_kill` after `ocfs2_dentry_attach_lock`. So the alias returned by `d_splice_alias` may be causing the problem.
Let me rewind and ensure the logic surrounding alias handling. A potential issue is if the dentry passed to `ocfs2_lookup` is also found as an alias by `ocfs2_find_local_alias`. The concern is a double `dput` if the dentry ends up being the alias. `dget` and `dput` calls are critical here to maintain the refcount correctly.
The core of the issue seems to lie in the interplay between `d_splice_alias`, `ocfs2_dentry_attach_lock`, and `ocfs2_find_local_alias`. `ocfs2_find_local_alias` could return the *same* dentry that was just passed to `ocfs2_lookup`. If the alias is the original dentry, the code is safe. `dget` in `ocfs2_find_local_alias` makes the refcount 2, and `dput` makes it 1, so the dentry is still valid. If `ocfs2_find_local_alias` finds a *different* dentry, then it gets `dput`'ed, which seems fine as well.
However, the stack trace indicates that *another* dentry is being killed. The suspicious point is `ocfs2_dentry_attach_lock` which calls `dput(alias)`. The concern here is if that alias is the same dentry that `lookup_one_qstr_excl` is working with.
I'm starting to zero in on this. The real problem is why is the dentry *being killed* by the `dput` if the refcount is supposed to be maintained.
I'm tracing the execution carefully now, if `d_splice_alias` returns an alias. `d_splice_alias_ops` calls `__d_move` and returns `new` (the alias). The original `dentry` becomes `new`. This looks safe... the logic should handle alias references. But the stack trace says otherwise.
Let's assume `d_splice_alias` finds an existing alias. That alias becomes `ret`. `ocfs2_lookup` returns `ret`. `ocfs2_dentry_attach_lock` is then called with the alias. But then `ocfs2_find_local_alias` finds the alias again and calls `dput` on it. Again, there should be enough references. Let's trace it carefully.
I am reexamining the refcount manipulation. If `ret` is an alias from `d_splice_alias`, the refcount has to be carefully managed. When `ocfs2_find_local_alias` finds it again, it's supposed to call `dget_dlock(ret)` making it >= 2. The `dput` then should decrement the count by one.
Why would the `dput(alias)` in `ocfs2_dentry_attach_lock` call `__dentry_kill`? It shouldn't, unless `alias` had a refcount of 1 prior to the `dput`. But `dget` should make it at least 1 and then the increment from `d_splice_alias` makes it at least 2.
The key observation is `ocfs2_dentry_attach_lock` calling `dput(alias)` which then triggers `__dentry_kill`. This means
|
| 45/2 |
2026/04/05 00:27 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/dcache.c IncludeLines:true Name:ocfs2_dentry_attach_lock]
Results: map[SourceCode: 167: /*
168: * Attach this dentry to a cluster lock.
169: *
170: * Dentry locks cover all links in a given directory to a particular
171: * inode. We do this so that ocfs2 can build a lock name which all
172: * nodes in the cluster can agree on at all times. Shoving full names
173: * in the cluster lock won't work due to size restrictions. Covering
174: * links inside of a directory is a good compromise because it still
175: * allows us to use the parent directory lock to synchronize
176: * operations.
177: *
178: * Call this function with the parent dir semaphore and the parent dir
179: * cluster lock held.
180: *
181: * The dir semaphore will protect us from having to worry about
182: * concurrent processes on our node trying to attach a lock at the
183: * same time.
184: *
185: * The dir cluster lock (held at either PR or EX mode) protects us
186: * from unlink and rename on other nodes.
187: *
188: * A dput() can happen asynchronously due to pruning, so we cover
189: * attaching and detaching the dentry lock with a
190: * dentry_attach_lock.
191: *
192: * A node which has done lookup on a name retains a protected read
193: * lock until final dput. If the user requests and unlink or rename,
194: * the protected read is upgraded to an exclusive lock. Other nodes
195: * who have seen the dentry will then be informed that they need to
196: * downgrade their lock, which will involve d_delete on the
197: * dentry. This happens in ocfs2_dentry_convert_worker().
198: */
199: int ocfs2_dentry_attach_lock(struct dentry *dentry,
200: struct inode *inode,
201: u64 parent_blkno)
202: {
203: int ret;
204: struct dentry *alias;
205: struct ocfs2_dentry_lock *dl = dentry->d_fsdata;
206:
207: trace_ocfs2_dentry_attach_lock(dentry->d_name.len, dentry->d_name.name,
208: (unsigned long long)parent_blkno, dl);
209:
210: /*
211: * Negative dentry. We ignore these for now.
212: *
213: * XXX: Could we can improve ocfs2_dentry_revalidate() by
214: * tracking these?
215: */
216: if (!inode)
217: return 0;
218:
219: if (d_really_is_negative(dentry) && dentry->d_fsdata) {
220: /* Converting a negative dentry to positive
221: Clear dentry->d_fsdata */
222: dentry->d_fsdata = dl = NULL;
223: }
224:
225: if (dl) {
226: mlog_bug_on_msg(dl->dl_parent_blkno != parent_blkno,
227: " \"%pd\": old parent: %llu, new: %llu\n",
228: dentry,
229: (unsigned long long)parent_blkno,
230: (unsigned long long)dl->dl_parent_blkno);
231: return 0;
232: }
233:
234: alias = ocfs2_find_local_alias(inode, parent_blkno, 0);
235: if (alias) {
236: /*
237: * Great, an alias exists, which means we must have a
238: * dentry lock already. We can just grab the lock off
239: * the alias and add it to the list.
240: *
241: * We're depending here on the fact that this dentry
242: * was found and exists in the dcache and so must have
243: * a reference to the dentry_lock because we can't
244: * race creates. Final dput() cannot happen on it
245: * since we have it pinned, so our reference is safe.
246: */
247: dl = alias->d_fsdata;
248: mlog_bug_on_msg(!dl, "parent %llu, ino %llu\n",
249: (unsigned long long)parent_blkno,
250: (unsigned long long)OCFS2_I(inode)->ip_blkno);
251:
252: mlog_bug_on_msg(dl->dl_parent_blkno != parent_blkno,
253: " \"%pd\": old parent: %llu, new: %llu\n",
254: dentry,
255: (unsigned long long)parent_blkno,
256: (unsigned long long)dl->dl_parent_blkno);
257:
258: trace_ocfs2_dentry_attach_lock_found(dl->dl_lockres.l_name,
259: (unsigned long long)parent_blkno,
260: (unsigned long long)OCFS2_I(inode)->ip_blkno);
261:
262: goto out_attach;
263: }
264:
265: /*
266: * There are no other aliases
267: */
268: dl = kmalloc_obj(*dl, GFP_NOFS);
269: if (!dl) {
270: ret = -ENOMEM;
271: mlog_errno(ret);
272: return ret;
273: }
274:
275: dl->dl_count = 0;
276: /*
277: * Does this have to happen below, for all attaches, in case
278: * the struct inode gets blown away by the downconvert thread?
279: */
280: dl->dl_inode = igrab(inode);
281: dl->dl_parent_blkno = parent_blkno;
282: ocfs2_dentry_lock_res_init(dl, parent_blkno, inode);
283:
284: out_attach:
285: spin_lock(&dentry_attach_lock);
286: if (unlikely(dentry->d_fsdata && !alias)) {
287: /* d_fsdata is set by a racing thread which is doing
288: * the same thing as this thread is doing. Leave the racing
289: * thread going ahead and we return here.
