| 0/0 |
2026/04/01 00:02 |
flow |
moderation |
1h01m
Results: map[Actionable:true Confident:true Explanation:The bug report is a highly consistent and actionable use-after-free (UAF)
report. It describes a race condition between `hiddev_disconnect` and
`hiddev_release` in the `hiddev` driver.
### Analysis of the Stack Traces:
1. **Access Stack Trace:** The UAF occurs in `__mutex_unlock_slowpath` (called
from `hiddev_disconnect` at `drivers/hid/usbhid/hiddev.c:940`). Specifically, it
crashes while trying to acquire `lock->wait_lock` (the `mutex`'s internal
spinlock) at `kernel/locking/mutex.c:963`. This indicates that the `mutex`
structure itself (part of `struct hiddev`) has been freed while `mutex_unlock`
was still executing.
2. **Allocation Stack Trace:** The object was allocated in `hiddev_connect` at
`drivers/hid/usbhid/hiddev.c:893` using `kzalloc_obj(struct hiddev)`.
3. **Free Stack Trace:** The object was freed in `hiddev_release` at
`drivers/hid/usbhid/hiddev.c:232` using `kfree(list->hiddev)`.
### Root Cause:
The `hiddev` structure uses a mutex named `existancelock` to synchronize access
to its state. When a USB device is disconnected, `hiddev_disconnect` is called.
It sets `hiddev->exist = 0` and, if the device is still open (`hiddev->open >
0`), it calls `mutex_unlock(&hiddev->existancelock)`.
If a user-space process closes the device file concurrently, `hiddev_release` is
called. It attempts to acquire the same `existancelock`. If `hiddev_disconnect`
is currently holding the lock, `hiddev_release` will wait (either by sleeping or
optimistic spinning).
When `hiddev_disconnect` calls `mutex_unlock`, it enters the slow path
(`__mutex_unlock_slowpath`) if there are waiters or if certain debug options are
enabled. In the slow path, it releases the mutex owner. At this point, the
waiting `hiddev_release` can acquire the lock. Since `hiddev->exist` was already
set to 0 by `hiddev_disconnect`, `hiddev_release` proceeds to decrement
`hiddev->open` to 0 and then calls `kfree(hiddev)`.
However, `hiddev_disconnect` is still inside `__mutex_unlock_slowpath`. After
releasing the owner, it attempts to access the mutex's internal `wait_lock` to
wake up any remaining waiters. Since `hiddev` has just been freed by
`hiddev_release`, this access results in a use-after-free.
### Consistency Check:
- The memory offset `0x1c` (28 bytes) into the 512-byte slab object matches the
location of the `magic` field in the `wait_lock` spinlock within the `mutex`
structure, accounting for 4 bytes of padding after the three `int` fields in
`struct hiddev`.
- All three stack traces point to the same driver (`hiddev`) and the same data
structure (`struct hiddev`).
- The scenario follows a well-known pattern of UAF in `mutex_unlock` when the
object's lifetime is tied to the lock itself without additional reference
counting.
This report provides all the necessary information to identify the bug and
implement a fix (e.g., by adding a `kref` to `struct hiddev` or ensuring the
object is not freed until `mutex_unlock` has safely returned).]
|
| 1/1 |
2026/04/01 00:02 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/b407e36884e568ba3a530d7884859f9ca6443b7c]
|
| 2/1 |
2026/04/01 00:03 |
action |
kernel-builder |
32m
Results: map[KernelObj:/app/workdir/cache/build/feed80300518ec1735ff11088d24dc5f9be7bf88]
|
| 3/1 |
2026/04/01 00:36 |
action |
codesearch-prepare |
25m
Results: map[Index:codesearch-index]
|
| 4/1 |
2026/04/01 01:01 |
agent |
expert |
2m
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:
usb 6-1: USB disconnect, device number 25
==================================================================
BUG: KASAN: slab-use-after-free in debug_spin_lock_before kernel/locking/spinlock_debug.c:86 [inline]
BUG: KASAN: slab-use-after-free in do_raw_spin_lock+0x23b/0x260 kernel/locking/spinlock_debug.c:115
Read of size 4 at addr ffff8880289f981c by task kworker/1:3/6000
CPU: 1 UID: 0 PID: 6000 Comm: kworker/1:3 Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Workqueue: usb_hub_wq hub_event
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0x156/0x4c9 mm/kasan/report.c:482
kasan_report+0xdf/0x1e0 mm/kasan/report.c:595
debug_spin_lock_before kernel/locking/spinlock_debug.c:86 [inline]
do_raw_spin_lock+0x23b/0x260 kernel/locking/spinlock_debug.c:115
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:133 [inline]
_raw_spin_lock_irqsave+0x42/0x60 kernel/locking/spinlock.c:162
__mutex_unlock_slowpath+0x18b/0x790 kernel/locking/mutex.c:963
hiddev_disconnect+0x15b/0x1d0 drivers/hid/usbhid/hiddev.c:940
hid_disconnect+0xbe/0x1b0 drivers/hid/hid-core.c:2353
hid_hw_stop drivers/hid/hid-core.c:2400 [inline]
hid_device_remove+0x1b2/0x270 drivers/hid/hid-core.c:2834
device_remove+0xcb/0x180 drivers/base/dd.c:631
__device_release_driver drivers/base/dd.c:1344 [inline]
device_release_driver_internal+0x44e/0x620 drivers/base/dd.c:1367
bus_remove_device+0x2bc/0x560 drivers/base/bus.c:657
device_del+0x376/0x9b0 drivers/base/core.c:3880
hid_remove_device drivers/hid/hid-core.c:3009 [inline]
hid_destroy_device+0x19c/0x240 drivers/hid/hid-core.c:3031
usbhid_disconnect+0xa0/0xe0 drivers/hid/usbhid/hid-core.c:1477
usb_unbind_interface+0x1dd/0x9e0 drivers/usb/core/driver.c:458
device_remove drivers/base/dd.c:633 [inline]
device_remove+0x12a/0x180 drivers/base/dd.c:625
__device_release_driver drivers/base/dd.c:1344 [inline]
device_release_driver_internal+0x44e/0x620 drivers/base/dd.c:1367
bus_remove_device+0x2bc/0x560 drivers/base/bus.c:657
device_del+0x376/0x9b0 drivers/base/core.c:3880
usb_disable_device+0x367/0x810 drivers/usb/core/message.c:1476
usb_disconnect+0x2e2/0x9a0 drivers/usb/core/hub.c:2345
hub_port_connect drivers/usb/core/hub.c:5407 [inline]
hub_port_connect_change drivers/usb/core/hub.c:5707 [inline]
port_event drivers/usb/core/hub.c:5871 [inline]
hub_event+0x1d0c/0x4af0 drivers/usb/core/hub.c:5953
process_one_work+0xa23/0x19a0 kernel/workqueue.c:3276
process_scheduled_works kernel/workqueue.c:3359 [inline]
worker_thread+0x5ef/0xe50 kernel/workqueue.c:3440
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x754/0xd80 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Allocated by task 24:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
kmalloc_noprof include/linux/slab.h:950 [inline]
kzalloc_noprof include/linux/slab.h:1188 [inline]
hiddev_connect+0x259/0x5d0 drivers/hid/usbhid/hiddev.c:893
hid_connect+0x241/0x1750 drivers/hid/hid-core.c:2267
hid_hw_start+0xaa/0x140 drivers/hid/hid-core.c:2380
plantronics_probe+0x2b9/0x3a0 drivers/hid/hid-plantronics.c:209
__hid_device_probe drivers/hid/hid-core.c:2776 [inline]
hid_device_probe+0x50e/0x800 drivers/hid/hid-core.c:2813
call_driver_probe drivers/base/dd.c:643 [inline]
really_probe+0x241/0xa60 drivers/base/dd.c:721
