[moderation] [ext4?] KCSAN: data-race in _copy_to_iter / string (7)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: f5098b6bae76 Linux 7.2-rc5
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14236499580000
kernel config: https://syzkaller.appspot.com/x/.config?x=84b3039e8461eef5
dashboard link: https://syzkaller.appspot.com/bug?extid=39c518c216ae4e2cc47a
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
CC: [adilger...@dilger.ca ja...@suse.cz liba...@linux.alibaba.com linux...@vger.kernel.org linux-...@vger.kernel.org oja...@linux.ibm.com rites...@gmail.com ty...@mit.edu yi.z...@huawei.com]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/7e252b1c6649/disk-f5098b6b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/fba0d14bebdb/vmlinux-f5098b6b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/424a9a3adf18/bzImage-f5098b6b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+39c518...@syzkaller.appspotmail.com

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

write to 0xffff88811ca0d050 of 1 bytes by task 3743 on cpu 0:
string_nocheck lib/vsprintf.c:652 [inline]
string+0x165/0x220 lib/vsprintf.c:730
bdev_name+0x12a/0x2b0 lib/vsprintf.c:971
pointer+0x5bc/0xc90 lib/vsprintf.c:2592
vsnprintf+0x49e/0x8c0 lib/vsprintf.c:2949
snprintf+0x86/0xb0 lib/vsprintf.c:3043
ext4_multi_mount_protect+0x4eb/0x600 fs/ext4/mmp.c:381
__ext4_fill_super fs/ext4/super.c:5501 [inline]
ext4_fill_super+0x2260/0x3760 fs/ext4/super.c:5828
get_tree_bdev_flags+0x291/0x300 fs/super.c:1640
get_tree_bdev+0x1f/0x30 fs/super.c:1663
ext4_get_tree+0x1c/0x30 fs/ext4/super.c:5860
vfs_get_tree+0x57/0x1d0 fs/super.c:1700
fc_mount fs/namespace.c:1198 [inline]
do_new_mount_fc fs/namespace.c:3765 [inline]
do_new_mount+0x288/0x8d0 fs/namespace.c:3841
path_mount+0x4ad/0xb90 fs/namespace.c:4161
do_mount fs/namespace.c:4174 [inline]
__do_sys_mount fs/namespace.c:4390 [inline]
__se_sys_mount+0x28c/0x2e0 fs/namespace.c:4367
__x64_sys_mount+0x67/0x80 fs/namespace.c:4367
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+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811ca0d000 of 512 bytes by task 3287 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+0x174/0x210 lib/iov_iter.c:374
copy_folio_to_iter include/linux/uio.h:204 [inline]
filemap_read+0x3ed/0xa20 mm/filemap.c:2865
blkdev_read_iter+0x21e/0x2d0 block/fops.c:830
new_sync_read fs/read_write.c:493 [inline]
vfs_read+0x699/0x7c0 fs/read_write.c:574
ksys_read+0xdc/0x1a0 fs/read_write.c:716
__do_sys_read fs/read_write.c:725 [inline]
__se_sys_read fs/read_write.c:723 [inline]
__x64_sys_read+0x40/0x50 fs/read_write.c:723
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+0x136/0x3c0 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: 3287 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(lazy)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
==================================================================


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