[moderation] [fs?] KCSAN: data-race in __follow_mount_rcu / do_new_mount

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: d8a9a4b11a13 Merge tag 'v7.0-rc6-smb3-client-fix' of git:/..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=138488f2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3a78dd265deac3a9
dashboard link: https://syzkaller.appspot.com/bug?extid=2137c95046b57e2ea8fa
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
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/d57ab69594ea/disk-d8a9a4b1.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/465f17bdc2a1/vmlinux-d8a9a4b1.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c658f4e05ddd/bzImage-d8a9a4b1.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2137c9...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __follow_mount_rcu / do_new_mount

read to 0xffff888106803cc0 of 4 bytes by task 5162 on cpu 1:
__follow_mount_rcu+0x15f/0x2b0 fs/namei.c:1709
handle_mounts fs/namei.c:1735 [inline]
step_into_slowpath+0x30c/0x4c0 fs/namei.c:2101
step_into fs/namei.c:2149 [inline]
walk_component fs/namei.c:2285 [inline]
lookup_last fs/namei.c:2786 [inline]
path_lookupat+0x330/0x500 fs/namei.c:2810
filename_lookup+0x190/0x390 fs/namei.c:2839
user_path_at+0x3e/0x100 fs/namei.c:3618
__do_sys_pivot_root fs/namespace.c:4738 [inline]
__se_sys_pivot_root fs/namespace.c:4726 [inline]
__x64_sys_pivot_root+0x9f/0x110 fs/namespace.c:4726
x64_sys_call+0x20a/0x3020 arch/x86/include/generated/asm/syscalls_64.h:156
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-write to 0xffff888106803cc0 of 4 bytes by task 5161 on cpu 0:
maybe_free_mountpoint fs/namespace.c:927 [inline]
unpin_mountpoint fs/namespace.c:942 [inline]
__unlock_mount fs/namespace.c:2792 [inline]
unlock_mount fs/namespace.c:2800 [inline]
do_new_mount_fc fs/namespace.c:3786 [inline]
do_new_mount+0x6a8/0x8d0 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 5161 Comm: syz.5.403 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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