__driver_probe_device+0x1de/0x400 drivers/base/dd.c:863
driver_probe_device+0x4c/0x1b0 drivers/base/dd.c:893
__device_attach_driver+0x1df/0x340 drivers/base/dd.c:1021
bus_for_each_drv+0x159/0x1e0 drivers/base/bus.c:500
__device_attach+0x1e4/0x4d0 drivers/base/dd.c:1093
device_initial_probe+0xaf/0xd0 drivers/base/dd.c:1148
bus_probe_device+0x64/0x160 drivers/base/bus.c:613
device_add+0x11d9/0x1950 drivers/base/core.c:3691
hid_add_device+0x2bf/0x440 drivers/hid/hid-core.c:2952
usbhid_probe+0xd57/0x1350 drivers/hid/usbhid/hid-core.c:1450
usb_probe_interface+0x303/0x8f0 drivers/usb/core/driver.c:396
call_driver_probe drivers/base/dd.c:643 [inline]
really_probe+0x241/0xa60 drivers/base/dd.c:721
__driver_probe_device+0x1de/0x400 drivers/base/dd.c:863
driver_probe_device+0x4c/0x1b0 drivers/base/dd.c:893
__device_attach_driver+0x1df/0x340 drivers/base/dd.c:1021
bus_for_each_drv+0x159/0x1e0 drivers/base/bus.c:500
__device_attach+0x1e4/0x4d0 drivers/base/dd.c:1093
device_initial_probe+0xaf/0xd0 drivers/base/dd.c:1148
bus_probe_device+0x64/0x160 drivers/base/bus.c:613
device_add+0x11d9/0x1950 drivers/base/core.c:3691
usb_set_configuration+0xd97/0x1c60 drivers/usb/core/message.c:2266
usb_generic_driver_probe+0xa1/0xe0 drivers/usb/core/generic.c:250
usb_probe_device+0xef/0x400 drivers/usb/core/driver.c:291
call_driver_probe drivers/base/dd.c:643 [inline]
really_probe+0x241/0xa60 drivers/base/dd.c:721
__driver_probe_device+0x1de/0x400 drivers/base/dd.c:863
driver_probe_device+0x4c/0x1b0 drivers/base/dd.c:893
__device_attach_driver+0x1df/0x340 drivers/base/dd.c:1021
bus_for_each_drv+0x159/0x1e0 drivers/base/bus.c:500
__device_attach+0x1e4/0x4d0 drivers/base/dd.c:1093
device_initial_probe+0xaf/0xd0 drivers/base/dd.c:1148
bus_probe_device+0x64/0x160 drivers/base/bus.c:613
device_add+0x11d9/0x1950 drivers/base/core.c:3691
usb_new_device.cold+0x685/0x115c drivers/usb/core/hub.c:2695
hub_port_connect drivers/usb/core/hub.c:5567 [inline]
hub_port_connect_change drivers/usb/core/hub.c:5707 [inline]
port_event drivers/usb/core/hub.c:5871 [inline]
hub_event+0x314d/0x4af0 drivers/usb/core/hub.c:5953
process_one_work+0xa23/0x19a0 kernel/workqueue.c:3276
process_scheduled_works kernel/workqueue.c:3359 [inline]
worker_thread+0x5ef/0xe50 kernel/workqueue.c:3440
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x754/0xd80 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
Freed by task 11803:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
kasan_save_free_info+0x3b/0x70 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5f/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]
kfree+0x1f6/0x6b0 mm/slub.c:6483
hiddev_release+0x40d/0x520 drivers/hid/usbhid/hiddev.c:232
__fput+0x3ff/0xb40 fs/file_table.c:469
task_work_run+0x150/0x240 kernel/task_work.c:233
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
__exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
exit_to_user_mode_loop+0x100/0x4a0 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:325 [inline]
do_syscall_64+0x67c/0xf80 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f
The buggy address belongs to the object at ffff8880289f9800
which belongs to the cache kmalloc-512 of size 512
The buggy address is located 28 bytes inside of
freed 512-byte region [ffff8880289f9800, ffff8880289f9a00)
The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff8880289f8c00 pfn:0x289f8
head: order:2 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0xfff00000000240(workingset|head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000240 ffff88801b842c80 ffffea0000b3f710 ffffea00015f9410
raw: ffff8880289f8c00 0000000800100008 00000000f5000000 0000000000000000
head: 00fff00000000240 ffff88801b842c80 ffffea0000b3f710 ffffea00015f9410
head: ffff8880289f8c00 0000000800100008 00000000f5000000 0000000000000000
head: 00fff00000000002 ffffea0000a27e01 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000004
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 2, migratetype Unmovable, gfp_mask 0x1d20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_HARDWALL), pid 10245, tgid 10245 (udevd), ts 189371739713, free_ts 189294134261
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x153/0x170 mm/page_alloc.c:1889
prep_new_page mm/page_alloc.c:1897 [inline]
get_page_from_freelist+0x111d/0x3140 mm/page_alloc.c:3962
__alloc_frozen_pages_noprof+0x27c/0x2ba0 mm/page_alloc.c:5250
alloc_slab_page mm/slub.c:3292 [inline]
allocate_slab mm/slub.c:3481 [inline]
new_slab+0xa6/0x6b0 mm/slub.c:3539
refill_objects+0x26b/0x400 mm/slub.c:7175
refill_sheaf mm/slub.c:2812 [inline]
__pcs_replace_empty_main+0x1ab/0x660 mm/slub.c:4615
alloc_from_pcs mm/slub.c:4717 [inline]
slab_alloc_node mm/slub.c:4851 [inline]
__kmalloc_cache_noprof+0x493/0x6f0 mm/slub.c:5375
kmalloc_noprof include/linux/slab.h:950 [inline]
kzalloc_noprof include/linux/slab.h:1188 [inline]
kernfs_fop_open+0x23d/0xd50 fs/kernfs/file.c:641
do_dentry_open+0x6d8/0x1660 fs/open.c:949
vfs_open+0x82/0x3f0 fs/open.c:1081
do_open fs/namei.c:4671 [inline]
path_openat+0x208c/0x31a0 fs/namei.c:4830
do_file_open+0x20e/0x430 fs/namei.c:4859
do_sys_openat2+0x10d/0x1e0 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+0x12d/0x210 fs/open.c:1383
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x106/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
page last free pid 11125 tgid 11125 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
__free_pages_prepare mm/page_alloc.c:1433 [inline]
__free_frozen_pages+0x7e1/0x10d0 mm/page_alloc.c:2978
stack_depot_save_flags+0x435/0x9d0 lib/stackdepot.c:735
kasan_save_stack+0x3f/0x50 mm/kasan/common.c:58
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
kmalloc_noprof include/linux/slab.h:950 [inline]
kzalloc_noprof include/linux/slab.h:1188 [inline]
ida_alloc_range+0x464/0x830 lib/idr.c:420
ida_alloc include/linux/idr.h:293 [inline]
create_worker+0x99/0x750 kernel/workqueue.c:2796
maybe_create_worker kernel/workqueue.c:3075 [inline]
manage_workers kernel/workqueue.c:3152 [inline]
worker_thread+0x8e2/0xe50 kernel/workqueue.c:3415
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x754/0xd80 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
Memory state around the buggy address:
ffff8880289f9700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8880289f9780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8880289f9800: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff8880289f9880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880289f9900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================
Reply: The bug report is a highly consistent and actionable use-after-free (UAF) report. It describes a race condition between `hiddev_disconnect` and `hiddev_release` in the `hiddev` driver.