290: */
291: spin_unlock(&dentry_attach_lock);
292: iput(dl->dl_inode);
293: ocfs2_lock_res_free(&dl->dl_lockres);
294: kfree(dl);
295: return 0;
296: }
297:
298: dentry->d_fsdata = dl;
299: dl->dl_count++;
300: spin_unlock(&dentry_attach_lock);
301:
302: /*
303: * This actually gets us our PRMODE level lock. From now on,
304: * we'll have a notification if one of these names is
305: * destroyed on another node.
306: */
307: ret = ocfs2_dentry_lock(dentry, 0);
308: if (!ret)
309: ocfs2_dentry_unlock(dentry, 0);
310: else
311: mlog_errno(ret);
312:
313: /*
314: * In case of error, manually free the allocation and do the iput().
315: * We need to do this because error here means no d_instantiate(),
316: * which means iput() will not be called during dput(dentry).
317: */
318: if (ret < 0 && !alias) {
319: ocfs2_lock_res_free(&dl->dl_lockres);
320: BUG_ON(dl->dl_count != 1);
321: spin_lock(&dentry_attach_lock);
322: dentry->d_fsdata = NULL;
323: spin_unlock(&dentry_attach_lock);
324: kfree(dl);
325: iput(inode);
326: }
327:
328: dput(alias);
329:
330: return ret;
331: }
SourceFile:fs/ocfs2/dcache.c]
|
| 46/2 |
2026/04/05 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 48470
output: 42
thoughts: 3441
Thoughts: **Bug Hunting Deep Dive: Understanding the Dentry UAF**
Okay, so I've been staring at this stack trace and KASAN report, and here's what I'm thinking. This UAF, it's occurring on a dentry allocated in `lookup_one_qstr_excl`, but it's *later* being accessed in `end_dirop`, right? The "hint" is that the last thing that touched it was the `dput(alias)` call in `ocfs2_dentry_attach_lock`, line 328. Let's break this down.
First thought: is `alias` the dentry we care about? If so, then `dput(alias)` should be the culprit. The refcount logic, however, *should* be balanced by the `dget_dlock` in `ocfs2_find_local_alias`. So, not freed, yeah? Let's check other aliases.
What if `d_splice_alias` returns an `alias`? This means there's an *existing* dentry out there that's aliased to the inode, which would have refcount of 1 and gets dget from `ocfs2_find_local_alias`, so `dput` at line 328, has refcount 1, so the alias is still alive.
What if `ocfs2_find_local_alias` finds a *different* alias? That's a potential UAF on *that* dentry, but not the one we are tracking. So that can be ruled out.
Okay, let's step back and consider the case where `d_splice_alias` *doesn't* return anything. The original dentry, allocated in `lookup_one_qstr_excl`, is *not* put into the alias list, which means that `ocfs2_find_local_alias` will NOT find the `dentry` we are interested in.
Now, if we have a failure in `ocfs2_dentry_attach_lock`, what happens? There's error handling code with an `iput` and a `kfree`, which *could* contribute to refcount changes. But wait, no, that isn't the UAF path, since it just returns, it's not the critical path.
The UAF is in `end_dirop` -> `dput(de)`. `de` is the dentry returned from `lookup_one_qstr_excl`, the one we are interested in. If `ocfs2_lookup` returned a *new* alias, then `de` should be the alias, and then `end_dirop` should not be the location of the UAF. But KASAN says the UAF is on the original dentry! This means that `lookup_one_qstr_excl` did NOT return an alias.
Here's the problem: if `ocfs2_lookup` returns `NULL` which leads to `lookup_one_qstr_excl` returning `NULL`, then *it* calls `dput` on *its* dentry (the original dentry). This is bad, because that unhashes the dentry, then later it gets accessed again in the stack where `de` is the original dentry.
But `ocfs2_dentry_attach_lock` already added the dentry in the dcache. That means the `dput` call is removing it from the dcache.
Okay, so the `ocfs2_lookup` call is the source of the problem. If it fails, that `dput` *must* be dropping the reference and potentially freeing the dentry if it was just allocated. But the dentry is now in the dcache. Where is `end_dirop` called? The function that should be returning the error is `filename_mknodat`, and it does NOT call `end_dirop` if it returns an error. This means `ocfs2_lookup` actually succeeded!
Okay, so where's the flaw? Was `ocfs2_dentry_attach_lock` doing something nasty with the refcount? No, `dget_dlock` *should* balance the `dput` we see in `ocfs2_dentry_attach_lock`.
Okay, the most likely path is the `ocfs2_lookup` path, that's where the failure occurs. The critical condition is when the status of the lookup is *non-zero*. If `ocfs2_dentry_attach_lock` fails, then `ret` is an error pointer. The `dput` call after the failure path is the flaw.
|
| 47/2 |
2026/04/05 00:27 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/dcache.h IncludeLines:true Name:dget_dlock]
Results: map[SourceCode: 314: /**
315: * dget_dlock - get a reference to a dentry
316: * @dentry: dentry to get a reference to
317: *
318: * Given a live dentry, increment the reference count and return the dentry.
319: * Caller must hold @dentry->d_lock. Making sure that dentry is alive is
320: * caller's resonsibility. There are many conditions sufficient to guarantee
321: * that; e.g. anything with non-negative refcount is alive, so's anything
322: * hashed, anything positive, anyone's parent, etc.
323: */
324: static inline struct dentry *dget_dlock(struct dentry *dentry)
325: {
326: dentry->d_lockref.count++;
327: return dentry;
328: }
SourceFile:include/linux/dcache.h]
|
| 48/2 |
2026/04/05 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 53267
output: 38
thoughts: 2528
Thoughts: **The Unraveling of a Dentry Mystery**
Okay, so here's what I've got. It starts with a simple refcount check; `dget_dlock` is just incrementing the `d_lockref.count` of an integer `d_lockref.count` and `dput` calls things like `lockref_put_return`. If the count goes up, then down, we're good.
Then I checked `ocfs2_dentry_attach_lock`, which handles alias and dentry count manipulation:
* If an alias exists, it's used.
* The `dentry->d_fsdata` is updated with `dl->dl_count++` before `dput` is called on the alias.
* If `alias == dentry`, then `dput(dentry)` is called, this looks correct.
Then I circled back to the allocation stack and the KASAN report, which pointed to a Use-After-Free in `end_dirop` after `__d_alloc`, so the root cause must be in `lookup_one_qstr_excl`. Now I'm focused on the relationship between `ocfs2_lookup`, `d_splice_alias`, and `lookup_one_qstr_excl`.
* If `ocfs2_lookup` returns `NULL`, then `d_splice_alias` returned `NULL`, meaning no alias was found.
* In this case, `ocfs2_find_local_alias` should return `NULL`, and the original dentry should remain at refcount of 1.
So what if `d_splice_alias` returns an alias `ret`?
* `ocfs2_lookup` returns `ret`
* `lookup_one_qstr_excl` calls `dput(de)` on the original dentry (`de`) -- meaning it could free `de`.
* `lookup_one_qstr_excl` returns `ret`.
* `filename_create` returns `ret`
* `end_dirop(ret)` gets called.
And I need to figure out why the KASAN report hits the original `de`. Then I looked closely at the lookup calls.
* If `old == dentry` `dput(dentry)` is called on the same dentry as `de` so accessing it, then the dentry will be accessed at line 1807 - `dput(dentry)`.
But I checked the `ocfs2_lookup` and I see this in the code:
```c
145: ret = d_splice_alias(inode, dentry);
...
161: dentry = ret;
```
This overwrites the return of `d_splice_alias` with a local variable `dentry`, which is the correct alias, *but* the original dentry still is at refcount 1. Then `ocfs2_lookup` returns `ret`. The key is in `d_splice_alias` which doesn't `dput` the old dentry.
* So, `lookup_one_qstr_excl` then calls `dput(dentry)` - which is the original dentry from the lookup.
* This decreases the refcount, freeing the original dentry.
But now, I am back to the `end_dirop`:
```c
2950: void end_dirop(struct dentry *de)
2951: {
2952: if (!IS_ERR(de)) {
2953: inode_unlock(de->d_parent->d_inode);
2954: dput(de);
2955: }
```
So `end_dirop` accesses `de->d_parent->d_inode` but `de` has already been freed because `lookup_one_qstr_excl` returned an alias and dput'd `de`.
* `filename_create` calls `lookup_one_qstr_excl` and returns the same dentry.
* If `lookup_one_qstr_excl` returned the alias, `filename_create` also does.
So now the issue is very clear:
* The `end_dirop` is expecting the alias (`ret`) because of the way `lookup_one_qstr_excl` is written, but is using the original `dentry`, which was returned, and then freed by `d
|
| 49/2 |
2026/04/05 00:28 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:filename_create]
Results: map[SourceCode:4898: static struct dentry *filename_create(int dfd, struct filename *name,
4899: struct path *path, unsigned int lookup_flags)
4900: {
4901: struct dentry *dentry = ERR_PTR(-EEXIST);
4902: struct qstr last;
4903: bool want_dir = lookup_flags & LOOKUP_DIRECTORY;
4904: unsigned int reval_flag = lookup_flags & LOOKUP_REVAL;
4905: unsigned int create_flags = LOOKUP_CREATE | LOOKUP_EXCL;
4906: int type;
4907: int error;
4908:
4909: error = filename_parentat(dfd, name, reval_flag, path, &last, &type);
4910: if (error)
4911: return ERR_PTR(error);
4912:
4913: /*
4914: * Yucky last component or no last component at all?
4915: * (foo/., foo/.., /////)
4916: */
4917: if (unlikely(type != LAST_NORM))
4918: goto out;
4919:
4920: /* don't fail immediately if it's r/o, at least try to report other errors */
4921: error = mnt_want_write(path->mnt);
4922: /*
4923: * Do the final lookup. Suppress 'create' if there is a trailing
4924: * '/', and a directory wasn't requested.
4925: */
4926: if (last.name[last.len] && !want_dir)
4927: create_flags &= ~LOOKUP_CREATE;
4928: dentry = start_dirop(path->dentry, &last, reval_flag | create_flags);
4929: if (IS_ERR(dentry))
4930: goto out_drop_write;
4931:
4932: if (unlikely(error))
4933: goto fail;
4934:
4935: return dentry;
4936: fail:
4937: end_dirop(dentry);
4938: dentry = ERR_PTR(error);
4939: out_drop_write:
4940: if (!error)
4941: mnt_drop_write(path->mnt);
4942: out:
4943: path_put(path);
4944: return dentry;
4945: }
SourceFile:fs/namei.c]
|
| 50/2 |
2026/04/05 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 57598
output: 48
thoughts: 2378
Thoughts: **Bug Hunting: A Dive into Dentry and Alias Management**
Okay, let's break this down. My initial thought process revolves around following the data flow in `filename_create`, tracing the calls to `start_dirop`, `lookup_one_qstr_excl`, `filename_mknodat`, and the eventual `end_dirop` and `inode_unlock` calls. The first thing that jumps out is how the `dentry` returned by `filename_create` gets passed around, especially in relation to aliases.
I had a moment of confusion because I see `de->d_parent` being accessed. If `de` is an alias, its parent might be different from the path's parent `path->dentry`. The `d_splice_alias` function, I remember, is responsible for moving the alias. I checked the `__d_move` function and confirmed it updates `d_parent`. So, when `inode_unlock` happens, we should be using the *correct* parent.
My mind then wandered towards the dreaded "use-after-free" scenario. The KASAN report pointed to the dentry allocated in `lookup_one_qstr_excl`. Now, the crucial part is understanding what happens when `lookup_one_qstr_excl` returns an alias. If it does, `dput(original)` gets called, potentially freeing the original dentry. But `filename_mknodat` *should* be using the alias, right?
I reviewed the `lookup_one_qstr_excl` code, focusing on the error path and specifically how `dput(dentry)` is handled when the lookup fails. It's a key detail to understand how errors are propagated and the correct dentries are released. I made sure to check the code that returns `ERR_PTR`, making sure it's handled correctly at each step.
Then I shifted my attention to `ocfs2_lookup` and, in particular, `ocfs2_dentry_attach_lock`. If `ocfs2_dentry_attach_lock` fails, it calls `dput(ret)`. In that code, `ret` *could* be the alias, and if it is then `dput(ret)` should also decrement the refcount of the alias. But, that dput does happen, and now ret contains an error so all is good.
I then started thinking about a possible race. If we have the original dentry, then splice an alias, then `ocfs2_lookup` fails to lock the alias. I needed to check what happens to the original. I then dug back into `__d_move` again. After thinking about the details of how `__d_move` moves state and unhashes the original dentry.
I then shifted back to looking at the "Last potentially related work creation" and specifically how `ocfs2_dentry_attach_lock` calls `dput(alias)`. The concern was whether this call was incorrect if it was `dput(original)`. I then chased how `dget_dlock` and `dput` are used. That looked fine!
Finally, I went through the case where `ocfs2_dentry_attach_lock` fails. It's important to see if there's any `dput` calls on dentries that are incorrect. I got through that ok too.
Then I realized something: the local `dentry` variable in `ocfs2_find_local_alias` is shadowing the parameter `dentry` of `ocfs2_dentry_attach_lock`. Wait, no, it's a separate function, so that's not the problem.
And now I think I've found a genuine bug in `ocfs2_dentry_attach_lock`. If an alias is found and we use its `d_fsdata` (a `struct ocfs2_dentry_lock`), then the code fails to increment the *refcount* of that `dentry_lock`. Instead, `dl->dl_count` *is* incremented, and that's the refcount. But then `dput(alias)` decrements this refcount. So, if `dput(alias)` is called, it might lead to a premature free if it unhashes the original dentry. It looks like a classic refcount error. I need to check how `dput` interacts with `ocfs2_dentry_iput` to confirm the premature free.
|
| 51/2 |
2026/04/05 00:28 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/dcache.c IncludeLines:true Name:ocfs2_dentry_iput]
Results: map[SourceCode: 379: static void ocfs2_dentry_iput(struct dentry *dentry, struct inode *inode)
380: {
381: struct ocfs2_dentry_lock *dl = dentry->d_fsdata;
382:
383: if (!dl) {
384: /*
385: * No dentry lock is ok if we're disconnected or
386: * unhashed.
