[moderation] [fs?] KCSAN: data-race in __lookup_mnt / umount_tree (7)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 20cb38a7af88 Merge tag 'for-6.9-rc2-tag' of git://git.kern..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=162923e3180000
kernel config: https://syzkaller.appspot.com/x/.config?x=ae557ecb5cabc4aa
dashboard link: https://syzkaller.appspot.com/bug?extid=c4b5521171c434a60c03
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
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/1c92ddc37dc2/disk-20cb38a7.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/2f582de438c0/vmlinux-20cb38a7.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c35d06026809/bzImage-20cb38a7.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c4b552...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __lookup_mnt / umount_tree

write to 0xffff888115bb4010 of 8 bytes by task 6801 on cpu 0:
unhash_mnt fs/namespace.c:891 [inline]
umount_mnt fs/namespace.c:906 [inline]
umount_tree+0x625/0x910 fs/namespace.c:1673
path_umount+0x991/0xa10 fs/namespace.c:1896
ksys_umount fs/namespace.c:1919 [inline]
__do_sys_umount fs/namespace.c:1924 [inline]
__se_sys_umount fs/namespace.c:1922 [inline]
__x64_sys_umount+0xb9/0xe0 fs/namespace.c:1922
do_syscall_64+0xd3/0x1d0
entry_SYSCALL_64_after_hwframe+0x72/0x7a

read to 0xffff888115bb4010 of 8 bytes by task 6803 on cpu 1:
__lookup_mnt+0x83/0xf0 fs/namespace.c:701
__follow_mount_rcu fs/namei.c:1506 [inline]
handle_mounts fs/namei.c:1536 [inline]
step_into+0x434/0x810 fs/namei.c:1842
walk_component+0x169/0x230 fs/namei.c:2010
lookup_last fs/namei.c:2461 [inline]
path_lookupat+0x10a/0x2b0 fs/namei.c:2485
filename_lookup+0x127/0x300 fs/namei.c:2514
user_path_at_empty+0x42/0x120 fs/namei.c:2921
user_path_at include/linux/namei.h:57 [inline]
ksys_umount fs/namespace.c:1916 [inline]
__do_sys_umount fs/namespace.c:1924 [inline]
__se_sys_umount fs/namespace.c:1922 [inline]
__x64_sys_umount+0x88/0xe0 fs/namespace.c:1922
do_syscall_64+0xd3/0x1d0
entry_SYSCALL_64_after_hwframe+0x72/0x7a

value changed: 0xffff888104009980 -> 0xffff888115bb4000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6803 Comm: syz-executor.4 Tainted: G W 6.9.0-rc3-syzkaller-00011-g20cb38a7af88 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
==================================================================


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