KASAN: use-after-free Read in __ext4_check_dir_entry

Status: upstream: reported C repro on 2026/02/22 03:13
Bug presence: origin:upstream
[Documentation on labels]
Reported-by: syzbot+99d5e4a655ffe696bea7@syzkaller.appspotmail.com
First crash: 169d, last: 169d
Fix bisection: failed (error log, bisect log)
  
▶ ▼ Bug presence (1)
Date Name Commit Repro Result
2026/02/22 upstream (ToT) fa5c82f4d2bb C [report] KASAN: use-after-free Read in __ext4_check_dir_entry
▶ ▼ Similar bugs (13)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
android-5-10 KASAN: use-after-free Read in __ext4_check_dir_entry (2) 19 syz, C 12 5d15h 568d 0/2 upstream: reported C repro on 2025/01/19 16:52
upstream KASAN: use-after-free Read in __ext4_check_dir_entry (3) ext4 19 syz, C inconclusive done 6 520d 673d 0/29 auto-obsoleted due to no activity on 2025/06/16 07:57
linux-4.14 KASAN: use-after-free Read in __ext4_check_dir_entry 19 syz, C inconclusive 6 2147d 2250d 0/1 upstream: reported C repro on 2020/06/12 02:23
upstream KASAN: use-after-free Read in __ext4_check_dir_entry (2) ext4 19 syz, C error 1 808d 818d 0/29 closed as dup on 2024/06/26 08:39
android-5-15 KASAN: use-after-free Read in __ext4_check_dir_entry (2) origin:lts 19 syz, C error 11 4d02h 488d 0/2 upstream: reported C repro on 2025/04/09 15:24
linux-5.15 KASAN: use-after-free Read in __ext4_check_dir_entry origin:upstream 19 syz, C error 10 23h03m 798d 0/3 upstream: reported C repro on 2024/06/03 01:15
android-5-15 KASAN: use-after-free Read in __ext4_check_dir_entry missing-backport origin:upstream 19 syz, C error 4 649d 773d 0/2 auto-obsoleted due to no activity on 2025/02/07 07:57
upstream KASAN: slab-use-after-free Read in __ext4_check_dir_entry prio:high ext4 19 syz, C error error 7 3d19h 406d 1/29 upstream: reported C repro on 2025/06/30 17:28
upstream KASAN: use-after-free Read in __ext4_check_dir_entry ext4 19 syz, C 8 3051d 3053d 22/29 closed as dup on 2018/03/31 22:38
android-5-10 KASAN: use-after-free Read in __ext4_check_dir_entry 19 syz, C error 2 704d 822d 0/2 auto-obsoleted due to no activity on 2024/12/26 18:56
android-6-1 KASAN: use-after-free Read in __ext4_check_dir_entry missing-backport origin:upstream 19 syz, C error 4 697d 821d 0/2 auto-obsoleted due to no activity on 2024/12/21 14:06
linux-6.1 KASAN: use-after-free Read in __ext4_check_dir_entry 19 15 761d 798d 0/3 auto-obsoleted due to no activity on 2024/09/17 20:54
android-6-1 KASAN: use-after-free Read in __ext4_check_dir_entry (2) origin:upstream 19 syz, C error 9 66d 492d 0/2 upstream: reported C repro on 2025/04/05 13:22

Sample crash report:
EXT4-fs error (device loop0): ext4_orphan_get:1403: comm syz.0.19: couldn't read orphan inode 15 (err -117)
EXT4-fs (loop0): mounted filesystem 00000000-0000-0000-0000-000000000000 r/w without journal. Quota mode: none.
EXT4-fs warning (device loop0): dx_probe:833: inode #2: comm syz.0.19: Unrecognised inode hash code 4
EXT4-fs warning (device loop0): dx_probe:966: inode #2: comm syz.0.19: Corrupt directory, running e2fsck is recommended
==================================================================
BUG: KASAN: use-after-free in __ext4_check_dir_entry+0x73c/0x8e0 fs/ext4/dir.c:85
Read of size 2 at addr ffff88805cc07003 by task syz.0.19/5921