### Analysis of the Stack Traces:
1. **Access Stack Trace:** The UAF occurs in `__mutex_unlock_slowpath` (called from `hiddev_disconnect` at `drivers/hid/usbhid/hiddev.c:940`). Specifically, it crashes while trying to acquire `lock->wait_lock` (the `mutex`'s internal spinlock) at `kernel/locking/mutex.c:963`. This indicates that the `mutex` structure itself (part of `struct hiddev`) has been freed while `mutex_unlock` was still executing.
2. **Allocation Stack Trace:** The object was allocated in `hiddev_connect` at `drivers/hid/usbhid/hiddev.c:893` using `kzalloc_obj(struct hiddev)`.
3. **Free Stack Trace:** The object was freed in `hiddev_release` at `drivers/hid/usbhid/hiddev.c:232` using `kfree(list->hiddev)`.
### Root Cause:
The `hiddev` structure uses a mutex named `existancelock` to synchronize access to its state. When a USB device is disconnected, `hiddev_disconnect` is called. It sets `hiddev->exist = 0` and, if the device is still open (`hiddev->open > 0`), it calls `mutex_unlock(&hiddev->existancelock)`.
If a user-space process closes the device file concurrently, `hiddev_release` is called. It attempts to acquire the same `existancelock`. If `hiddev_disconnect` is currently holding the lock, `hiddev_release` will wait (either by sleeping or optimistic spinning).
When `hiddev_disconnect` calls `mutex_unlock`, it enters the slow path (`__mutex_unlock_slowpath`) if there are waiters or if certain debug options are enabled. In the slow path, it releases the mutex owner. At this point, the waiting `hiddev_release` can acquire the lock. Since `hiddev->exist` was already set to 0 by `hiddev_disconnect`, `hiddev_release` proceeds to decrement `hiddev->open` to 0 and then calls `kfree(hiddev)`.
However, `hiddev_disconnect` is still inside `__mutex_unlock_slowpath`. After releasing the owner, it attempts to access the mutex's internal `wait_lock` to wake up any remaining waiters. Since `hiddev` has just been freed by `hiddev_release`, this access results in a use-after-free.
### Consistency Check:
- The memory offset `0x1c` (28 bytes) into the 512-byte slab object matches the location of the `magic` field in the `wait_lock` spinlock within the `mutex` structure, accounting for 4 bytes of padding after the three `int` fields in `struct hiddev`.
- All three stack traces point to the same driver (`hiddev`) and the same data structure (`struct hiddev`).
- The scenario follows a well-known pattern of UAF in `mutex_unlock` when the object's lifetime is tied to the lock itself without additional reference counting.
This report provides all the necessary information to identify the bug and implement a fix (e.g., by adding a `kref` to `struct hiddev` or ensuring the object is not freed until `mutex_unlock` has safely returned).
|
| 5/2 |
2026/04/01 01:01 |
llm |
expert |
0m
Model:
Tokens:
input: 8709
output: 25
thoughts: 924
Thoughts: **Analyzing a KASAN Slab-Use-After-Free Bug in the HID Subsystem**
Okay, here's what I'm seeing. We have a KASAN slab-use-after-free bug, and it seems to be related to the `hiddev` driver, specifically within the USB HID subsystem. The stack traces provided are what I'm going to start with.
First, the access stack trace points to a read of size 4 at a specific address, triggered by `kworker/1:3/6000`. The critical call chain I'm seeing is: `hiddev_disconnect` -> `mutex_unlock` -> `__mutex_unlock_slowpath` -> `do_raw_spin_lock`. The bug is flagged within the `do_raw_spin_lock` function, which is pretty low-level. It's interesting that the access happens during a spinlock operation, which, when triggered through `__mutex_unlock_slowpath`, suggests a potential synchronization issue related to the mutex in `hiddev_disconnect`.
Next, I've got the allocation stack trace. The memory in question was allocated in `hiddev_connect`. The allocation uses `kzalloc_noprof`, a zeroed allocation. This is typical for device drivers to ensure that the memory is initialized correctly.
Finally, we have the free stack trace. The allocated memory is freed by `hiddev_release`, specifically, at line 232 within `drivers/hid/usbhid/hiddev.c`. I see `kfree` being called, which makes sense.
Now, let's put it all together. The code seems to be allocating memory in `hiddev_connect`, using it, and then freeing it in `hiddev_release`. The use-after-free, however, occurs in `hiddev_disconnect`. I can also see a mutex lock and unlock operation. Given these three points, I need to look closely at the `hiddev` structure and the interactions between `connect`, `disconnect`, and `release`. Specifically, is the `hiddev` structure being correctly managed with the reference count or using the mutex for synchronization during disconnect after release?
The crucial function seems to be `hiddev_disconnect`. It's calling `mutex_unlock(&hiddev->exist_lock)`. So, the `exist_lock` field within the `hiddev` struct is the key. Let's start by listing the contents of the directory `drivers/hid/usbhid/`, and then dive into `hiddev.c` to understand the `hiddev` structure and how `exist_lock` is being used and initialized. I need to understand the lifecycle of the `hiddev` structure, how its locks are handled, and how it is managed. This is where the root cause probably lies.
|
| 6/2 |
2026/04/01 01:01 |
tool |
codesearch-dir-index |
0m
Args: map[Dir:drivers/hid/usbhid/]
Results: map[Files:[hid-core.c hid-pidff.c hid-pidff.h hiddev.c usbhid.h usbkbd.c usbmouse.c] Subdirs:<nil>]
|
| 7/2 |
2026/04/01 01:01 |
llm |
expert |
0m
Model:
Tokens:
input: 10333
output: 44
thoughts: 225
Thoughts: **Analyzing Potential Resource Contention in `hiddev.c`**
Okay, let's dive in. I'm going to focus on those specific lines within `drivers/hid/usbhid/hiddev.c` that the stack trace pointed to. The memory allocation happens at `hiddev_connect+0x259`, the freeing at `hiddev_release+0x40d`, and, critically, access (as indicated by the stack) at `hiddev_disconnect+0x15b`.
The stack trace reveals something interesting. It indicates a `mutex_unlock_slowpath` call right before the `hiddev_disconnect` function. That strongly suggests we're dealing with a lock-related issue, a potential point of contention where a mutex is being released, possibly after a period of access. Time to examine the code around `hiddev.c:940` to see what's happening. I need to understand what's being protected by that mutex and whether its usage is causing the observed behavior. My initial hypothesis is that we're seeing some type of resource access issue given where the locking/unlocking is occurring.
|
| 8/2 |
2026/04/01 01:01 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/hid/usbhid/hiddev.c IncludeLines:true Name:hiddev_disconnect]
Results: map[SourceCode: 927: void hiddev_disconnect(struct hid_device *hid)
928: {
929: struct hiddev *hiddev = hid->hiddev;
930: struct usbhid_device *usbhid = hid->driver_data;
931:
932: usb_deregister_dev(usbhid->intf, &hiddev_class);
933:
934: mutex_lock(&hiddev->existancelock);
935: hiddev->exist = 0;
936:
937: if (hiddev->open) {
938: hid_hw_close(hiddev->hid);
939: wake_up_interruptible(&hiddev->wait);
940: mutex_unlock(&hiddev->existancelock);
941: } else {
942: mutex_unlock(&hiddev->existancelock);
943: kfree(hiddev);
944: }
945: }
SourceFile:drivers/hid/usbhid/hiddev.c]
|
| 9/2 |
2026/04/01 01:01 |
llm |
expert |
0m
Model:
Tokens:
input: 11158
output: 44
thoughts: 385
Thoughts: **Debugging a Use-After-Free in the HID Driver**
Okay, so I've got a use-after-free error. The key line, according to the stack trace, is `mutex_unlock(&hiddev->existancelock);` at line 940 of `hiddev.c`. I see the KASAN report, specifically a slab-use-after-free, triggered by a read access in `__mutex_unlock_slowpath` which then calls `_raw_spin_lock_irqsave(&lock->wait_lock, flags)`. Alright, so the bad access is on `hiddev->existancelock.wait_lock`. That confirms my initial suspicion; `hiddev` is being freed prematurely. It's being used by `hiddev_disconnect` even after it should have been released.
