[moderation] [exfat?] KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e92b2872d0b1 Merge tag 'rust-fixes-7.1' of git://git.kerne..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=135cead2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=2e40c0f41e01837e
dashboard link: https://syzkaller.appspot.com/bug?extid=6244b6da154efe0aedb6
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [hiro...@mail.parknet.co.jp linki...@kernel.org linux-...@vger.kernel.org linux-...@vger.kernel.org sj155...@samsung.com]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/dccc5f5f0aa0/disk-e92b2872.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/806f97c3379b/vmlinux-e92b2872.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b449b06ca9a5/bzImage-e92b2872.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+6244b6...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync

write to 0xffff88811b0b8a80 of 8 bytes by task 5174 on cpu 0:
__remove_assoc_queue fs/buffer.c:495 [inline]
mmb_sync+0x14d/0x600 fs/buffer.c:566
fat_file_fsync+0xba/0x110 fs/fat/file.c:197
vfs_fsync_range+0xc5/0xe0 fs/sync.c:186
generic_write_sync include/linux/fs.h:2654 [inline]
generic_file_write_iter+0x1ba/0x310 mm/filemap.c:4472
iter_file_splice_write+0x6c4/0xa80 fs/splice.c:736
do_splice_from fs/splice.c:936 [inline]
direct_splice_actor+0x156/0x2a0 fs/splice.c:1159
splice_direct_to_actor+0x311/0x670 fs/splice.c:1103
do_splice_direct_actor fs/splice.c:1202 [inline]
do_splice_direct+0x119/0x1a0 fs/splice.c:1228
do_sendfile+0x382/0x650 fs/read_write.c:1372
__do_sys_sendfile64 fs/read_write.c:1433 [inline]
__se_sys_sendfile64 fs/read_write.c:1419 [inline]
__x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1419
x64_sys_call+0x2dc4/0x3020 arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811b0b8a80 of 8 bytes by task 5175 on cpu 1:
mmb_mark_buffer_dirty+0x2e/0x140 fs/buffer.c:720
fat16_ent_put+0x52/0x60 fs/fat/fatent.c:186
fat_alloc_clusters+0x57b/0xc50 fs/fat/fatent.c:508
fat_add_cluster fs/fat/inode.c:108 [inline]
__fat_get_block fs/fat/inode.c:155 [inline]
fat_get_block+0x25e/0x5f0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2042
block_write_begin fs/buffer.c:2153 [inline]
cont_write_begin+0x5fe/0x970 fs/buffer.c:2491
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
cont_expand_zero fs/buffer.c:2419 [inline]
cont_write_begin+0x1b0/0x970 fs/buffer.c:2481
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2382
fat_cont_expand+0x3e/0x170 fs/fat/file.c:226
fat_setattr+0x2a4/0x870 fs/fat/file.c:516
notify_change+0x872/0x8f0 fs/attr.c:556
do_truncate+0x14f/0x190 fs/open.c:68
vfs_truncate+0x1e8/0x220 fs/open.c:118
ksys_truncate+0xa2/0x140 fs/open.c:142
__do_sys_truncate fs/open.c:154 [inline]
__se_sys_truncate fs/open.c:152 [inline]
__x64_sys_truncate+0x31/0x40 fs/open.c:152
x64_sys_call+0x19fb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:77
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff88811b0e31f8 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 5175 Comm: syz.4.473 Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
==================================================================


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