[syzbot] [exfat?] KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

[syzbot] [exfat?] KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    de0674d9bc69 Merge tag 'for-6.19-rc8-tag' of git://git.ker..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15f240aa580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8e27f4588a0f2183
dashboard link: https://syzkaller.appspot.com/bug?extid=51b4c65bb770155d058f
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/bfedab2f6279/disk-de0674d9.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f012a4cb8d82/vmlinux-de0674d9.xz
kernel image: https://storage.googleapis.com/syzbot-assets/acb727c49110/bzImage-de0674d9.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+51b4c65bb770155d058f@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

read-write to 0xffff888129151032 of 1 bytes by task 4937 on cpu 1:
 fat12_ent_put+0xc4/0x170 fs/fat/fatent.c:165
 fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:417
 fat_chain_add+0x16c/0x490 fs/fat/misc.c:136
 fat_add_cluster fs/fat/inode.c:113 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x46c/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x183/0x490 mm/filemap.c:4314
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x5a6/0x9f0 fs/read_write.c:686
 ksys_write+0xdc/0x1a0 fs/read_write.c:738
 __do_sys_write fs/read_write.c:749 [inline]
 __se_sys_write fs/read_write.c:746 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:746
 x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888129151000 of 512 bytes by task 4940 on cpu 0:
 fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
 fat_alloc_clusters+0xb48/0xc50 fs/fat/fatent.c:543
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x183/0x490 mm/filemap.c:4314
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
 __kernel_write_iter+0x319/0x590 fs/read_write.c:619
 dump_emit_page fs/coredump.c:1298 [inline]
 dump_user_range+0xa7d/0xdb0 fs/coredump.c:1372
 elf_core_dump+0x21a2/0x2330 fs/binfmt_elf.c:2111
 coredump_write+0xacf/0xdf0 fs/coredump.c:1049
 do_coredump fs/coredump.c:1126 [inline]
 vfs_coredump+0x26bc/0x3120 fs/coredump.c:1200
 get_signal+0xd7b/0xf60 kernel/signal.c:3019
 arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
 __exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:75 [inline]
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
 irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
 irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
 irqentry_exit+0xf7/0x510 kernel/entry/common.c:196
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 4940 Comm: syz.3.398 Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================
syz.3.398 (4940) used greatest stack depth: 8696 bytes left


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
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If the report is already addressed, let syzbot know by replying with:
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If you want to overwrite report's subsystems, reply with:
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(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

Re: [syzbot] [exfat?] KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

[Dmitry Vyukov]

On Fri, 6 Feb 2026 at 07:43, syzbot
<syzbot+51b4c65bb770155d058f@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    de0674d9bc69 Merge tag 'for-6.19-rc8-tag' of git://git.ker..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15f240aa580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8e27f4588a0f2183
> dashboard link: https://syzkaller.appspot.com/bug?extid=51b4c65bb770155d058f
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/bfedab2f6279/disk-de0674d9.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/f012a4cb8d82/vmlinux-de0674d9.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/acb727c49110/bzImage-de0674d9.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+51b4c65bb770155d058f@syzkaller.appspotmail.com
>
> ==================================================================
> BUG: KCSAN: data-race in fat12_ent_put / fat_mirror_bhs
>
> read-write to 0xffff888129151032 of 1 bytes by task 4937 on cpu 1:
>  fat12_ent_put+0xc4/0x170 fs/fat/fatent.c:165
>  fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:417
>  fat_chain_add+0x16c/0x490 fs/fat/misc.c:136
>  fat_add_cluster fs/fat/inode.c:113 [inline]
>  __fat_get_block fs/fat/inode.c:155 [inline]
>  fat_get_block+0x46c/0x5e0 fs/fat/inode.c:190
>  __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
>  block_write_begin fs/buffer.c:2256 [inline]
>  cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
>  fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
>  generic_perform_write+0x183/0x490 mm/filemap.c:4314
>  __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
>  generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
>  new_sync_write fs/read_write.c:593 [inline]
>  vfs_write+0x5a6/0x9f0 fs/read_write.c:686
>  ksys_write+0xdc/0x1a0 fs/read_write.c:738
>  __do_sys_write fs/read_write.c:749 [inline]
>  __se_sys_write fs/read_write.c:746 [inline]
>  __x64_sys_write+0x40/0x50 fs/read_write.c:746
>  x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> read to 0xffff888129151000 of 512 bytes by task 4940 on cpu 0:
>  fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
>  fat_alloc_clusters+0xb48/0xc50 fs/fat/fatent.c:543
>  fat_add_cluster fs/fat/inode.c:108 [inline]
>  __fat_get_block fs/fat/inode.c:155 [inline]
>  fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
>  __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
>  block_write_begin fs/buffer.c:2256 [inline]
>  cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
>  fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
>  generic_perform_write+0x183/0x490 mm/filemap.c:4314
>  __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
>  generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
>  __kernel_write_iter+0x319/0x590 fs/read_write.c:619
>  dump_emit_page fs/coredump.c:1298 [inline]
>  dump_user_range+0xa7d/0xdb0 fs/coredump.c:1372
>  elf_core_dump+0x21a2/0x2330 fs/binfmt_elf.c:2111
>  coredump_write+0xacf/0xdf0 fs/coredump.c:1049
>  do_coredump fs/coredump.c:1126 [inline]
>  vfs_coredump+0x26bc/0x3120 fs/coredump.c:1200
>  get_signal+0xd7b/0xf60 kernel/signal.c:3019
>  arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
>  __exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
>  exit_to_user_mode_loop kernel/entry/common.c:75 [inline]
>  __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
>  irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
>  irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
>  irqentry_exit+0xf7/0x510 kernel/entry/common.c:196
>  asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 UID: 0 PID: 4940 Comm: syz.3.398 Not tainted syzkaller #0 PREEMPT(voluntary)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
> ==================================================================


