[moderation] [fs?] KCSAN: data-race in __umount_mnt / choose_mountpoint_rcu

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 266ee584e55e Merge tag 'arm64-fixes' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13e9ae7c580000
kernel config: https://syzkaller.appspot.com/x/.config?x=68b6063e8fdfb838
dashboard link: https://syzkaller.appspot.com/bug?extid=82d39a5a82eb46c13fc2
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [bra...@kernel.org ja...@suse.cz linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/dbfb165178bb/disk-266ee584.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/4ad0c5419223/vmlinux-266ee584.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c704ac2f93b5/bzImage-266ee584.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+82d39a...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __umount_mnt / choose_mountpoint_rcu

write to 0xffff88811a499c90 of 8 bytes by task 12153 on cpu 0:
__umount_mnt+0x4e/0x2c0 fs/namespace.c:1008
umount_mnt fs/namespace.c:1023 [inline]
umount_tree+0x547/0x7f0 fs/namespace.c:1818
do_umount fs/namespace.c:-1 [inline]
path_umount+0x7c8/0x7e0 fs/namespace.c:2041
ksys_umount fs/namespace.c:2064 [inline]
__do_sys_umount fs/namespace.c:2069 [inline]
__se_sys_umount fs/namespace.c:2067 [inline]
__x64_sys_umount+0xb6/0xe0 fs/namespace.c:2067
x64_sys_call+0xdd2/0x3000 arch/x86/include/generated/asm/syscalls_64.h:167
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811a499c90 of 8 bytes by task 12145 on cpu 1:
mnt_has_parent fs/mount.h:119 [inline]
choose_mountpoint_rcu+0x2a/0x130 fs/namei.c:1385
follow_dotdot_rcu fs/namei.c:2033 [inline]
handle_dots+0x520/0x750 fs/namei.c:2108
walk_component fs/namei.c:2145 [inline]
link_path_walk+0x6d3/0x900 fs/namei.c:2519
path_lookupat+0x63/0x2a0 fs/namei.c:2675
filename_lookup+0x147/0x340 fs/namei.c:2705
user_path_at+0x3e/0x130 fs/namei.c:3215
ksys_umount fs/namespace.c:2061 [inline]
__do_sys_umount fs/namespace.c:2069 [inline]
__se_sys_umount fs/namespace.c:2067 [inline]
__x64_sys_umount+0x85/0xe0 fs/namespace.c:2067
x64_sys_call+0xdd2/0x3000 arch/x86/include/generated/asm/syscalls_64.h:167
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff88811a488a80 -> 0xffff88811a499c80

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 12145 Comm: syz.5.2878 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup