[v6.1] kernel BUG in fat_mirror_bhs

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 4931e0e1673d Linux 6.1.170
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=14b50a36580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b1adc0bfde2d8a4a
dashboard link: https://syzkaller.appspot.com/bug?extid=4b0f8cca630d37e12565
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
userspace arch: arm64
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12862ece580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16862ece580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1d2e800805ca/disk-4931e0e1.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/86557170127a/vmlinux-4931e0e1.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f9b3a6753f2d/Image-4931e0e1.gz.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/afe80d678a7e/mount_0.gz
fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=14862ece580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+4b0f8c...@syzkaller.appspotmail.com

------------[ cut here ]------------
kernel BUG at fs/buffer.c:579!
Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 4513 Comm: syz.0.17 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
pstate: 82400005 (Nzcv daif +PAN -UAO +TCO -DIT -SSBS BTYPE=--)
pc : mark_buffer_dirty_inode+0x288/0x28c fs/buffer.c:579
lr : mark_buffer_dirty_inode+0x288/0x28c fs/buffer.c:579
sp : ffff800021627100
x29: ffff800021627110 x28: ffff8000216273c0 x27: 0000000000000000
x26: ffff0000f3d58368 x25: dfff800000000000 x24: ffff0000f1bb2018
x23: 0000000000000000 x22: 0000000000000200 x21: 1fffe0001e7ab0b1
x20: ffff0000f3d58588 x19: ffff0000e1f3e000 x18: ffff800011b9bf60
x17: 1fffe00033ea637e x16: ffff8000082d92bc x15: 0000000000000000
x14: 0000000000000001 x13: 1fffff8000796ce0 x12: 0000000000ff0100
x11: ff00800008a8f4e4 x10: 0000000000000000 x9 : ffff800008a8f4e4
x8 : ffff0000d7089bc0 x7 : ffff800008937890 x6 : 0000000000000000
x5 : 0000000000000080 x4 : 0000000000000001 x3 : ffff800008a8f738
x2 : 0000000000000001 x1 : ffff800011b9ebc0 x0 : 0000000000000000
Call trace:
mark_buffer_dirty_inode+0x288/0x28c fs/buffer.c:579
fat_mirror_bhs+0x30c/0x484 fs/fat/fatent.c:398
fat_alloc_clusters+0x8fc/0xb64 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+0x33c/0x930 fs/fat/inode.c:190
__block_write_begin_int+0x350/0x1388 fs/buffer.c:1991
__block_write_begin fs/buffer.c:2041 [inline]
block_write_begin fs/buffer.c:2102 [inline]
cont_write_begin+0x53c/0x780 fs/buffer.c:2456
fat_write_begin+0x98/0x150 fs/fat/inode.c:235
generic_perform_write+0x234/0x4f4 mm/filemap.c:3862
__generic_file_write_iter+0x130/0x250 mm/filemap.c:3960
generic_file_write_iter+0xb4/0x2b0 mm/filemap.c:3989
call_write_iter include/linux/fs.h:2265 [inline]
new_sync_write fs/read_write.c:491 [inline]
vfs_write+0x3ec/0x7f0 fs/read_write.c:584
ksys_pwrite64 fs/read_write.c:699 [inline]
__do_sys_pwrite64 fs/read_write.c:709 [inline]
__se_sys_pwrite64 fs/read_write.c:706 [inline]
__arm64_sys_pwrite64+0x174/0x20c fs/read_write.c:706
__invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
invoke_syscall+0x98/0x2b4 arch/arm64/kernel/syscall.c:52
el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
do_el0_svc+0x58/0x130 arch/arm64/kernel/syscall.c:204
el0_svc+0x58/0x128 arch/arm64/kernel/entry-common.c:637
el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585
Code: 9101c3ff d50323bf d65f03c0 97e8704e (d4210000)
---[ end trace 0000000000000000 ]---


---
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 syzk...@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 syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

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