CPU: 0 PID: 5921 Comm: syz.0.19 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0x18c/0x250 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:364 [inline]
 print_report+0xa8/0x210 mm/kasan/report.c:468
 kasan_report+0x117/0x150 mm/kasan/report.c:581
 __ext4_check_dir_entry+0x73c/0x8e0 fs/ext4/dir.c:85
 ext4_readdir+0x11f7/0x3a80 fs/ext4/dir.c:261
 iterate_dir+0x1c2/0x580 fs/readdir.c:106
 __do_sys_getdents64 fs/readdir.c:405 [inline]
 __se_sys_getdents64+0xf6/0x270 fs/readdir.c:390
 do_syscall_x64 arch/x86/entry/common.c:46 [inline]
 do_syscall_64+0x55/0xa0 arch/x86/entry/common.c:76
 entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7f430df9c629
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:00007ffe7186ec68 EFLAGS: 00000246 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 00007f430e215fa0 RCX: 00007f430df9c629
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000006
RBP: 00007f430e032b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f430e215fac R14: 00007f430e215fa0 R15: 00007f430e215fa0
 </TASK>

The buggy address belongs to the physical page:
page:ffffea00017301c0 refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x5cc07
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
page_type: 0xffffffff()
raw: 00fff00000000000 ffffea0001b6c588 ffffea0001730008 0000000000000000
raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as freed
page last allocated via order 0, migratetype Movable, gfp_mask 0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid 5916, tgid 5916 (syz.0.17), ts 87922739760, free_ts 88114513540
 set_page_owner include/linux/page_owner.h:31 [inline]
 post_alloc_hook+0x1c1/0x200 mm/page_alloc.c:1581
 prep_new_page mm/page_alloc.c:1588 [inline]
 get_page_from_freelist+0x1951/0x19e0 mm/page_alloc.c:3220
 __alloc_pages+0x1f0/0x460 mm/page_alloc.c:4486
 __folio_alloc+0x10/0x20 mm/page_alloc.c:4518
 vma_alloc_folio+0x47a/0x8f0 mm/mempolicy.c:2242
 shmem_alloc_folio+0x1a9/0x2a0 mm/shmem.c:1679
 shmem_alloc_and_acct_folio+0x1e6/0x6d0 mm/shmem.c:1704
 shmem_get_folio_gfp+0xcde/0x2aa0 mm/shmem.c:2041
 shmem_get_folio mm/shmem.c:2164 [inline]
 shmem_write_begin+0xf2/0x420 mm/shmem.c:2706
 generic_perform_write+0x2fe/0x5c0 mm/filemap.c:4031
 shmem_file_write_iter+0xfb/0x120 mm/shmem.c:2883
 call_write_iter include/linux/fs.h:2018 [inline]
 new_sync_write fs/read_write.c:491 [inline]
 vfs_write+0x46c/0x990 fs/read_write.c:584
 ksys_write+0x150/0x260 fs/read_write.c:637
 do_syscall_x64 arch/x86/entry/common.c:46 [inline]
 do_syscall_64+0x55/0xa0 arch/x86/entry/common.c:76
 entry_SYSCALL_64_after_hwframe+0x68/0xd2
page last free stack trace:
 reset_page_owner include/linux/page_owner.h:24 [inline]
 free_pages_prepare mm/page_alloc.c:1181 [inline]
 free_unref_page_prepare+0x7b2/0x8c0 mm/page_alloc.c:2365
 free_unref_page_list+0xbe/0x860 mm/page_alloc.c:2504
 release_pages+0x1f7a/0x2200 mm/swap.c:1022
 __folio_batch_release+0x71/0xe0 mm/swap.c:1042
 folio_batch_release include/linux/pagevec.h:83 [inline]
 shmem_undo_range+0x630/0x1b20 mm/shmem.c:1026
 shmem_truncate_range mm/shmem.c:1135 [inline]
 shmem_evict_inode+0x245/0x9e0 mm/shmem.c:1264
 evict+0x4ca/0x8d0 fs/inode.c:705
 __dentry_kill+0x431/0x650 fs/dcache.c:611
 dentry_kill+0xb8/0x290 fs/dcache.c:-1
 dput+0xfe/0x1e0 fs/dcache.c:918
 __fput+0x5e5/0x970 fs/file_table.c:392
 task_work_run+0x1d4/0x260 kernel/task_work.c:245
 resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
 exit_to_user_mode_loop+0xe6/0x110 kernel/entry/common.c:177
 exit_to_user_mode_prepare+0xee/0x180 kernel/entry/common.c:210
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x1a/0x50 kernel/entry/common.c:302
 do_syscall_64+0x61/0xa0 arch/x86/entry/common.c:82

Memory state around the buggy address:
 ffff88805cc06f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff88805cc06f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff88805cc07000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff88805cc07080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88805cc07100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/02/22 03:12 linux-6.6.y 7a137e9bfa0e 6e7b5511 .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-linux-6-6-kasan KASAN: use-after-free Read in __ext4_check_dir_entry
* Struck through repros no longer work on HEAD.