Now, I need to trace the allocation and deallocation to understand the timing issue. Let's see... the `hiddev` structure is allocated in `hiddev_connect+0x259/0x5d0` at line 893, and it's being freed in `hiddev_release+0x40d/0x520` at line 232.
Okay, let me start by taking a closer look at `hiddev_connect`. I'm starting to get a picture of what might be happening here, so I'll need to dig deeper into the code around those points to track down the race condition.
|
| 10/2 |
2026/04/01 01:01 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/hid/usbhid/hiddev.c IncludeLines:true Name:hiddev_connect]
Results: map[SourceCode: 872: /*
873: * This is where hid.c calls us to connect a hid device to the hiddev driver
874: */
875: int hiddev_connect(struct hid_device *hid, unsigned int force)
876: {
877: struct hiddev *hiddev;
878: struct usbhid_device *usbhid = hid->driver_data;
879: int retval;
880:
881: if (!force) {
882: unsigned int i;
883: for (i = 0; i < hid->maxcollection; i++)
884: if (hid->collection[i].type ==
885: HID_COLLECTION_APPLICATION &&
886: !IS_INPUT_APPLICATION(hid->collection[i].usage))
887: break;
888:
889: if (i == hid->maxcollection)
890: return -EINVAL;
891: }
892:
893: if (!(hiddev = kzalloc_obj(struct hiddev)))
894: return -ENOMEM;
895:
896: init_waitqueue_head(&hiddev->wait);
897: INIT_LIST_HEAD(&hiddev->list);
898: spin_lock_init(&hiddev->list_lock);
899: mutex_init(&hiddev->existancelock);
900: hid->hiddev = hiddev;
901: hiddev->hid = hid;
902: hiddev->exist = 1;
903: retval = usb_register_dev(usbhid->intf, &hiddev_class);
904: if (retval) {
905: hid_err(hid, "Not able to get a minor for this device\n");
906: hid->hiddev = NULL;
907: kfree(hiddev);
908: return retval;
909: }
910:
911: /*
912: * If HID_QUIRK_NO_INIT_REPORTS is set, make sure we don't initialize
913: * the reports.
914: */
915: hiddev->initialized = hid->quirks & HID_QUIRK_NO_INIT_REPORTS;
916:
917: hiddev->minor = usbhid->intf->minor;
918:
919: return 0;
920: }
SourceFile:drivers/hid/usbhid/hiddev.c]
|
| 11/2 |
2026/04/01 01:01 |
llm |
expert |
0m
Model:
Tokens:
input: 12650
output: 44
thoughts: 11
Thoughts: Okay, here's my attempt at summarizing that thought process, keeping in mind I'm speaking to a fellow expert:
**Checking `hiddev_release`**
Alright, let's take a look at `hiddev_release`. It's a critical function, no doubt. Understanding the releasing aspect is key when working with HID devices, especially in kernel space. I need to make sure the resources are properly freed, and Iām considering the potential impact on device stability if this function isn't perfectly implemented. We all know how easily these drivers can get into trouble. I'm focusing on its interactions with the device file descriptor, any associated memory, and interrupt handling. I need to ensure there are no race conditions or dangling pointers lurking within its logic. Any errors that are overlooked here can lead to issues downstream, whether that's memory leaks, system instability, or even security vulnerabilities. So, this warrants a deep dive into its implementation and any associated documentation, and I will cross-check the associated function to make sure that it's all handled.
|
| 12/2 |
2026/04/01 01:02 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/hid/usbhid/hiddev.c IncludeLines:true Name:hiddev_release]
Results: map[SourceCode: 213: /*
214: * release file op
215: */
216: static int hiddev_release(struct inode * inode, struct file * file)
217: {
218: struct hiddev_list *list = file->private_data;
219: unsigned long flags;
220:
221: spin_lock_irqsave(&list->hiddev->list_lock, flags);
222: list_del(&list->node);
223: spin_unlock_irqrestore(&list->hiddev->list_lock, flags);
224:
225: mutex_lock(&list->hiddev->existancelock);
226: if (!--list->hiddev->open) {
227: if (list->hiddev->exist) {
228: hid_hw_close(list->hiddev->hid);
229: hid_hw_power(list->hiddev->hid, PM_HINT_NORMAL);
230: } else {
231: mutex_unlock(&list->hiddev->existancelock);
232: kfree(list->hiddev);
233: vfree(list);
234: return 0;
235: }
236: }
237:
238: mutex_unlock(&list->hiddev->existancelock);
239: vfree(list);
240:
241: return 0;
242: }
SourceFile:drivers/hid/usbhid/hiddev.c]
|
| 13/2 |
2026/04/01 01:02 |
llm |
expert |
0m
Model:
Tokens:
input: 13383
output: 39
thoughts: 1318
Thoughts: **Analyzing a Potential Use-After-Free Vulnerability in HID Device Disconnect**
Okay, so I've been digging into this `hiddev` code, and I think I've spotted a potential race condition and a possible Use-After-Free (UAF) vulnerability. Let's break down my thought process:
First, the core issue seems to revolve around the interaction between `hiddev_disconnect` and `hiddev_release`. From what I can tell, the `hiddev` structure is freed in `hiddev_release` under specific conditions: `list->hiddev->open` becomes 0 and `list->hiddev->exist` is also 0.
Now, let's look at `hiddev_disconnect`. It appears to be a disconnect function for a HID device. It acquires the `hiddev->existancelock` mutex, sets `hiddev->exist` to 0, and then checks `hiddev->open`. Crucially, if `hiddev->open` is 1, it calls `hid_hw_close`, then wakes up a waiting process via `wake_up_interruptible(&hiddev->wait)`, and *then* unlocks the `existancelock` mutex.
The vulnerability stems from concurrency. Imagine that `hiddev_release` is called concurrently, potentially because the user has closed the device. Let's trace this specific scenario where `hiddev_release` is running at the same time:
1. `hiddev_disconnect` is running and has the lock on `existancelock`.
2. `hiddev_release` is now called and attempts to acquire the `existancelock`. It blocks.
3. `hiddev_disconnect` calls `mutex_unlock(&hiddev->existancelock)` which is line 940, the alleged point of the UAF.
4. `hiddev_release` wakes up.
5. `hiddev_release` *then* acquires the lock, decrements `open` and checks `exist`, which it finds is 0.
6. `hiddev_release` unlocks the mutex, and then, crucially, it calls `kfree(list->hiddev)`, freeing the `hiddev` structure.
7. `hiddev_disconnect` has *already* released the lock and is potentially now running the slowpath in unlock, but the `hiddev` structure might be freed at this point.
The stack trace points to `__mutex_unlock_slowpath` in `kernel/locking/mutex.c` as the culprit. I believe that specifically, the `spin_lock_irqsave(&lock->wait_lock, flags);` line is where the UAF is triggered. The lock pointer is `&hiddev->existancelock`.
