[moderation] [exfat?] KCSAN: data-race in _copy_to_iter / fat32_ent_put (6)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: b4e07588e743 tracing: tell git to ignore the generated 'un..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10dc47f6580000
kernel config: https://syzkaller.appspot.com/x/.config?x=ff1bc04bab908995
dashboard link: https://syzkaller.appspot.com/bug?extid=23cbb05142ae920887fd
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/71c6f85eeaf3/disk-b4e07588.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/9da381bc6ca7/vmlinux-b4e07588.xz
kernel image: https://storage.googleapis.com/syzbot-assets/2bd28557b04a/bzImage-b4e07588.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+23cbb0...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in _copy_to_iter / fat32_ent_put

read-write to 0xffff88813fb44028 of 4 bytes by task 12527 on cpu 1:
fat32_ent_put+0x4e/0x90 fs/fat/fatent.c:194
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:2035
block_write_begin fs/buffer.c:2146 [inline]
cont_write_begin+0x5fe/0x970 fs/buffer.c:2484
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
cont_expand_zero fs/buffer.c:2412 [inline]
cont_write_begin+0x1b0/0x970 fs/buffer.c:2474
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
generic_perform_write+0x183/0x490 mm/filemap.c:4325
__generic_file_write_iter+0xec/0x120 mm/filemap.c:4439
generic_file_write_iter+0x8d/0x310 mm/filemap.c:4468
do_iter_readv_writev+0x4fd/0x5a0 fs/read_write.c:-1
vfs_writev+0x2e1/0x900 fs/read_write.c:1059
do_pwritev fs/read_write.c:1155 [inline]
__do_sys_pwritev2 fs/read_write.c:1213 [inline]
__se_sys_pwritev2+0xfc/0x1c0 fs/read_write.c:1204
__x64_sys_pwritev2+0x67/0x80 fs/read_write.c:1204
x64_sys_call+0xe07/0x3020 arch/x86/include/generated/asm/syscalls_64.h:329
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 0xffff88813fb44000 of 512 bytes by task 3889 on cpu 0:
instrument_copy_to_user include/linux/instrumented.h:130 [inline]
copy_to_user_iter lib/iov_iter.c:24 [inline]
iterate_ubuf include/linux/iov_iter.h:30 [inline]
iterate_and_advance2 include/linux/iov_iter.h:302 [inline]
iterate_and_advance include/linux/iov_iter.h:330 [inline]
_copy_to_iter+0x130/0xea0 lib/iov_iter.c:197
copy_page_to_iter+0x178/0x210 lib/iov_iter.c:374
copy_folio_to_iter include/linux/uio.h:204 [inline]
filemap_read+0x407/0xa10 mm/filemap.c:2857
blkdev_read_iter+0x22d/0x2e0 block/fops.c:855
new_sync_read fs/read_write.c:493 [inline]
vfs_read+0x6c0/0x7f0 fs/read_write.c:574
ksys_read+0xdc/0x1a0 fs/read_write.c:717
__do_sys_read fs/read_write.c:726 [inline]
__se_sys_read fs/read_write.c:724 [inline]
__x64_sys_read+0x40/0x50 fs/read_write.c:724
x64_sys_call+0x2886/0x3020 arch/x86/include/generated/asm/syscalls_64.h:1
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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 3889 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
==================================================================


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