FWIW this is how LLM explained why this race is harmful. It does not
look totally bogus to me.

=======

The data race occurs between `fat12_ent_put()` and `fat_mirror_bhs()`
in the FAT filesystem implementation.

### Analysis:
1.  **Task 1 (`fat12_ent_put`)**: This function is responsible for
updating a 12-bit FAT entry. Because FAT12 entries are 1.5 bytes long
and can span byte boundaries, updating an entry involves a
read-modify-write on two bytes. This is protected by the
`fat12_entry_lock` spinlock to ensure atomicity against other FAT12
entry updates. In the reported stack trace, it is called from
`fat_alloc_clusters()`, which also holds the `lock_fat` mutex.
2.  **Task 2 (`fat_mirror_bhs`)**: This function copies the "main" FAT
blocks (held in `buffer_head`s) to the backup FAT copies (mirrors) on
the disk. It uses `memcpy()` to copy the entire block data. In the
reported stack trace, it is called from `fat_alloc_clusters()`.
3.  **The Race**: In `fat_alloc_clusters()`, the `lock_fat` mutex is
released *before* calling `fat_mirror_bhs()`. This allows another task
(like Task 1) to acquire the `lock_fat` mutex and begin modifying the
same FAT block that Task 2 is currently mirroring. Since
`fat_mirror_bhs()` does not take the `fat12_entry_lock` spinlock, and
`memcpy()` is not atomic, Task 2 can read a FAT entry in a partially
updated (torn) state.
4.  **Impact**: If a torn FAT entry is copied to the mirror FAT and
subsequently written to disk, the filesystem metadata becomes
corrupted. In FAT12, a torn read can result in a cluster pointer that
is neither the old nor the new value, potentially pointing to a random
cluster. This leads to cross-linked files, lost data, or filesystem
inconsistency. While the main FAT might eventually be updated
correctly, there is a window where the mirror is corrupted on disk,
and if a crash occurs, the filesystem is left in an inconsistent
state. A similar race condition exists for `fat_sync_bhs()`, which
flushes the main FAT block to disk.

### Conclusion:
This data race is **harmful**. It involves concurrent access to shared
filesystem metadata without sufficient synchronization, leading to the
possibility of writing inconsistent/corrupted metadata to the disk.
Filesystem metadata integrity is critical, and torn writes are a
well-known source of serious filesystem corruption. The inconsistency
in locking between `fat_free_clusters()` (which holds the lock during
mirroring) and `fat_alloc_clusters()` (which does not) further
indicates that this is an unsafe oversight.