I know that `mutex_unlock` first *attempts* a fast release, but, if there are waiters, then it goes into the slow path and wakes up waiters. It seems that `hiddev_release` could be woken up just as `hiddev_disconnect` begins its unlock process. If `hiddev_release` then frees `hiddev`, and then `__mutex_unlock_slowpath` attempts to use `lock` (which is now an invalid pointer to freed memory), we have our UAF. So even if the fast path is successful, the slow path is still potentially unsafe.
So, in summary: the race condition lies in the potential for `hiddev_release` to free the `hiddev` structure *while* `hiddev_disconnect` is in the process of unlocking the `existancelock` mutex. The `__mutex_unlock_slowpath` function, in particular, becomes vulnerable when it tries to operate on the freed memory, leading to the Use-After-Free condition when it calls the `spin_lock_irqsave` function. The vulnerability is subtle, and the timing has to be just right, but the analysis is sound: the memory can be freed before `hiddev_disconnect` is done with the mutex.
|
| 14/2 |
2026/04/01 01:02 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/mutex.c IncludeLines:true Name:mutex_unlock]
Results: map[SourceCode: 530: /**
531: * mutex_unlock - release the mutex
532: * @lock: the mutex to be released
533: *
534: * Unlock a mutex that has been locked by this task previously.
535: *
536: * This function must not be used in interrupt context. Unlocking
537: * of a not locked mutex is not allowed.
538: *
539: * The caller must ensure that the mutex stays alive until this function has
540: * returned - mutex_unlock() can NOT directly be used to release an object such
541: * that another concurrent task can free it.
542: * Mutexes are different from spinlocks & refcounts in this aspect.
543: *
544: * This function is similar to (but not equivalent to) up().
545: */
546: void __sched mutex_unlock(struct mutex *lock)
547: {
548: #ifndef CONFIG_DEBUG_LOCK_ALLOC
549: if (__mutex_unlock_fast(lock))
550: return;
551: #endif
552: __mutex_unlock_slowpath(lock, _RET_IP_);
553: }
SourceFile:kernel/locking/mutex.c]
|
| 15/2 |
2026/04/01 01:02 |
llm |
expert |
0m
Model:
Tokens:
input: 15986
output: 43
thoughts: 764
Thoughts: **Analyzing a Potential Race Condition in the HID Driver**
Okay, so I'm looking at this code and immediately see a potential issue with how mutexes are being used, specifically in the context of releasing a `hiddev` object. The comment here in the code is the key: it explicitly states that you *cannot* use `mutex_unlock()` directly to release an object if another concurrent task might free it. That's a huge red flag, and it seems like we're doing exactly that.
Let me break down my thinking step-by-step. First, I see that `hiddev_disconnect` calls `mutex_unlock(&hiddev->existancelock)`. Simultaneously, `hiddev_release` is waiting on the same `existancelock`. Once that lock is released, `hiddev_release` resumes. The sequence of events looks like this: `hiddev_disconnect` unlocks, `hiddev_release` acquires the lock and sees `hiddev->exist` is 0.
Now, here's where it gets interesting: if `hiddev->exist` is 0 (as set by `hiddev_disconnect`), then `hiddev_release` unlocks *again* and frees `hiddev`. That's the problem.
In `hiddev_disconnect`, we have the following scenario: If `hiddev->open` is 1, `hiddev_disconnect` *also* calls `mutex_unlock`.
The danger is this: If `hiddev_release` is waiting on that lock, then it's woken up by `hiddev_disconnect`'s unlock. It can acquire the lock, and then, if the condition `hiddev->exist` is met, `hiddev_release` will free `hiddev`. But `hiddev_disconnect` is still inside `__mutex_unlock_slowpath` where it *may* still be using the `mutex` structure after the lock has been released. This is a classic race condition and a use-after-free scenario. The critical thing here is that the freeing operation in `hiddev_release` is *directly* triggered by the unlock in `hiddev_disconnect`. The comment about the danger of directly freeing the object with unlock suddenly makes perfect sense.
My gut feeling, considering the comment about mutexes, is that the system could be accessing the `mutex` structure in `__mutex_unlock_slowpath` after the lock is released, and since `hiddev` may already be freed by then, it could lead to memory corruption.
I need to look deeper into `__mutex_unlock_slowpath` to confirm my suspicion, but the potential is definitely there.
|
| 16/2 |
2026/04/01 01:02 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/mutex.c IncludeLines:true Name:__mutex_unlock_slowpath]
Results: map[SourceCode: 928: /*
929: * Release the lock, slowpath:
930: */
931: static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)
932: {
933: struct task_struct *next = NULL;
934: DEFINE_WAKE_Q(wake_q);
935: unsigned long owner;
936: unsigned long flags;
937:
938: mutex_release(&lock->dep_map, ip);
939:
940: /*
941: * Release the lock before (potentially) taking the spinlock such that
942: * other contenders can get on with things ASAP.
943: *
944: * Except when HANDOFF, in that case we must not clear the owner field,
945: * but instead set it to the top waiter.
946: */
947: owner = atomic_long_read(&lock->owner);
948: for (;;) {
949: MUTEX_WARN_ON(__owner_task(owner) != current);
950: MUTEX_WARN_ON(owner & MUTEX_FLAG_PICKUP);
951:
952: if (owner & MUTEX_FLAG_HANDOFF)
953: break;
954:
955: if (atomic_long_try_cmpxchg_release(&lock->owner, &owner, __owner_flags(owner))) {
956: if (owner & MUTEX_FLAG_WAITERS)
957: break;
958:
959: return;
960: }
961: }
962:
963: raw_spin_lock_irqsave(&lock->wait_lock, flags);
964: debug_mutex_unlock(lock);
965: if (!list_empty(&lock->wait_list)) {
966: /* get the first entry from the wait-list: */
967: struct mutex_waiter *waiter =
968: list_first_entry(&lock->wait_list,
969: struct mutex_waiter, list);
970:
971: next = waiter->task;
972:
973: debug_mutex_wake_waiter(lock, waiter);
974: __clear_task_blocked_on(next, lock);
975: wake_q_add(&wake_q, next);
976: }
977:
978: if (owner & MUTEX_FLAG_HANDOFF)
979: __mutex_handoff(lock, next);
980:
981: raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
982: }
SourceFile:kernel/locking/mutex.c]
|
| 17/2 |
2026/04/01 01:02 |
llm |
expert |
0m
Model:
Tokens:
input: 18186
output: 42
thoughts: 1915
Thoughts: **UAF Analysis in HID Driver Mutex Unlock**
Okay, so I'm looking at this kernel stack trace and immediately see a slab-use-after-free error within `__mutex_unlock_slowpath`, specifically when attempting to take the `wait_lock`. The trace suggests the crash occurs *after* the `owner` has been cleared but *before* the `wait_lock` is taken.
My initial thought is, `hiddev_disconnect` is the culprit, and it seems to have released the lock's ownership (via `atomic_long_try_cmpxchg_release` at line 955) but hasn't yet entered the critical section protected by the `wait_lock` (line 963). This release allows a window for another task, say, a `hiddev_release`, which was waiting for the lock, to swoop in and acquire it.
The crux of the matter seems to revolve around the interaction between `hiddev_disconnect` and `hiddev_release`, particularly around the `existancelock` mutex. `hiddev_release` gets called when a file is closed, and it's quite likely that it's waiting on this lock. If the `hiddev->open` counter hits zero within `hiddev_release` (implying the last file handle is closed), `hiddev` is `kfree`'d.
Let's break this down. Line 955 in `hiddev_disconnect` *releases* the lock by clearing the owner atomically. A waiting `hiddev_release` could be in `__mutex_lock_slowpath` or blocked waiting on the `wait_lock`. If it gets the chance, it will happily take the lock.