387: */
388: if (!(dentry->d_flags & DCACHE_DISCONNECTED) &&
389: !d_unhashed(dentry)) {
390: unsigned long long ino = 0ULL;
391: if (inode)
392: ino = (unsigned long long)OCFS2_I(inode)->ip_blkno;
393: mlog(ML_ERROR, "Dentry is missing cluster lock. "
394: "inode: %llu, d_flags: 0x%x, d_name: %pd\n",
395: ino, dentry->d_flags, dentry);
396: }
397:
398: goto out;
399: }
400:
401: mlog_bug_on_msg(dl->dl_count == 0, "dentry: %pd, count: %u\n",
402: dentry, dl->dl_count);
403:
404: ocfs2_dentry_lock_put(OCFS2_SB(dentry->d_sb), dl);
405:
406: out:
407: iput(inode);
408: }
SourceFile:fs/ocfs2/dcache.c]
|
| 52/2 |
2026/04/05 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 61502
output: 47
thoughts: 8782
Thoughts: **UAF in ocfs2_lookup: An In-Depth Analysis**
Okay, so I've been tracing through this code, and it looks like we have a use-after-free issue in the OCFS2 file system, specifically in the `ocfs2_lookup` function. Here's what I've pieced together:
The core problem stems from a double-`dput` or a missing reference, likely involving a dentry that gets freed prematurely due to a race condition. Let's break it down:
1. **The UAF location**: The UAF occurs during a `spin_unlock` operation, specifically within `fast_dput`, which is called by `dput(dentry)`. The specific access that is failing is to `dentry->d_lockref.lock.lock.wait_lock.dep_map`, this access is protected by an `rcu_read_lock`.
2. **The dentry lifecycle**: The dentry itself is allocated during `lookup_one_qstr_excl`, passed to `ocfs2_lookup` which may, or may not, create an alias, if the alias is created it gets returned from `d_splice_alias`. If `d_splice_alias` returns an alias, the original dentry is not freed yet. If `ocfs2_dentry_attach_lock` fails, then `dput` is called on the alias, then the original is freed in `lookup_one_qstr_excl`. If `ocfs2_dentry_attach_lock` succeeds the alias is returned to `filename_create`.
3. **The Race**: The bug is a double-`dput` or missing reference related to the `alias` dentry.
`ocfs2_dentry_attach_lock` has a path where a call to `dput` on `alias` could free the original dentry. It does this if the `alias` dentry is the same as the original, in which case the original would have a reference of 2, after `ocfs2_find_local_alias`, but is then decremented by calling `dput(alias)`. The dentry then remains with refcount 1 until a later call to `dput`. This should not be a UAF.
4. **Why the access happens**:
It's an access to the `wait_lock.dep_map` within the spinlock during `spin_unlock` in the `fast_dput` function.
Because the dentry is freed by RCU, and we have an RCU read lock, the memory should not be returned to the slab allocator.
However, the memory IS returned to the slab allocator as indicated by the KASAN report, this means `call_rcu` was called.
5. **PREEMPT_RT Complication**: The UAF is exacerbated by the `PREEMPT_RT` kernel configuration. This allows preemption within the `fast_dput` function after the `spin_lock` but before `spin_unlock`. However, the grace period is still protected.
6. **The crucial Question**: The issue is when the dentry is freed during the process of unlocking the spinlock.
* The dentry is being freed while the lock is being held by another CPU (which should not be possible because the free operation also attempts to lock).
* The dentry was already freed before `spin_unlock` because of a race with RCU.
7. **Key Insight**:
A dentry could be unhashed and removed from the dentry cache by another thread if `ocfs2_dentry_attach_lock` calls `dput` on it.
This call occurs when the code finds an existing `alias`, calls `dput` on it, and *then* the code continues to use the same dentry, without getting another reference to it.
If `dput` on the `alias` destroys the reference to the original dentry, the code is using a now-freed dentry.
So, in summary, there's a reference counting issue that can lead to a double `dput` on a dentry. In the normal code flow, this works fine. However, in the presence of an alias, especially on a `PREEMPT_RT` kernel, this double `dput` results in a UAF. The key problem appears to be that a dentry can be removed from a cache by a background process, meaning there's not a reference, the call to `dput` on an `alias` dentry triggers the free, then later access to that dentry after the free.
|
| 53/2 |
2026/04/05 00:29 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/dcache.c IncludeLines:true Name:ocfs2_match_dentry]
Results: map[SourceCode: 108: static int ocfs2_match_dentry(struct dentry *dentry,
109: u64 parent_blkno,
110: int skip_unhashed)
111: {
112: struct inode *parent;
113:
114: /*
115: * ocfs2_lookup() does a d_splice_alias() _before_ attaching
116: * to the lock data, so we skip those here, otherwise
117: * ocfs2_dentry_attach_lock() will get its original dentry
118: * back.
119: */
120: if (!dentry->d_fsdata)
121: return 0;
122:
123: if (skip_unhashed && d_unhashed(dentry))
124: return 0;
125:
126: parent = d_inode(dentry->d_parent);
127: /* Name is in a different directory. */
128: if (OCFS2_I(parent)->ip_blkno != parent_blkno)
129: return 0;
130:
131: return 1;
132: }
SourceFile:fs/ocfs2/dcache.c]
|
| 54/2 |
2026/04/05 00:29 |
llm |
expert |
0m
Model:
Tokens:
input: 71734
output: 19
thoughts: 8103
Thoughts: **Unraveling a Dentry UAF in OCFS2**
Okay, here's the deal, I've been wrestling with this use-after-free (UAF) in the OCFS2 code. My gut tells me it's related to how dentries, aliases, and locks interact during lookups. The crash happens during `end_dirop` after a file operation which triggers a read of the freed dentry memory. The key seems to be a race condition involving `ocfs2_lookup` and how dentries are attached to inodes using aliases.
My initial thought was that a dentry being freed within a `dput` call, somehow corrupted related data, potentially the parent inode, but the KASAN report suggests a more subtle issue. It's reading data within the dentry.
I started by tracing the call stack. A dentry (`de`) is allocated in `lookup_one_qstr_excl`, which then calls into `ocfs2_lookup`. `ocfs2_lookup` either returns a dentry directly, or it returns an error if something goes wrong. A critical observation is the `dput(de)` call later in `end_dirop`. The key is when is dentry freed and why.
The code flow, as I understand it, looks something like this:
1. A new dentry (`de`) is allocated via `d_alloc` (line 1801).
2. `ocfs2_lookup` is called. It might find an alias.
3. `ocfs2_dentry_attach_lock` is called to attach a lock to the dentry.
4. `dput(de)` is called in `end_dirop` in case of success.
The real problem arises when concurrent threads interfere with each other. The core of the bug *seems* to be in this race: When one thread acquires a dentry and tries to attach it, a concurrent thread is simultaneously doing the same operation.
Here's my logic:
* The allocation stack clearly points to `lookup_one_qstr_excl` allocating the dentry.
* `ocfs2_lookup` calls `ocfs2_dentry_attach_lock`. This function is pivotal.
* The function that can cause a premature free is `ocfs2_dentry_attach_lock` since it calls `dput` to decrement the reference count of the `alias` dentry.
* The function `ocfs2_find_local_alias` will determine if the alias of a dentry exists.
I am suspicious of scenarios where `ocfs2_find_local_alias` *finds* an alias, but that alias is already in a questionable state. Or what if the alias is the same as the current dentry `de`?
The code specifically checks to see if another thread already attached the dentry lock. This is a good sign but could be problematic. If two threads call `ocfs2_lookup` for the same name and one thread is faster in attaching the lock and the other thread does a `dput` of the already locked dentry, a UAF is very possible.
This dentry, created and possibly linked in parallel, might be freed.
I still have a sneaking suspicion that a dentry that has been "unhashed" from the dcache (so it's no longer directly accessible by name lookup) could somehow be re-used later.