Re: [syzbot] [exfat?] KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

[OGAWA Hirofumi]

syzbot <syzbot+51b4c65bb770155d058f@syzkaller.appspotmail.com> writes:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    de0674d9bc69 Merge tag 'for-6.19-rc8-tag' of git://git.ker..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15f240aa580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8e27f4588a0f2183
> dashboard link: https://syzkaller.appspot.com/bug?extid=51b4c65bb770155d058f
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/bfedab2f6279/disk-de0674d9.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/f012a4cb8d82/vmlinux-de0674d9.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/acb727c49110/bzImage-de0674d9.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+51b4c65bb770155d058f@syzkaller.appspotmail.com

This looks like same with the following thread.

  https://lore.kernel.org/all/20250902081727.7146-1-anmuxixixi@gmail.com/

And this temporary inconsistency will be fixed until unmount, so it
should be no corruption by this race. Let me know if corruption remained
after unmount.

Thanks.

> ==================================================================
> BUG: KCSAN: data-race in fat12_ent_put / fat_mirror_bhs
>
> read-write to 0xffff888129151032 of 1 bytes by task 4937 on cpu 1:
>  fat12_ent_put+0xc4/0x170 fs/fat/fatent.c:165
>  fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:417
>  fat_chain_add+0x16c/0x490 fs/fat/misc.c:136
>  fat_add_cluster fs/fat/inode.c:113 [inline]
>  __fat_get_block fs/fat/inode.c:155 [inline]
>  fat_get_block+0x46c/0x5e0 fs/fat/inode.c:190
>  __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
>  block_write_begin fs/buffer.c:2256 [inline]
>  cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
>  fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
>  generic_perform_write+0x183/0x490 mm/filemap.c:4314
>  __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
>  generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
>  new_sync_write fs/read_write.c:593 [inline]
>  vfs_write+0x5a6/0x9f0 fs/read_write.c:686
>  ksys_write+0xdc/0x1a0 fs/read_write.c:738
>  __do_sys_write fs/read_write.c:749 [inline]
>  __se_sys_write fs/read_write.c:746 [inline]
>  __x64_sys_write+0x40/0x50 fs/read_write.c:746
>  x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> read to 0xffff888129151000 of 512 bytes by task 4940 on cpu 0:
>  fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
>  fat_alloc_clusters+0xb48/0xc50 fs/fat/fatent.c:543
>  fat_add_cluster fs/fat/inode.c:108 [inline]
>  __fat_get_block fs/fat/inode.c:155 [inline]
>  fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
>  __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
>  block_write_begin fs/buffer.c:2256 [inline]
>  cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
>  fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
>  generic_perform_write+0x183/0x490 mm/filemap.c:4314
>  __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
>  generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
>  __kernel_write_iter+0x319/0x590 fs/read_write.c:619
>  dump_emit_page fs/coredump.c:1298 [inline]
>  dump_user_range+0xa7d/0xdb0 fs/coredump.c:1372
>  elf_core_dump+0x21a2/0x2330 fs/binfmt_elf.c:2111
>  coredump_write+0xacf/0xdf0 fs/coredump.c:1049
>  do_coredump fs/coredump.c:1126 [inline]
>  vfs_coredump+0x26bc/0x3120 fs/coredump.c:1200
>  get_signal+0xd7b/0xf60 kernel/signal.c:3019
>  arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
>  __exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
>  exit_to_user_mode_loop kernel/entry/common.c:75 [inline]
>  __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
>  irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
>  irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
>  irqentry_exit+0xf7/0x510 kernel/entry/common.c:196
>  asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 UID: 0 PID: 4940 Comm: syz.3.398 Not tainted syzkaller #0 PREEMPT(voluntary) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
> ==================================================================
> syz.3.398 (4940) used greatest stack depth: 8696 bytes left
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
>
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
>
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
>
> If you want to undo deduplication, reply with:
> #syz undup

-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

Re: [syzbot] [exfat?] KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

[Dmitry Vyukov]

On Fri, 6 Feb 2026 at 08:40, OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote:
>
> syzbot <syzbot+51b4c65bb770155d058f@syzkaller.appspotmail.com> writes:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    de0674d9bc69 Merge tag 'for-6.19-rc8-tag' of git://git.ker..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15f240aa580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=8e27f4588a0f2183
> > dashboard link: https://syzkaller.appspot.com/bug?extid=51b4c65bb770155d058f
> > compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/bfedab2f6279/disk-de0674d9.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/f012a4cb8d82/vmlinux-de0674d9.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/acb727c49110/bzImage-de0674d9.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+51b4c65bb770155d058f@syzkaller.appspotmail.com
>
> This looks like same with the following thread.
>
>   https://lore.kernel.org/all/20250902081727.7146-1-anmuxixixi@gmail.com/
>
> And this temporary inconsistency will be fixed until unmount, so it
> should be no corruption by this race. Let me know if corruption remained
> after unmount.

