KASAN: slab-use-after-free Write in do_split

Status: upstream: reported C repro on 2026/09/28 21:54
Subsystems: ext4
Labels: prio:high
[Documentation on labels]
Reported-by: syzbot+09bec78ee77613a3efdd@syzkaller.appspotmail.com
First crash: 26d, last: 26d
Cause bisection: introduced by (bisect log) :
commit 3f757b56f1c4579fe32b810bce1d39f202964412
Author: Filipe Manana <fdmanana@suse.com>
Date: Fri May 16 16:07:40 2025 +0000

  btrfs: unfold transaction aborts at btrfs_create_new_inode()

Crash: KASAN: slab-out-of-bounds Write in do_split (log)
Repro: C syz .config
  
▶ ▼ ✨ AI Jobs (1)
ID Workflow Result Correct Bug Created Started Finished Revision
1702c32d assessment-security DenialOfService: ✅ Exploitable: ✅ FilesystemTrigger: ✅ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌ ❓ KASAN: slab-use-after-free Write in do_split 2026/09/29 05:13 2026/09/29 05:13 2026/09/29 05:14 7bf4ea23
▶ ▼ Discussions (3)
Title Replies (including bot) Last reply
[PATCH v2] ext4: don't append a directory block already mapped in the inode 3 (3) 2026/10/05 11:15
[syzbot] [ext4?] KASAN: slab-use-after-free Write in do_split 1 (9) 2026/10/05 01:01
[PATCH] ext4: wipe moved dirents with their real length 4 (4) 2026/10/05 00:33
▶ ▼ Similar bugs (4)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
android-5-10 BUG: unable to handle kernel paging request in do_split 19 syz, C error 25 21d 806d 0/2 upstream: reported C repro on 2024/08/09 00:52
linux-6.1 KASAN: use-after-free Read in do_split 19 1 715d 715d 0/3 auto-obsoleted due to no activity on 2025/02/15 19:28
upstream BUG: unable to handle kernel paging request in do_split ext4 8 syz, C inconclusive 17 834d 847d 27/29 fixed on 2024/08/14 03:44
android-5-15 KASAN: slab-out-of-bounds Read in do_split origin:lts 22 syz, C error 26 5h15m 204d 0/2 upstream: reported C repro on 2026/04/03 11:26
▶ ▼ Last patch testing requests (6)
Created Duration User Patch Repo Result
2026/10/05 01:01 24m adrianox@gmail.com patch https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e OK job log log
2026/10/04 23:01 41m adrianox@gmail.com patch git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e OK job log log
2026/10/04 18:49 27m adrianox@gmail.com patch upstream OK job log log
2026/10/01 21:30 26m adrianox@gmail.com patch upstream OK job log log
2026/10/01 20:34 29m adrianox@gmail.com patch upstream OK job log log
2026/10/01 19:48 22m adrianox@gmail.com patch upstream report job log log

Sample crash report:
==================================================================
BUG: KASAN: slab-use-after-free in dx_move_dirents fs/ext4/namei.c:1863 [inline]
BUG: KASAN: slab-use-after-free in do_split+0x1241/0x1e90 fs/ext4/namei.c:1993
Write of size 90458 at addr ffff88803b38ac6e by task syz.0.17/6003

CPU: 1 UID: 0 PID: 6003 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_address_description+0x55/0x1e0 mm/kasan/report.c:378
 print_report+0x58/0x70 mm/kasan/report.c:482
 kasan_report+0x117/0x150 mm/kasan/report.c:595
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
 __asan_memset+0x22/0x50 mm/kasan/shadow.c:84
 dx_move_dirents fs/ext4/namei.c:1863 [inline]
 do_split+0x1241/0x1e90 fs/ext4/namei.c:1993
 make_indexed_dir+0xdab/0x1360 fs/ext4/namei.c:2328
 __ext4_add_entry+0x1e77/0x1ff0 fs/ext4/namei.c:2432
 ext4_add_entry fs/ext4/namei.c:2469 [inline]
 ext4_rename fs/ext4/namei.c:3939 [inline]
 ext4_rename2+0x1f71/0x3270 fs/ext4/namei.c:4218
 vfs_rename+0xbd9/0xef0 fs/namei.c:6208
 filename_renameat2+0x533/0x9b0 fs/namei.c:6326
 __do_sys_rename fs/namei.c:6370 [inline]
 __se_sys_rename+0x55/0x2c0 fs/namei.c:6366
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe7da7be159
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:00007fe7d9dfd028 EFLAGS: 00000246 ORIG_RAX: 0000000000000052
RAX: ffffffffffffffda RBX: 00007fe7daa46090 RCX: 00007fe7da7be159
RDX: 0000000000000000 RSI: 0000200000000780 RDI: 0000200000000100
RBP: 00007fe7da85506b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fe7daa46128 R14: 00007fe7daa46090 R15: 00007fff45d807e8
 </TASK>