Crucially, the code allows `hiddev_release` to decrement the `open` count, and potentially `kfree` the `hiddev` structure. But `hiddev_disconnect` is still chugging along! Because `hiddev_disconnect` hasn't taken `wait_lock` yet, the `hiddev` structure, and crucially the `wait_lock` *within* it, could be freed by another task. Then, when `hiddev_disconnect` *finally* tries to take that same `wait_lock` at line 963, it's accessing freed memory ā a textbook use-after-free condition.
The problem lies in the fact that `hiddev` can be freed (by `hiddev_release`) while `hiddev_disconnect` is still in the process of unlocking the mutex. The `mutex_unlock` documentation states the caller must ensure the mutex stays alive. We're violating that.
The fix seems to be: either use reference counting to ensure the `hiddev` structure remains alive for the duration of the mutex unlock or rethink the synchronization strategy entirely. It would be wise to ensure `hiddev` can't be freed while `hiddev_disconnect` is still executing. A deeper look at the `struct hiddev` definition would be beneficial.
|
| 18/2 |
2026/04/01 01:02 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/hid/usbhid/hiddev.c IncludeLines:true Name:hiddev]
Results: map[SourceCode: 23: struct hiddev {
24: int minor;
25: int exist;
26: int open;
27: struct mutex existancelock;
28: wait_queue_head_t wait;
29: struct hid_device *hid;
30: struct list_head list;
31: spinlock_t list_lock;
32: bool initialized;
33: };
SourceFile:include/linux/hiddev.h]
|
| 19/2 |
2026/04/01 01:02 |
llm |
expert |
0m
Model:
Tokens:
input: 20943
output: 30
thoughts: 117
Thoughts: **Navigating the HID Development Files**
Okay, so I'm diving into this HID-related code and the first thing that jumps out is this reference to `include/linux/hiddev.h`. Immediately, my brain starts to piece things together. *Where* exactly is this file? Let me mentally confirm the file path.
Right, I seem to recall that the `hiddev.c` file ā the meat of the `hiddev` driver ā should be lurking in the `drivers/hid/usbhid/` directory. So, it's probably `drivers/hid/usbhid/hiddev.c`. I'm pretty sure that's where the definition of the `struct hiddev` is located as well. But just to be absolutely certain, let me cross-reference that with any prior searches.
Aha! The previous "codesearch-definition-source" for `hiddev_disconnect` also pointed to the same `drivers/hid/usbhid/hiddev.c` file. Alright, let's just quickly confirm by inspecting the file content itself. Time to verify that `struct hiddev` is indeed declared there. Let's make sure.
|
| 20/2 |
2026/04/01 01:02 |
tool |
codesearch-file-index |
0m
Args: map[SourceFile:drivers/hid/usbhid/hiddev.c]
Results: map[Entities:[map[Kind:function Name:__hiddev_open] map[Kind:function Name:hiddev_connect] map[Kind:function Name:hiddev_devnode] map[Kind:function Name:hiddev_disconnect] map[Kind:function Name:hiddev_fasync] map[Kind:function Name:hiddev_hid_event] map[Kind:function Name:hiddev_ioctl] map[Kind:function Name:hiddev_ioctl_string] map[Kind:function Name:hiddev_ioctl_usage] map[Kind:function Name:hiddev_lookup_report] map[Kind:function Name:hiddev_lookup_usage] map[Kind:function Name:hiddev_open] map[Kind:function Name:hiddev_poll] map[Kind:function Name:hiddev_read] map[Kind:function Name:hiddev_release] map[Kind:function Name:hiddev_report_event] map[Kind:function Name:hiddev_send_event] map[Kind:function Name:hiddev_write] map[Kind:struct Name:hiddev_list] map[Kind:global_variable Name:__UNIQUE_ID_addressable_hiddev_hid_event_1022] map[Kind:global_variable Name:__gendwarfksyms_ptr_hiddev_hid_event] map[Kind:global_variable Name:hiddev_class] map[Kind:global_variable Name:hiddev_fops]]]
|
| 21/2 |
2026/04/01 01:02 |
llm |
expert |
0m
Model:
Tokens:
input: 21750
output: 23
thoughts: 39
Thoughts: Okay, here's my interpretation of those thoughts, tailored for an expert audience, presented in the first person:
**Navigating the HID Device Landscape**
Alright, hang on a second. I'm looking for the `struct hiddev` definition, and I was *certain* it resided within `drivers/hid/usbhid/hiddev.c`. That's where I would have expected to find the core device interface implementation. But it's not there. Hmm, that's unusual, and it throws off my mental map of the driver structure a bit.
Okay, deep breath. Let's backtrack and approach this methodically. If the structure itself isn't in the implementation file, the logical next step is to check the related header files. I'm going to jump to `include/linux/hiddev.h`. That's the expected place for the main data structure definition, and where any necessary typedefs or extern declarations will live. I'll take a look in there and make sure that this is the correct header and that the definition is what I expect. I'll need to figure out where that declaration is.
|
| 22/2 |
2026/04/01 01:02 |
tool |
read-file |
0m
Args: map[File:include/linux/hiddev.h]
Results: map[Contents:/* SPDX-License-Identifier: GPL-2.0-or-later */
/*
* Copyright (c) 1999-2000 Vojtech Pavlik
*
* Sponsored by SuSE
*/
/*
*
* Should you need to contact me, the author, you can do so either by
* e-mail - mail your message to <vojtech@suse.cz>, or by paper mail:
* Vojtech Pavlik, Ucitelska 1576, Prague 8, 182 00 Czech Republic
*/
#ifndef _HIDDEV_H
#define _HIDDEV_H
#include <uapi/linux/hiddev.h>
/*
* In-kernel definitions.
*/
struct hiddev {
int minor;
int exist;
int open;
struct mutex existancelock;
wait_queue_head_t wait;
struct hid_device *hid;
struct list_head list;
spinlock_t list_lock;
bool initialized;
};
struct hid_device;
struct hid_usage;
struct hid_field;
struct hid_report;
#ifdef CONFIG_USB_HIDDEV
int hiddev_connect(struct hid_device *hid, unsigned int force);
void hiddev_disconnect(struct hid_device *);
void hiddev_hid_event(struct hid_device *hid, struct hid_field *field,
struct hid_usage *usage, __s32 value);
void hiddev_report_event(struct hid_device *hid, struct hid_report *report);
#else
static inline int hiddev_connect(struct hid_device *hid,
unsigned int force)
{ return -1; }
static inline void hiddev_disconnect(struct hid_device *hid) { }
static inline void hiddev_hid_event(struct hid_device *hid, struct hid_field *field,
struct hid_usage *usage, __s32 value) { }
static inline void hiddev_report_event(struct hid_device *hid, struct hid_report *report) { }
#endif
#endif
]
|
| 23/2 |
2026/04/01 01:02 |
llm |
expert |
0m
Model:
Tokens:
input: 22560
output: 33
thoughts: 3935
Thoughts: **Analysis of a Potential Use-After-Free Vulnerability in the HID Device Subsystem**
Alright, I've got a handle on this. The code in `hiddev.c` is looking like it has a classic use-after-free issue, and I think I've traced it down with pretty good precision.
The crux of the problem lies around how `hiddev` objects are managed and, in particular, when they are freed relative to the `existancelock` mutex. `struct hiddev` itself, as I see it, doesn't have a reference count, only `open` and `exist`. The current logic, or at least how I see it, is vulnerable.
Specifically, the sequence in `hiddev_disconnect` at line 940, where `mutex_unlock(&hiddev->existancelock)` is called, has the potential to cause problems if `hiddev_release` gets to the object first. I can see that `hiddev_release` is *waiting* on the same `existancelock`, which is a critical piece of information. The problem here, is that the `mutex_unlock` routine itself can have a slow path where the object can be freed before the unlock is done.