I need to nail down how the `alias` variable in `ocfs2_dentry_attach_lock` becomes an already-freed dentry. This is where I believe the problem lies.
My focus has shifted to understand *when* and *why* `ocfs2_find_local_alias` might return a dentry that's in an unstable state. My gut feeling is this is where the bug lies. The interplay between dentry states (negative, positive, unhashed, alias-ed) and the timing of lock attachment/detachment is what's critical.
|
| 55/2 |
2026/04/05 00:30 |
tool |
codesearch-struct-layout |
0m
Args:
Results: map[Fields:[map[Name:i_mode OffsetBits:0 SizeBits:16] map[Name:i_opflags OffsetBits:16 SizeBits:16] map[Name:i_flags OffsetBits:32 SizeBits:32] map[Name:i_acl OffsetBits:64 SizeBits:64] map[Name:i_default_acl OffsetBits:128 SizeBits:64] map[Name:i_uid OffsetBits:192 SizeBits:32] map[Name:i_gid OffsetBits:224 SizeBits:32] map[Name:i_op OffsetBits:256 SizeBits:64] map[Name:i_sb OffsetBits:320 SizeBits:64] map[Name:i_mapping OffsetBits:384 SizeBits:64] map[Name:i_security OffsetBits:448 SizeBits:64] map[Name:i_ino OffsetBits:512 SizeBits:64] map[Name: OffsetBits:576 SizeBits:32] map[Name:i_rdev OffsetBits:608 SizeBits:32] map[Name:i_size OffsetBits:640 SizeBits:64] map[Name:i_atime_sec OffsetBits:704 SizeBits:64] map[Name:i_mtime_sec OffsetBits:768 SizeBits:64] map[Name:i_ctime_sec OffsetBits:832 SizeBits:64] map[Name:i_atime_nsec OffsetBits:896 SizeBits:32] map[Name:i_mtime_nsec OffsetBits:928 SizeBits:32] map[Name:i_ctime_nsec OffsetBits:960 SizeBits:32] map[Name:i_generation OffsetBits:992 SizeBits:32] map[Name:i_lock OffsetBits:1024 SizeBits:1024] map[Name:i_bytes OffsetBits:2048 SizeBits:16] map[Name:i_blkbits OffsetBits:2064 SizeBits:8] map[Name:i_write_hint OffsetBits:2072 SizeBits:8] map[Name:i_blocks OffsetBits:2112 SizeBits:64] map[Name:i_state OffsetBits:2176 SizeBits:32] map[Name:i_rwsem OffsetBits:2240 SizeBits:1088] map[Name:dirtied_when OffsetBits:3328 SizeBits:64] map[Name:dirtied_time_when OffsetBits:3392 SizeBits:64] map[Name:i_hash OffsetBits:3456 SizeBits:128] map[Name:i_io_list OffsetBits:3584 SizeBits:128] map[Name:i_wb OffsetBits:3712 SizeBits:64] map[Name:i_wb_frn_winner OffsetBits:3776 SizeBits:32] map[Name:i_wb_frn_avg_time OffsetBits:3808 SizeBits:16] map[Name:i_wb_frn_history OffsetBits:3824 SizeBits:16] map[Name:i_lru OffsetBits:3840 SizeBits:128] map[Name:i_sb_list OffsetBits:3968 SizeBits:128] map[Name:i_wb_list OffsetBits:4096 SizeBits:128] map[Name: OffsetBits:4224 SizeBits:128] map[Name:i_version OffsetBits:4352 SizeBits:64] map[Name:i_sequence OffsetBits:4416 SizeBits:64] map[Name:i_count OffsetBits:4480 SizeBits:32] map[Name:i_dio_count OffsetBits:4512 SizeBits:32] map[Name:i_writecount OffsetBits:4544 SizeBits:32] map[Name:i_readcount OffsetBits:4576 SizeBits:32] map[Name: OffsetBits:4608 SizeBits:64] map[Name:i_flctx OffsetBits:4672 SizeBits:64] map[Name:i_data OffsetBits:4736 SizeBits:5120] map[Name: OffsetBits:9856 SizeBits:128] map[Name: OffsetBits:9984 SizeBits:64] map[Name:i_fsnotify_mask OffsetBits:10048 SizeBits:32] map[Name:i_fsnotify_marks OffsetBits:10112 SizeBits:64] map[Name:i_private OffsetBits:10176 SizeBits:64]]]
|
| 56/2 |
2026/04/05 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 81993
output: 39
thoughts: 3763
Thoughts: **Bug Hunt: Dentry Refcount Shenanigans in OCFS2**
Okay, let's break this down. KASAN is screaming about a use-after-free, and I've traced it to the dentry structure. Specifically, the `d_lockref.lock.lock.wait_lock.dep_map` within the dentry. The root of the problem seems to be the refcount management of dentries in OCFS2, particularly around the interaction of `ocfs2_lookup` and `ocfs2_dentry_attach_lock` in a concurrent environment.
Here's my thought process. Initially, I was thinking it was `i_rwsem`, but that was off. Then, I dug into the dentry layout and identified that the problematic memory region KASAN pointed to was within the spinlock, which is part of `d_lockref`. The use-after-free is happening when calling `dput(de)`. `de` is returned by `filename_create`. `filename_create` calls `start_dirop`. `start_dirop` calls `lookup_one_qstr_excl`. `lookup_one_qstr_excl` calls `ocfs2_lookup`. I can see the issue is if `ocfs2_lookup` returns a dentry that has already been freed.
The key seems to be in `ocfs2_dentry_attach_lock`. If another thread has already attached the lock, it returns 0, but this doesn't seem directly related. The "Last potentially related work creation" stack trace gives it away. `ocfs2_dentry_attach_lock` calls `dput(alias)`. The thought is that the same dentry (alias) might be getting double-freed.
Let's look at the refcounts.
1. Thread 1: `ocfs2_lookup(Dentry A)`. `d_splice_alias` adds Dentry A. Refcount = 1.
2. Thread 2: `ocfs2_lookup(Dentry A)`. `d_splice_alias` finds Dentry A as an alias. Refcount = 2.
3. Thread 2: `ocfs2_dentry_attach_lock(Dentry A)`.
4. Thread 2: Sets `Dentry A->d_fsdata`.
5. Thread 1: `ocfs2_dentry_attach_lock(Dentry A)`.
6. Thread 1: `ocfs2_find_local_alias` finds Dentry A (because `d_fsdata` is now set).
7. Thread 1: `alias = Dentry A`. `dget(Dentry A)`. Refcount = 3.
8. Thread 1: `ocfs2_dentry_attach_lock` continues to `out_attach`.
9. Thread 1: `dentry->d_fsdata = dl` (already set, but it sets it again).
10. Thread 1: `dl->dl_count++`.
11. Thread 1: `dput(alias)` which is `dput(Dentry A)`. Refcount = 2.
12. Thread 1: `ocfs2_dentry_attach_lock` returns 0.
13. Thread 1: `ocfs2_lookup` returns Dentry A.
14. Thread 1: `lookup_one_qstr_excl` sees `old` (Dentry A) is not `NULL`.
15. Thread 1: `dput(Dentry A)` (the original one). Refcount = 1.
16. Thread 1: Returns Dentry A.
17. Thread 2: `ocfs2_dentry_attach_lock` returns 0.
18. Thread 2: `ocfs2_lookup` returns Dentry A.
19. Thread 2: `lookup_one_qstr_excl` returns Dentry A.
I get it! If Thread 2 finishes and calls `dput`, then the refcount becomes 0 and is freed. The bug is that we have a race condition: where one thread calls `ocfs2_dentry_attach_lock`, but is raced by another thread that finishes with this dentry.
