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

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: ea1013c15392 Merge tag 'bpf-fixes' of git://git.kernel.org..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=149d8d58580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b319ff1b6a2797ca
dashboard link: https://syzkaller.appspot.com/bug?extid=e18a2411961a6168ae7e
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/e71363ad5fa5/disk-ea1013c1.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/7d02687e9370/vmlinux-ea1013c1.xz
kernel image: https://storage.googleapis.com/syzbot-assets/1289ed40062b/bzImage-ea1013c1.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e18a24...@syzkaller.appspotmail.com

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

write to 0xffff88811da2ed90 of 8 bytes by task 7838 on cpu 1:
mnt_set_mountpoint fs/namespace.c:1025 [inline]
attach_mnt fs/namespace.c:1059 [inline]
mnt_change_mountpoint+0x1ab/0x490 fs/namespace.c:1071
reparent fs/pnode.c:641 [inline]
propagate_umount+0xd24/0xf30 fs/pnode.c:682
umount_tree+0x2f8/0x800 fs/namespace.c:1790
do_umount fs/namespace.c:-1 [inline]
path_umount+0x7c0/0x810 fs/namespace.c:2038
ksys_umount fs/namespace.c:2061 [inline]
__do_sys_umount fs/namespace.c:2066 [inline]
__se_sys_umount fs/namespace.c:2064 [inline]
__x64_sys_umount+0xb6/0xe0 fs/namespace.c:2064
x64_sys_call+0xd6a/0x3000 arch/x86/include/generated/asm/syscalls_64.h:167
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd8/0x2c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811da2ed90 of 8 bytes by task 7837 on cpu 0:
mnt_has_parent fs/mount.h:120 [inline]
choose_mountpoint_rcu+0x2a/0x130 fs/namei.c:1425
follow_dotdot_rcu fs/namei.c:2103 [inline]
handle_dots+0x63d/0x8a0 fs/namei.c:2178
walk_component fs/namei.c:2215 [inline]
link_path_walk+0x8bb/0xe30 fs/namei.c:2589
path_lookupat+0x8c/0x500 fs/namei.c:2745
filename_lookup+0x147/0x340 fs/namei.c:2775
user_path_at+0x3e/0x130 fs/namei.c:3567
ksys_umount fs/namespace.c:2058 [inline]
__do_sys_umount fs/namespace.c:2066 [inline]
__se_sys_umount fs/namespace.c:2064 [inline]
__x64_sys_umount+0x85/0xe0 fs/namespace.c:2064
x64_sys_call+0xd6a/0x3000 arch/x86/include/generated/asm/syscalls_64.h:167
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd8/0x2c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff888104640300 -> 0xffff888104641380

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 7837 Comm: syz.5.1204 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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