The code flow, as I've walked through it, is this: `hiddev_disconnect` sets `exist` to 0, which is important. If `hiddev->open` is 0, it *itself* frees the object. If, however, `hiddev->open` is greater than 0, it calls `mutex_unlock` and then, in theory, *expects* `hiddev_release` to do the freeing. The issue is that `hiddev_release` also tries to acquire the same `existancelock` and can, if the stars align, free the object *before* the `mutex_unlock` in `hiddev_disconnect` completes its work.
The potential for the UAF is there because the `mutex_unlock` call *might* access freed memory due to optimistic spinning. Specifically, if an optimistic spinner, like `hiddev_release`, comes along and grabs the lock after it has been released and *before* all of the release code is complete, then the potential is there.
I've checked the line numbers and addresses against my local copies of the code, so I'm sure that the vulnerability is where I think it is.
The report is clearly actionable, and I think I understand the root cause. This has the tell-tale signs of a "caller must ensure that the mutex stays alive until this function has returned" problem. We need to be able to protect the lifetime of `hiddev` across the `mutex_unlock`.
I'm thinking one likely fix is going to be the introduction of a reference count using `kref` so we can prevent the object from being prematurely freed. We can also use an alternative approach and ensure that `hiddev_disconnect` does not access the freed object at all by refactoring to protect the call by ensuring the lock is held.
The "Tainted: \[L]=SOFTLOCKUP" message isn't the primary issue but could be a symptom of the race, leading to contention on a corrupted lock. The crash likely points to a read from the freed memory after the unlock call, as I verified by checking the object's layout. The offset matches perfectly: 28 decimal is in the mutex structure. I'm confident in my analysis.
|
| 24/2 |
2026/04/01 01:03 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:include/linux/hiddev.h Name:mutex]
Results: map[Fields:[map[Name:owner OffsetBits:0 SizeBits:64] map[Name:wait_lock OffsetBits:64 SizeBits:512] map[Name:osq OffsetBits:576 SizeBits:32] map[Name:wait_list OffsetBits:640 SizeBits:128] map[Name:magic OffsetBits:768 SizeBits:64] map[Name:dep_map OffsetBits:832 SizeBits:320]]]
|
| 25/2 |
2026/04/01 01:03 |
llm |
expert |
0m
Model:
Tokens:
input: 27400
output: 32
thoughts: 153
Thoughts: **Deconstructing the Memory Layout**
Okay, so I'm looking at this structure and trying to understand where these locks are positioned in memory. I see `owner` is right at the start, at offset 0, which is standard. Now, `wait_lock` is apparently 64 bits (8 bytes) further in. If `existancelock` is at offset 12 within the `hiddev` structure, that means `wait_lock` should be located at offset 12 plus 8, which gets me to offset 20. Converting that to hexadecimal, 20 is 0x14. Then, I see the problematic access is at offset 0x1c (28 in decimal). To figure out where that is relative to the `existancelock` member, I just subtract 12. That means this access must be 16 bytes further in. Since `wait_lock` is at offset 8 within `struct mutex`, and the problematic access is 8 bytes further in, the access must be within the lock itself. So, I need to check the definition of `struct spinlock` to understand the layout and what's being accessed there. Time to dive into that definition and see what's what.
|
| 26/2 |
2026/04/01 01:03 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:kernel/locking/mutex.c Name:spinlock]
Results: map[Fields:[map[Name: OffsetBits:0 SizeBits:512]]]
|
| 27/2 |
2026/04/01 01:03 |
llm |
expert |
0m
Model:
Tokens:
input: 27882
output: 34
thoughts: 22
Thoughts: Here's my attempt at summarizing that thought process, tailored for an expert audience:
**Spinlock Conundrum**
Okay, hold on. My initial gut feeling is nagging at me. This `spinlock`... isn't it potentially a union, or something similar, under the hood? I need to clarify my understanding of its internal representation. The behavior I'm seeing doesn't quite jive with a simple atomic integer, and I need to rule out any funky memory layout tricks. I should probably double-check the definition of `struct raw_spinlock`. That's the key; that's where the implementation details are hidden. I need to refresh my mental model of that structure to see what's actually going on. This could be important to understanding the performance characteristics of this code.
|
| 28/2 |
2026/04/01 01:03 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:kernel/locking/mutex.c Name:raw_spinlock]
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]]]
|
| 29/2 |
2026/04/01 01:03 |
llm |
expert |
0m
Model:
Tokens:
input: 28216
output: 45
thoughts: 285
Thoughts: **Analyzing Spinlock Access**
Okay, let's break this down. From the structure definitions, `raw_spinlock` has `raw_lock` at offset 0, then a `magic` field at offset 4, and `owner_cpu` at offset 8. The sizes are all 32 bits, which is 4 bytes each, so that lines up.
Now, I'm tracing through this, and it looks like `wait_lock` is at offset 8 within a `struct mutex`, and `existancelock` is at offset 12 in a `struct hiddev`. That means, the way this all stacks up, `wait_lock` is effectively at offset 20 within a `struct hiddev` instance.
So, if `wait_lock` starts at offset 20, then its `magic` field, `wait_lock.magic`, would be at offset 20 + 4 = 24. And `wait_lock.owner_cpu` is at offset 20 + 8 = 28. Perfect, the calculation tracks the location. Offset 28 (0x1c hex).
Now, the memory access is at address `ffff8880289f981c`. The object's starting address is `ffff8880289f9800`. Doing the math, that access is at offset 28 from the beginning of the object. Bingo, that offset 28 is precisely where `wait_lock.owner_cpu` should be! This is looking good.
Wait a second...I'm pretty sure `debug_spin_lock_before` in `kernel/locking/spinlock_debug.c`, line 86, is going to be checking the `owner_cpu` value. Let's make sure that's the area the code is operating in, and double-check it. Need to verify that's the source of the access. Gotta go look at that line of code.
|
| 30/2 |
2026/04/01 01:03 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/spinlock_debug.c IncludeLines:true Name:debug_spin_lock_before]
Results: map[SourceCode: 83: static inline void
84: debug_spin_lock_before(raw_spinlock_t *lock)
85: {
86: SPIN_BUG_ON(READ_ONCE(lock->magic) != SPINLOCK_MAGIC, lock, "bad magic");
87: SPIN_BUG_ON(READ_ONCE(lock->owner) == current, lock, "recursion");
88: SPIN_BUG_ON(READ_ONCE(lock->owner_cpu) == raw_smp_processor_id(),
89: lock, "cpu recursion");
90: }
SourceFile:kernel/locking/spinlock_debug.c]
|
| 31/2 |
2026/04/01 01:03 |
llm |
expert |
0m
Model:
Tokens:
input: 29166
output: 19
thoughts: 1581
Thoughts: **Bug Analysis: A Deep Dive into a Kernel Use-After-Free**
Okay, let's break this down. First, I need to figure out this memory access. The report's highlighting an access to `lock->magic` at offset 0x1c. I started by checking the offsets in `raw_spinlock_t` and `mutex`, thinking there might be some alignment issues I was missing. I initially got the `magic` offset wrong, I was calculating `magic` as residing at offset 24 (0x18), but the access was at 0x1c (28 decimal). I quickly identified that this was incorrect given my understanding of the relevant data structures. I cross-referenced the line of code from the stack trace and the code, and realized that it was looking at the magic number.