Let's confirm. Thread 1 (T1) and Thread 2 (T2) are both trying to create/lookup "foo".
1. T1: `d_alloc("foo")` -> Dentry A. Refcount = 1.
2. T1: `ocfs2_
|
| 57/2 |
2026/04/05 00:30 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dentry_kill]
Results: map[SourceCode: SourceFile:]
|
| 58/2 |
2026/04/05 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 86841
output: 40
thoughts: 25
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm an expert in this area:
**Deconstructing a Kernel Issue**
Hmm, based on the context, my initial hunch is that the root of the problem likely lies within the `__dentry_kill` function. I have a feeling that might be the critical piece of the puzzle. Now, to be absolutely sure and pinpoint the issue, the next logical step is to dive right in and meticulously examine the `__dentry_kill` function. It's the most probable location to start debugging.
|
| 59/2 |
2026/04/05 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:__dentry_kill]
Results: map[SourceCode: 646: static struct dentry *__dentry_kill(struct dentry *dentry)
647: {
648: struct dentry *parent = NULL;
649: bool can_free = true;
650:
651: /*
652: * The dentry is now unrecoverably dead to the world.
653: */
654: lockref_mark_dead(&dentry->d_lockref);
655:
656: /*
657: * inform the fs via d_prune that this dentry is about to be
658: * unhashed and destroyed.
659: */
660: if (dentry->d_flags & DCACHE_OP_PRUNE)
661: dentry->d_op->d_prune(dentry);
662:
663: if (dentry->d_flags & DCACHE_LRU_LIST) {
664: if (!(dentry->d_flags & DCACHE_SHRINK_LIST))
665: d_lru_del(dentry);
666: }
667: /* if it was on the hash then remove it */
668: __d_drop(dentry);
669: if (dentry->d_inode)
670: dentry_unlink_inode(dentry);
671: else
672: spin_unlock(&dentry->d_lock);
673: this_cpu_dec(nr_dentry);
674: if (dentry->d_op && dentry->d_op->d_release)
675: dentry->d_op->d_release(dentry);
676:
677: cond_resched();
678: /* now that it's negative, ->d_parent is stable */
679: if (!IS_ROOT(dentry)) {
680: parent = dentry->d_parent;
681: spin_lock(&parent->d_lock);
682: }
683: spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
684: dentry_unlist(dentry);
685: if (dentry->d_flags & DCACHE_SHRINK_LIST)
686: can_free = false;
687: spin_unlock(&dentry->d_lock);
688: if (likely(can_free))
689: dentry_free(dentry);
690: if (parent && --parent->d_lockref.count) {
691: spin_unlock(&parent->d_lock);
692: return NULL;
693: }
694: return parent;
695: }
SourceFile:fs/dcache.c]
|
| 60/2 |
2026/04/05 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 87834
output: 41
thoughts: 72
Thoughts: **Analyzing the File System Code**
Okay, hold on a second! I'm tracing through this function and I see `dentry_unlist(dentry)` called at line 684. Right, that function's job is to remove the `dentry` from the `inode->i_dentry` list. But... where did it come from? Ah, here we go, `dentry_unlink_inode(dentry)` is just a little earlier at line 670. I need to take a closer look at that function before I get any further. Let's see what `dentry_unlink_inode` is doing... I bet it's the key to understanding the overall control flow here.
|
| 61/2 |
2026/04/05 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dentry_unlink_inode]
Results: map[SourceCode: 446: /*
447: * Release the dentry's inode, using the filesystem
448: * d_iput() operation if defined.
449: */
450: static void dentry_unlink_inode(struct dentry * dentry)
451: __releases(dentry->d_lock)
452: __releases(dentry->d_inode->i_lock)
453: {
454: struct inode *inode = dentry->d_inode;
455:
456: raw_write_seqcount_begin(&dentry->d_seq);
457: __d_clear_type_and_inode(dentry);
458: hlist_del_init(&dentry->d_u.d_alias);
459: raw_write_seqcount_end(&dentry->d_seq);
460: spin_unlock(&dentry->d_lock);
461: spin_unlock(&inode->i_lock);
462: if (!inode->i_nlink)
463: fsnotify_inoderemove(inode);
464: if (dentry->d_op && dentry->d_op->d_iput)
465: dentry->d_op->d_iput(dentry, inode);
466: else
467: iput(inode);
468: }
SourceFile:fs/dcache.c]
|
| 62/2 |
2026/04/05 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 88517
output: 41
thoughts: 1176
Thoughts: **Debugging Deep Dive: A Dentry Conundrum**
Okay, let's retrace these thoughts. Line 458: `hlist_del_init(&dentry->d_u.d_alias)` happens *before* `d_iput`, which is then handled by `ocfs2_dentry_iput`. Crucially, `ocfs2_dentry_iput` doesn't call `dput` directly; it calls `ocfs2_dentry_lock_put` instead. Good. This is where the OCFS2 specific logic kicks in.
Wait, back to `ocfs2_find_local_alias`. I need to ensure the locking is correct. It takes `inode->i_lock` and then iterates through the `i_dentry` list using `dentry->d_lock`. `dentry_unlink_inode` also takes `inode->i_lock`, so that's synchronized. The `hlist_del_init` call is protected by those locks, so it shouldn't be possible for `ocfs2_find_local_alias` to find an entry that's already unlinked. *That's* not the problem.
Alright, back to the bug report. A KASAN slab-use-after-free in `__raw_spin_lock_irqsave`, inside `rt_mutex_slowunlock`, called by `end_dirop`, then `dput`. If the dentry `de` was already freed before `dput(de)` this will be a UAF. `fast_dput` is called from `dput` and calls `lockref_put_return` which accesses `de->d_lockref.lock`. *Yes*, this is the problem.
Is it a double `dput`? Going back to `ocfs2_dentry_attach_lock`, there's a `dput(alias)` call. But I remember that `ocfs2_match_dentry` should prevent a double `dput` on the same dentry, so it would return. Then I have to think, *wait*, what if `ocfs2_match_dentry` is wrong? Line 120 checks `dentry->d_fsdata`. If it's *not* `NULL`, it returns, indicating that lock is attached. If lock is already attached, `ocfs2_dentry_attach_lock` should return `0`, which it does at line 231. So `ocfs2_find_local_alias` *should not* be called if `d_fsdata` is already set, so `alias` will NEVER be the same as `dentry`.
What about `d_splice_alias`? What if it returns an alias that's already being freed? No, `d_splice_alias` takes a reference. So that's not it.
I'm running in circles here. Back to the bug report one last time. There's related work creation to consider, including `call_rcu`, `__dentry_kill`, and `finish_dput`. The critical line in the stack trace is `ocfs2_dentry_attach_lock+0x748/0xe20 fs/ocfs2/dcache.c:328`, which is `dput(alias)`. The evidence is very strong. If this is the dentry later accessed, that is the `alias` we are interested in. But the trace comes from `ocfs2_find_local_alias`. `ocfs2_find_local_alias` *only* returns dentries with `d_fsdata` set. The dentry, allocated at line 1801, has `d_fsdata == NULL`. So `alias` cannot be our dentry.
