[moderation] [ext4?] KCSAN: data-race in _copy_to_iter / vsnprintf

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8a30aeb0d1b4 Merge tag 'nfsd-7.0-2' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14171e02580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3a78dd265deac3a9
dashboard link: https://syzkaller.appspot.com/bug?extid=be7bd9b03a9be40d9ac5
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [adilger...@dilger.ca linux...@vger.kernel.org linux-...@vger.kernel.org ty...@mit.edu]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/d210c1d5bea9/disk-8a30aeb0.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/0f40c70c1c61/vmlinux-8a30aeb0.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0a2ee2732326/bzImage-8a30aeb0.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+be7bd9...@syzkaller.appspotmail.com

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

write to 0xffff888105f0b055 of 1 bytes by task 6874 on cpu 0:
vsnprintf+0x7ee/0x860 lib/vsprintf.c:2980
snprintf+0x86/0xb0 lib/vsprintf.c:3042
ext4_multi_mount_protect+0x4e4/0x600 fs/ext4/mmp.c:382
__ext4_fill_super fs/ext4/super.c:5464 [inline]
ext4_fill_super+0x22e6/0x37c0 fs/ext4/super.c:5791
get_tree_bdev_flags+0x291/0x300 fs/super.c:1694
get_tree_bdev+0x1f/0x30 fs/super.c:1717
ext4_get_tree+0x1c/0x30 fs/ext4/super.c:5823
vfs_get_tree+0x57/0x1d0 fs/super.c:1754
fc_mount fs/namespace.c:1193 [inline]
do_new_mount_fc fs/namespace.c:3763 [inline]
do_new_mount+0x288/0x8d0 fs/namespace.c:3839
path_mount+0x4d0/0xbc0 fs/namespace.c:4159
do_mount fs/namespace.c:4172 [inline]
__do_sys_mount fs/namespace.c:4361 [inline]
__se_sys_mount+0x28c/0x2e0 fs/namespace.c:4338
__x64_sys_mount+0x67/0x80 fs/namespace.c:4338
x64_sys_call+0x2d61/0x3020 arch/x86/include/generated/asm/syscalls_64.h:166
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888105f0b000 of 512 bytes by task 6698 on cpu 1:
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+0x18f/0x2d0 lib/iov_iter.c:374
copy_folio_to_iter include/linux/uio.h:204 [inline]
filemap_read+0x407/0xa10 mm/filemap.c:2856
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/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 6698 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================


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