I need to confirm if these structures are exactly as I'm thinking they are. I went back and re-examined `struct mutex` and `struct raw_spinlock`, paying close attention to the byte sizes of each member and the alignment. `owner` of type `long` needs to be 8-byte aligned, thus, `wait_lock` of type `raw_spinlock_t` is at offset 8 bytes within the `struct mutex` structure. Given this layout, the `magic` field of the `wait_lock` is at 12 bytes. If the `existancelock` member is 12 bytes in, then `magic` would reside at offset 24. It still wasn't right. I went back to the drawing board.
Then it hit me: padding in `struct hiddev`. I need to account for any potential padding in the `hiddev` structure. Let's lay it out: `minor`, `exist`, and `open` are all `int`s (4 bytes each). That's 12 bytes total. The `existancelock` is a `struct mutex`, and it's 8-byte aligned because of `owner`. Therefore, there's 4 bytes of padding after `open` to align `existancelock`. So, `existancelock` starts at offset 16 (12 + 4). Then, the embedded `wait_lock` starts at 16 + 8 = 24. Consequently, `magic` is at 24 + 4 = 28 (0x1c)! Bingo! That lines up perfectly with the memory access in the report.
So, the access at 0x1c is indeed `lock->magic`, as the stack trace suggests. Line 86 in `kernel/locking/mutex.c` is the key.
This confirms the bug: a use-after-free in `struct hiddev`. `mutex_unlock` is being called on `hiddev->existancelock` in `hiddev_disconnect` *while* another task can potentially free `hiddev` as soon as it grabs the lock.
Okay, let's verify my assessment. Are there:
* Three stacks (Access, Allocation, Free): Yes.
* The same object type? Yes, `hiddev`.
* The same subsystem? Yes, `hiddev`.
* Consistent explanation? Yes, the scenario I constructed explains everything.
Now, checking the allocation stack, I need to look closer at `hiddev_connect+0x259/0x5d0`. Given that the stack is showing that line 893 is the line of code, this seems a little odd. Let's see if line 893 is consistent.
The code does say that at line 893 is indeed where the kzalloc function is. That's fine.
The free stack shows `kfree` called from `hiddev_release`. Line 232 is where list->hiddev is being freed. Perfect.
The access stack shows `__mutex_unlock_slowpath` calling `hiddev_disconnect` which is calling `mutex_unlock`. Line 940 of the code matches this. Great.
The report is definitely actionable.
Let me think about the call sequence and root cause:
`hiddev_disconnect` is called from `usbhid_disconnect` -> `hid_destroy_device` -> `hid_remove_device` -> `device_del` -> ... -> `usb_disconnect`. This happens when the USB device is unplugged.
`hiddev_release` is called when the device file `/dev/usb/hiddevX` is closed.
Now, if a program has the device file open, and the USB device is unplugged:
1. `hiddev_disconnect` is called (due to unplug).
2. It sets `exist = 0`.
3.
|
| 32/2 |
2026/04/01 01:03 |
tool |
set-results |
0m
Args: map[Actionable:true Confident:true]
Results: map[Actionable:true Confident:true]
|
| 33/2 |
2026/04/01 01:03 |
llm |
expert |
0m
Model:
Tokens:
input: 31832
output: 803
thoughts: 244
Thoughts: Here's my analysis of the situation:
**Understanding the UAF in `hiddev_disconnect`**
Okay, the analysis is done, and it's looking actionable and consistent. It's that familiar User-After-Free (UAF) in `mutex_unlock` that's rearing its head again. Specifically, it's occurring with `hiddev->existancelock` within the `hiddev_disconnect` function. I've tracked it back: the `hiddev` object was allocated during `hiddev_connect` and later freed in `hiddev_release`. Now, let's break down the timing. The sequence is clear:
1. `hiddev_disconnect` is holding the `existancelock` mutex.
2. Meanwhile, `hiddev_release` (or another contending task) is waiting to acquire the same lock, or maybe even optimistically spinning.
3. `hiddev_disconnect` calls `mutex_unlock` to release the mutex.
4. Then, things get tricky: `hiddev_disconnect` enters the `__mutex_unlock_slowpath` path and releases the lock's ownership.
5. Critically, `hiddev_release` *grabs* the `existancelock`, and goes ahead and *frees* `hiddev`.
6. The problem is, `hiddev_disconnect` is still inside `__mutex_unlock_slowpath`, and it's trying to access `hiddev->existancelock.wait_lock` *after* the object has been freed. That's the UAF.
This is a known pattern with `mutex_unlock` where the next thread to acquire the lock then proceeds to free the object. The standard fix? Ensure that the object remains valid all the way until `mutex_unlock` *completely* returns. I'm ready to craft the final response with the fix now.
|
| 34/1 |
2026/04/01 01:03 |
action |
format-explanation |
0m
Results: map[Explanation:The bug report is a highly consistent and actionable use-after-free (UAF)
report. It describes a race condition between `hiddev_disconnect` and
`hiddev_release` in the `hiddev` driver.
### Analysis of the Stack Traces:
1. **Access Stack Trace:** The UAF occurs in `__mutex_unlock_slowpath` (called
from `hiddev_disconnect` at `drivers/hid/usbhid/hiddev.c:940`). Specifically, it
crashes while trying to acquire `lock->wait_lock` (the `mutex`'s internal
spinlock) at `kernel/locking/mutex.c:963`. This indicates that the `mutex`
structure itself (part of `struct hiddev`) has been freed while `mutex_unlock`
was still executing.
2. **Allocation Stack Trace:** The object was allocated in `hiddev_connect` at
`drivers/hid/usbhid/hiddev.c:893` using `kzalloc_obj(struct hiddev)`.
3. **Free Stack Trace:** The object was freed in `hiddev_release` at
`drivers/hid/usbhid/hiddev.c:232` using `kfree(list->hiddev)`.
### Root Cause:
The `hiddev` structure uses a mutex named `existancelock` to synchronize access
to its state. When a USB device is disconnected, `hiddev_disconnect` is called.
It sets `hiddev->exist = 0` and, if the device is still open (`hiddev->open >
0`), it calls `mutex_unlock(&hiddev->existancelock)`.
If a user-space process closes the device file concurrently, `hiddev_release` is
called. It attempts to acquire the same `existancelock`. If `hiddev_disconnect`
is currently holding the lock, `hiddev_release` will wait (either by sleeping or
optimistic spinning).
When `hiddev_disconnect` calls `mutex_unlock`, it enters the slow path
(`__mutex_unlock_slowpath`) if there are waiters or if certain debug options are
enabled. In the slow path, it releases the mutex owner. At this point, the
waiting `hiddev_release` can acquire the lock. Since `hiddev->exist` was already
set to 0 by `hiddev_disconnect`, `hiddev_release` proceeds to decrement
`hiddev->open` to 0 and then calls `kfree(hiddev)`.
However, `hiddev_disconnect` is still inside `__mutex_unlock_slowpath`. After
releasing the owner, it attempts to access the mutex's internal `wait_lock` to
wake up any remaining waiters. Since `hiddev` has just been freed by
`hiddev_release`, this access results in a use-after-free.
### Consistency Check:
- The memory offset `0x1c` (28 bytes) into the 512-byte slab object matches the
location of the `magic` field in the `wait_lock` spinlock within the `mutex`
structure, accounting for 4 bytes of padding after the three `int` fields in
`struct hiddev`.
- All three stack traces point to the same driver (`hiddev`) and the same data
structure (`struct hiddev`).
- The scenario follows a well-known pattern of UAF in `mutex_unlock` when the
object's lifetime is tied to the lock itself without additional reference
counting.
This report provides all the necessary information to identify the bug and
implement a fix (e.g., by adding a `kref` to `struct hiddev` or ensuring the
object is not freed until `mutex_unlock` has safely returned).]
|