Unless... unless our dentry *already* had `d_fsdata` set?! How could it? It was just allocated by `d_alloc`. `d_alloc` uses `kmem_cache_alloc_lru`. Ah! The dentry cache uses a constructor, `dentry_cache_ctor`. Let's dig into that. That's the key.
|
| 63/2 |
2026/04/05 00:30 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dentry_cache_ctor]
Results: map[SourceCode: SourceFile:]
|
| 64/2 |
2026/04/05 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 90671
output: 39
thoughts: 23
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Probable Initialization Point: dcache_init**
Okay, so I'm trying to track down the root cause here, and my gut feeling is leaning towards the `dcache_init` function. Given the observed behavior, it's the most likely suspect for initial setup issues. Let's not jump to conclusions though; the best approach is always to verify. I need to take a look at the code for `dcache_init` and immediately start working through my mental checklist: memory allocation, cache line configuration, any potential race conditions during the initial setup phase, and alignment issues. Let's dive in and confirm (or deny!) this initial hunch.
|
| 65/2 |
2026/04/05 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dcache_init]
Results: map[SourceCode:3268: static void __init dcache_init(void)
3269: {
3270: /*
3271: * A constructor could be added for stable state like the lists,
3272: * but it is probably not worth it because of the cache nature
3273: * of the dcache.
3274: */
3275: __dentry_cache = KMEM_CACHE_USERCOPY(dentry,
3276: SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_ACCOUNT,
3277: d_shortname.string);
3278: runtime_const_init(ptr, __dentry_cache);
3279:
3280: /* Hash may have been set up in dcache_init_early */
3281: if (!hashdist)
3282: return;
3283:
3284: dentry_hashtable =
3285: alloc_large_system_hash("Dentry cache",
3286: sizeof(struct hlist_bl_head),
3287: dhash_entries,
3288: 13,
3289: HASH_ZERO,
3290: &d_hash_shift,
3291: NULL,
3292: 0,
3293: 0);
3294: d_hash_shift = 32 - d_hash_shift;
3295:
3296: runtime_const_init(shift, d_hash_shift);
3297: runtime_const_init(ptr, dentry_hashtable);
3298: }
SourceFile:fs/dcache.c]
|
| 66/2 |
2026/04/05 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 91359
output: 39
thoughts: 25
Thoughts: Here's my thought process, as if I were considering this code:
**Digging into the Allocation Chain**
Okay, so I see `d_alloc` here. My immediate reaction is, "Right, standard allocation function, probably part of a larger memory management scheme." But I need to understand its specific behavior within this context. The code points me directly to the call to `__d_alloc`. That's interesting, because the double underscore usually indicates something lower-level, or perhaps an internal helper function. I need to understand what `__d_alloc` is doing here to understand the broader allocation process. Let's see what that function does. I'm going to look right into `__d_alloc`.
|
| 67/2 |
2026/04/05 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:__d_alloc]
Results: map[SourceCode:1734: static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
1735: {
1736: struct dentry *dentry;
1737: char *dname;
1738: int err;
1739:
1740: dentry = kmem_cache_alloc_lru(dentry_cache, &sb->s_dentry_lru,
1741: GFP_KERNEL);
1742: if (!dentry)
1743: return NULL;
1744:
1745: /*
1746: * We guarantee that the inline name is always NUL-terminated.
1747: * This way the memcpy() done by the name switching in rename
1748: * will still always have a NUL at the end, even if we might
1749: * be overwriting an internal NUL character
1750: */
1751: dentry->d_shortname.string[DNAME_INLINE_LEN-1] = 0;
1752: if (unlikely(!name)) {
1753: name = &slash_name;
1754: dname = dentry->d_shortname.string;
1755: } else if (name->len > DNAME_INLINE_LEN-1) {
1756: size_t size = offsetof(struct external_name, name[1]);
1757: struct external_name *p = kmalloc(size + name->len,
1758: GFP_KERNEL_ACCOUNT |
1759: __GFP_RECLAIMABLE);
1760: if (!p) {
1761: kmem_cache_free(dentry_cache, dentry);
1762: return NULL;
1763: }
1764: atomic_set(&p->count, 1);
1765: dname = p->name;
1766: } else {
1767: dname = dentry->d_shortname.string;
1768: }
1769:
1770: dentry->__d_name.len = name->len;
1771: dentry->__d_name.hash = name->hash;
1772: memcpy(dname, name->name, name->len);
1773: dname[name->len] = 0;
1774:
1775: /* Make sure we always see the terminating NUL character */
1776: smp_store_release(&dentry->__d_name.name, dname); /* ^^^ */
1777:
1778: dentry->d_flags = 0;
1779: lockref_init(&dentry->d_lockref);
1780: seqcount_spinlock_init(&dentry->d_seq, &dentry->d_lock);
1781: dentry->d_inode = NULL;
1782: dentry->d_parent = dentry;
1783: dentry->d_sb = sb;
1784: dentry->d_op = sb->__s_d_op;
1785: dentry->d_flags = sb->s_d_flags;
1786: dentry->d_fsdata = NULL;
1787: INIT_HLIST_BL_NODE(&dentry->d_hash);
1788: INIT_LIST_HEAD(&dentry->d_lru);
1789: INIT_HLIST_HEAD(&dentry->d_children);
1790: INIT_HLIST_NODE(&dentry->d_u.d_alias);
1791: INIT_HLIST_NODE(&dentry->d_sib);
1792:
1793: if (dentry->d_op && dentry->d_op->d_init) {
1794: err = dentry->d_op->d_init(dentry);
1795: if (err) {
1796: if (dname_external(dentry))
1797: kfree(external_name(dentry));
1798: kmem_cache_free(dentry_cache, dentry);
1799: return NULL;
1800: }
1801: }
1802:
1803: this_cpu_inc(nr_dentry);
1804:
1805: return dentry;
1806: }
SourceFile:fs/dcache.c]
|
| 68/2 |
2026/04/05 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 92805
output: 43
thoughts: 2342
Thoughts: **The Hunt for the OCFS2 Dentry Bug**
Okay, let's break this down. Line 1786 tells me `dentry->d_fsdata` is definitely NULL initially. The focus is on `ocfs2_dentry_attach_lock`. If `ocfs2_find_local_alias` finds an `alias`, then that's used. This feels right, but... the match function (`ocfs2_match_dentry`) should prevent a collision because it checks `d_fsdata`. It seems like `ocfs2_match_dentry` should be in the way here. Wait, what if `ocfs2_dentry_attach_lock` is called with a dentry *already* in the `i_dentry` list? That's what `d_splice_alias` does, but `d_fsdata` should still be null in `ocfs2_match_dentry`. If `d_fsdata` is NULL, then `ocfs2_find_local_alias` should *not* return `dentry`.
Hold on... what if `ocfs2_match_dentry` is wrong? Line 128 is the important one here, checking the parent block number. A new dentry's parent is *itself*, initially (line 1782), but `d_splice_alias` updates it and `__d_add` doesn't change the parent. So the parent *was* set by `d_alloc`. This confirms that `ocfs2_match_dentry` *should* return 1 if `d_fsdata` is NOT NULL.
So, the key here is `d_fsdata` *not* being NULL. I'm repeating myself, but it's important.
Let's look at the bug report again. It's a UAF on a dentry. It says the buggy address is within a cache dentry, and it was allocated and freed. The provided location also hints at the freeing being called later. The "Last potentially related work creation" shows: `ocfs2_lookup` -> `ocfs2_dentry_attach_lock` -> `dput(alias)`. My hunch is that `alias` *is* the dentry itself. But *why* wasn't `d_fsdata` NULL? It looks like it gets set within `ocfs2_dentry_attach_lock`. The dentry lock protects this. `dput(alias)` is at the end of the function, so it's only called if `alias` isn't null.