The buggy address belongs to the physical page:
page: refcount:2 mapcount:0 mapping:ffff888023717420 index:0x8 pfn:0x3b38a
memcg:ffff88801c2dc200
aops:def_blk_aops ino:700000 dentry name(?):""
flags: 0x8000000000493c(referenced|uptodate|dirty|lru|active|owner_2|private|node=0|zone=1)
raw: 008000000000493c ffffea0000efb748 ffffea0000d68388 ffff888023717420
raw: 0000000000000008 ffff8880235dbb40 00000002ffffffff ffff88801c2dc200
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x152c40(GFP_NOFS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_HARDWALL), pid 5836, tgid 5836 (udevd), ts 120465350871
 set_page_owner include/linux/page_owner.h:33 [inline]
 post_alloc_hook+0x1f9/0x250 mm/page_alloc.c:1871
 prep_new_page mm/page_alloc.c:1879 [inline]
 get_page_from_freelist+0x2591/0x2600 mm/page_alloc.c:3943
 __alloc_frozen_pages_noprof+0x230/0x5c0 mm/page_alloc.c:5436
 alloc_pages_mpol+0xd4/0x280 mm/mempolicy.c:2486
 alloc_frozen_pages_noprof mm/mempolicy.c:2557 [inline]
 alloc_pages_noprof+0xd2/0x2f0 mm/mempolicy.c:2577
 folio_alloc_noprof+0x22/0xc0 mm/mempolicy.c:2587
 filemap_alloc_folio_noprof+0x111/0x4c0 mm/filemap.c:1019
 ractl_alloc_folio mm/readahead.c:200 [inline]
 page_cache_ra_unbounded+0x2d6/0x8f0 mm/readahead.c:288
 do_page_cache_ra mm/readahead.c:348 [inline]
 force_page_cache_ra+0x26f/0x2e0 mm/readahead.c:378
 filemap_get_pages+0x493/0x1e80 mm/filemap.c:2714
 filemap_read+0x429/0x11e0 mm/filemap.c:2824
 blkdev_read_iter+0x2f8/0x430 block/fops.c:832
 new_sync_read fs/read_write.c:493 [inline]
 vfs_read+0x59e/0xa90 fs/read_write.c:574
 ksys_read+0x156/0x270 fs/read_write.c:716
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
page last free pid 28 tgid 28 ts 120462276719 stack trace:
 reset_page_owner include/linux/page_owner.h:26 [inline]
 __free_pages_prepare mm/page_alloc.c:1418 [inline]
 __free_frozen_pages+0xf8e/0x1070 mm/page_alloc.c:2962
 __tlb_remove_table_free mm/mmu_gather.c:228 [inline]
 tlb_remove_table_rcu+0x85/0x100 mm/mmu_gather.c:291
 rcu_do_batch kernel/rcu/tree.c:2650 [inline]
 rcu_core kernel/rcu/tree.c:2919 [inline]
 rcu_cpu_kthread+0xb2c/0x1740 kernel/rcu/tree.c:3007
 smpboot_thread_fn+0x565/0xa70 kernel/smpboot.c:160
 kthread+0x38b/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 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:
 ffff88803b38af00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff88803b38af80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff88803b38b000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff88803b38b080: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fa fb
 ffff88803b38b100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Crashes (6):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/09/28 13:32 upstream 72d3fcf802c4 a3c7249d .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (clean fs)] ci2-upstream-fs KASAN: slab-use-after-free Write in do_split
2026/09/28 11:06 upstream 72d3fcf802c4 a3c7249d .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (clean fs)] ci2-upstream-fs KASAN: slab-use-after-free Write in do_split
2026/09/28 08:40 upstream 72d3fcf802c4 a3c7249d .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (clean fs)] ci2-upstream-fs KASAN: slab-use-after-free Write in do_split
2026/09/28 12:20 upstream 72d3fcf802c4 a3c7249d .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (clean fs)] ci2-upstream-fs KASAN: slab-out-of-bounds Write in do_split
2026/09/28 09:52 upstream 72d3fcf802c4 a3c7249d .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (clean fs)] ci2-upstream-fs KASAN: slab-out-of-bounds Write in do_split
2026/09/28 07:07 upstream 72d3fcf802c4 a3c7249d .config console log report info [disk image] [vmlinux] [kernel image] ci2-upstream-fs KASAN: slab-use-after-free Write in do_split
* Struck through repros no longer work on HEAD.