[moderation] [exfat?] KCSAN: data-race in __filemap_add_folio / file_write_and_wait_range (8)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 623fb9912f6a Merge tag 'pinctrl-v6.19-2' of git://git.kern..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12f4b19a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b319ff1b6a2797ca
dashboard link: https://syzkaller.appspot.com/bug?extid=29aa8c66783fd2588e08
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
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/e5f3a7186ee7/disk-623fb991.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5008371e363b/vmlinux-623fb991.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5710a258830a/bzImage-623fb991.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+29aa8c...@syzkaller.appspotmail.com

netlink: 4 bytes leftover after parsing attributes in process `syz.0.5751'.
FAT-fs (loop0): error, invalid access to FAT (entry 0x0fff0000)
FAT-fs (loop0): Filesystem has been set read-only
==================================================================
BUG: KCSAN: data-race in __filemap_add_folio / file_write_and_wait_range

read-write to 0xffff888107321d60 of 8 bytes by task 29785 on cpu 1:
__filemap_add_folio+0x5b9/0x7d0 mm/filemap.c:919
filemap_add_folio+0x1d9/0x360 mm/filemap.c:966
__filemap_get_folio_mpol+0x326/0x650 mm/filemap.c:2016
__filemap_get_folio include/linux/pagemap.h:763 [inline]
block_write_begin fs/buffer.c:2251 [inline]
cont_write_begin+0x5cb/0x970 fs/buffer.c:2594
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
generic_perform_write+0x184/0x490 mm/filemap.c:4314
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4457
iter_file_splice_write+0x66b/0xa20 fs/splice.c:738
do_splice_from fs/splice.c:938 [inline]
direct_splice_actor+0x156/0x2a0 fs/splice.c:1161
splice_direct_to_actor+0x312/0x680 fs/splice.c:1105
do_splice_direct_actor fs/splice.c:1204 [inline]
do_splice_direct+0xda/0x150 fs/splice.c:1230
do_sendfile+0x380/0x650 fs/read_write.c:1370
__do_sys_sendfile64 fs/read_write.c:1431 [inline]
__se_sys_sendfile64 fs/read_write.c:1417 [inline]
__x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1417
x64_sys_call+0x2db1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888107321d60 of 8 bytes by task 29779 on cpu 0:
mapping_needs_writeback mm/filemap.c:634 [inline]
file_write_and_wait_range+0x65/0x2c0 mm/filemap.c:785
__generic_file_fsync+0x46/0x160 fs/libfs.c:1558
fat_file_fsync+0x49/0x100 fs/fat/file.c:191
vfs_fsync_range+0x10d/0x130 fs/sync.c:188
generic_write_sync include/linux/fs.h:2616 [inline]
generic_file_write_iter+0x1b8/0x2f0 mm/filemap.c:4461
iter_file_splice_write+0x66b/0xa20 fs/splice.c:738
do_splice_from fs/splice.c:938 [inline]
direct_splice_actor+0x156/0x2a0 fs/splice.c:1161
splice_direct_to_actor+0x312/0x680 fs/splice.c:1105
do_splice_direct_actor fs/splice.c:1204 [inline]
do_splice_direct+0xda/0x150 fs/splice.c:1230
do_sendfile+0x380/0x650 fs/read_write.c:1370
__do_sys_sendfile64 fs/read_write.c:1431 [inline]
__se_sys_sendfile64 fs/read_write.c:1417 [inline]
__x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1417
x64_sys_call+0x2db1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x0000000000000003 -> 0x0000000000000004

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 29779 Comm: syz.0.5751 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================
syz.0.5751: attempt to access beyond end of device
loop0: rw=8390657, sector=128, nr_sectors = 1 limit=128
Buffer I/O error on dev loop0, logical block 128, lost async page write


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