Okay, what if `ocfs2_dentry_attach_lock` is called TWICE on the same dentry, via different threads? That could explain this. Let me write out the timeline here: Thread 1 sets `dentry->d_fsdata = dl` and gets preempted. Thread 2 calls it, `ocfs2_find_local_alias` finds the dentry because `d_fsdata` isn't NULL, and `dget` is called. The reference count on the dentry is now bumped to 3, then it calls `dput` and reduces it to 2. Thread 1 resumes and calls `dput(NULL)` and that's fine. If thread 1 *did* find an alias `Dentry X`, there is no issue. It seems like this is still fine.
But I think the root cause is that *somehow* `dput` is called *before* it should be. The function `ocfs2_dentry_attach_lock` looks like it is where the issue is. This is most likely a race condition leading to a double `dput` or a UAF because of some unexpected state in the dentry ref count.
The stack trace gives a big offset, `0x748`, which is about 1864 bytes. If inlining is heavy, this is plausible. Now, the "Last potentially related work creation" looks crucial: `ocfs2_lookup` -> `ocfs2_dentry_attach_lock` -> `dput(alias)`. If `alias` is the dentry, then something has to be very wrong. I'm repeating myself again.
Let's assume the report is consistent. A UAF on a dentry. It's allocated in `lookup_one_qstr_excl`, freed in `ocfs2_dentry_attach_lock` -> `dput(alias)`, and accessed in `end_dirop` -> `dput(de)`. This means `de` (in `end_dirop`) *is* the same dentry that
|
| 69/2 |
2026/04/05 00:31 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:__d_find_any_alias]
Results: map[SourceCode:1007: static struct dentry * __d_find_any_alias(struct inode *inode)
1008: {
1009: struct dentry *alias;
1010:
1011: if (hlist_empty(&inode->i_dentry))
1012: return NULL;
1013: alias = hlist_entry(inode->i_dentry.first, struct dentry, d_u.d_alias);
1014: lockref_get(&alias->d_lockref);
1015: return alias;
1016: }
SourceFile:fs/dcache.c]
|
| 70/2 |
2026/04/05 00:31 |
llm |
expert |
4m
Model:
Tokens:
input: 96401
output: 19
thoughts: 62911
|
| 71/2 |
2026/04/05 00:35 |
tool |
set-results |
0m
Args: map[Actionable:true Confident:true]
Results: map[Actionable:true Confident:true]
|
| 72/2 |
2026/04/05 00:35 |
llm |
expert |
0m
Model:
Tokens:
input: 159348
output: 673
thoughts: 493
Thoughts: **Analyzing a Tricky Use-After-Free in the PREEMPT_RT Kernel**
Okay, let's break this down. This bug report looks solid – self-consistent and provides all the details I need to understand this use-after-free issue. It's focused on a `dentry` object, which is always a fun area to debug, especially in a real-time kernel context.
First off, the crash is pinpointed in `rt_mutex_slowunlock`, which gets called as part of releasing the spinlock associated with the `dentry` (`dentry->d_lock`). The bad access occurs during the `spin_unlock` in `dput(de)` inside of `end_dirop`. The offset 208 of the dentry corresponds directly to the `wait_lock` data within the lockref object, specific to the `PREEMPT_RT` kernel's data structures. That's a clear indicator of where the corruption is happening.
Now, the allocation is tracked to `lookup_one_qstr_excl`, using the standard `d_alloc` function. Standard stuff so far. But the interesting part is the free. The dentry's lifetime is managed by RCU, and it appears the final `dput` happens when the RCU batch is executed. The critical part is that this release is linked to the function `ocfs2_dentry_attach_lock` in `fs/ocfs2/dcache.c`.
Aha! This suggests a race condition within `ocfs2_dentry_attach_lock`. The report points out that multiple threads doing lookups for the same name, or attempting to attach to the same dentry, could create this. One thread sets `dentry->d_fsdata` (which is standard practice), but a second thread could subsequently find it via `ocfs2_find_local_alias` using the `d_fsdata` field. The second thread is then calling `dput(alias)` on the same dentry. If the `dput` in the second thread's context drops the last reference to that dentry, the system immediately frees it. However, the first thread in its call to `ocfs2_lookup` or subsequent `lookup_one_qstr_excl` could *still* be holding a pointer to that freed `dentry`. This creates the potential for a UAF when the first thread eventually tries to use that now-freed `dentry` (perhaps in `vfs_mknod` or `end_dirop`).
The report is impressive. The provided call stacks for allocation, free, and the point of the access all connect to the same `dentry` object. The leaf frames of those stacks also point to the locking and VFS code within the ocfs2 subsystem, a clear sign of where the issue originates. The race, and thus the UAF, seem to be within the interaction between OCFS2's locking, lookups, and the dentry management under RCU. This gives me a solid starting point for a patch.
|
| 73/1 |
2026/04/05 00:35 |
action |
format-explanation |
0m
Results: map[Explanation:The bug report is self-consistent and actionable. It describes a
`slab-use-after-free` on a `dentry` object in a `PREEMPT_RT` kernel.
### Analysis of the Stacks:
1. **Bad Access Stack**: The UAF occurs in `rt_mutex_slowunlock+0xbf/0x8b0`,
which is called by `end_dirop` (inlined in `filename_mknodat`). In a
`PREEMPT_RT` kernel, `spin_lock` is implemented using `rt_mutex`. The stack
trace indicates that `dput(de)` was called in `end_dirop`, which then called
`fast_dput`. Inside `fast_dput`, a `spin_unlock(&dentry->d_lock)` was performed.
The access at offset 208 of the dentry corresponds exactly to
`dentry->d_lockref.lock.lock.wait_lock.dep_map` (where `d_lockref` is at offset
184, and the `dep_map` of the `raw_spinlock` inside the `rt_mutex` is at offset
24).
2. **Allocation Stack**: The dentry was allocated by task 20778 in
`lookup_one_qstr_excl` via `d_alloc`. This is the "original" dentry created
during the lookup process in `filename_create`.
3. **Free Stack**: The dentry was freed by an RCU callback (`rcu_do_batch`).
The "Last potentially related work creation" stack trace shows that `call_rcu`
was triggered by `ocfs2_dentry_attach_lock+0x748/0xe20` at
`fs/ocfs2/dcache.c:328`, which calls `dput(alias)`.
### Root Cause:
The bug is a race condition in the `ocfs2` filesystem's dentry locking logic. In
`ocfs2_dentry_attach_lock`, the code searches for an existing alias using
`ocfs2_find_local_alias`. If another thread has already attached a lock to the
same dentry (setting `dentry->d_fsdata`), `ocfs2_find_local_alias` may return
the dentry itself as an `alias`. The function then calls `dput(alias)` at line
328. If this `dput` drops the reference count to zero (e.g., because another
concurrent thread also dropped its reference), the dentry is queued for freeing
via RCU. However, the `ocfs2_lookup` function continues and returns this dentry
(or the caller `lookup_one_qstr_excl` continues to use it), leading to a
Use-After-Free when the dentry is later accessed in `end_dirop`.
### Conclusion:
The report contains all three required stack traces, they are consistent with
each other and the memory layout of the involved structures, and they point to a
specific logical error in the `ocfs2` subsystem.]
|