[v6.6] KASAN: use-after-free Read in __ext4_check_dir_entry

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 7a137e9bfa0e Linux 6.6.127
git tree: linux-6.6.y
console output: https://syzkaller.appspot.com/x/log.txt?x=1754fc02580000
kernel config: https://syzkaller.appspot.com/x/.config?x=2a950bf7c0bff9f9
dashboard link: https://syzkaller.appspot.com/bug?extid=99d5e4a655ffe696bea7
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=175ad722580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10dad722580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/04a02efb4a79/disk-7a137e9b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f7b848f192f0/vmlinux-7a137e9b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/77a981f90b3d/bzImage-7a137e9b.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/0b33353ce258/mount_0.gz
fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=10b4d95a580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+99d5e4...@syzkaller.appspotmail.com

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
==================================================================


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