Thanks for the link. I don't have means to test that, but the
explanation in the thread you referenced looks reasonable. So let's
just keep an AI to properly annotate the race according to LKMM rules.


> > ==================================================================
> > BUG: KCSAN: data-race in fat12_ent_put / fat_mirror_bhs
> >
> > read-write to 0xffff888129151032 of 1 bytes by task 4937 on cpu 1:
> >  fat12_ent_put+0xc4/0x170 fs/fat/fatent.c:165
> >  fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:417
> >  fat_chain_add+0x16c/0x490 fs/fat/misc.c:136
> >  fat_add_cluster fs/fat/inode.c:113 [inline]
> >  __fat_get_block fs/fat/inode.c:155 [inline]
> >  fat_get_block+0x46c/0x5e0 fs/fat/inode.c:190
> >  __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
> >  block_write_begin fs/buffer.c:2256 [inline]
> >  cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
> >  fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
> >  generic_perform_write+0x183/0x490 mm/filemap.c:4314
> >  __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
> >  generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
> >  new_sync_write fs/read_write.c:593 [inline]
> >  vfs_write+0x5a6/0x9f0 fs/read_write.c:686
> >  ksys_write+0xdc/0x1a0 fs/read_write.c:738
> >  __do_sys_write fs/read_write.c:749 [inline]
> >  __se_sys_write fs/read_write.c:746 [inline]
> >  __x64_sys_write+0x40/0x50 fs/read_write.c:746
> >  x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > read to 0xffff888129151000 of 512 bytes by task 4940 on cpu 0:
> >  fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
> >  fat_alloc_clusters+0xb48/0xc50 fs/fat/fatent.c:543
> >  fat_add_cluster fs/fat/inode.c:108 [inline]
> >  __fat_get_block fs/fat/inode.c:155 [inline]
> >  fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
> >  __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
> >  block_write_begin fs/buffer.c:2256 [inline]
> >  cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
> >  fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
> >  generic_perform_write+0x183/0x490 mm/filemap.c:4314
> >  __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
> >  generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
> >  __kernel_write_iter+0x319/0x590 fs/read_write.c:619
> >  dump_emit_page fs/coredump.c:1298 [inline]
> >  dump_user_range+0xa7d/0xdb0 fs/coredump.c:1372
> >  elf_core_dump+0x21a2/0x2330 fs/binfmt_elf.c:2111
> >  coredump_write+0xacf/0xdf0 fs/coredump.c:1049
> >  do_coredump fs/coredump.c:1126 [inline]
> >  vfs_coredump+0x26bc/0x3120 fs/coredump.c:1200
> >  get_signal+0xd7b/0xf60 kernel/signal.c:3019
> >  arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
> >  __exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
> >  exit_to_user_mode_loop kernel/entry/common.c:75 [inline]
> >  __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
> >  irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
> >  irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
> >  irqentry_exit+0xf7/0x510 kernel/entry/common.c:196
> >  asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 0 UID: 0 PID: 4940 Comm: syz.3.398 Not tainted syzkaller #0 PREEMPT(voluntary)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
> > ==================================================================
> > syz.3.398 (4940) used greatest stack depth: 8696 bytes left
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@googlegroups.com.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> >
> > If the report is already addressed, let syzbot know by replying with:
> > #syz fix: exact-commit-title
> >
> > If you want to overwrite report's subsystems, reply with:
> > #syz set subsystems: new-subsystem
> > (See the list of subsystem names on the web dashboard)
> >
> > If the report is a duplicate of another one, reply with:
> > #syz dup: exact-subject-of-another-report
> >
> > If you want to undo deduplication, reply with:
> > #syz undup