[syzbot] [fs?] KCSAN: data-race in choose_mountpoint_rcu / umount_tree

[syzbot <syzbot+81fdaf0f522d5c5e41fb@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    a33b5a08cbbd Merge tag 'sched_ext-for-6.15-rc3-fixes' of g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1058f26f980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=85dd0f8b81b9d41f
dashboard link: https://syzkaller.appspot.com/bug?extid=81fdaf0f522d5c5e41fb
compiler:       Debian clang version 15.0.6, Debian LLD 15.0.6

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/718e6f7bde0a/disk-a33b5a08.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/20f5e402fb15/vmlinux-a33b5a08.xz
kernel image: https://storage.googleapis.com/syzbot-assets/2dd06e277fc7/bzImage-a33b5a08.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+81fdaf0f522d5c5e41fb@syzkaller.appspotmail.com

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

write to 0xffff888108512d98 of 8 bytes by task 21705 on cpu 1:
 unhash_mnt fs/namespace.c:1050 [inline]
 umount_mnt fs/namespace.c:1064 [inline]
 umount_tree+0x6f0/0xb10 fs/namespace.c:1922
 do_umount fs/namespace.c:-1 [inline]
 path_umount+0x9c1/0xa40 fs/namespace.c:2144
 ksys_umount fs/namespace.c:2167 [inline]
 __do_sys_umount fs/namespace.c:2172 [inline]
 __se_sys_umount fs/namespace.c:2170 [inline]
 __x64_sys_umount+0xb7/0xe0 fs/namespace.c:2170
 x64_sys_call+0x2883/0x2e10 arch/x86/include/generated/asm/syscalls_64.h:167
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0x1a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888108512d98 of 8 bytes by task 21704 on cpu 0:
 choose_mountpoint_rcu+0x3d/0x130 fs/namei.c:1386
 follow_dotdot_rcu fs/namei.c:2020 [inline]
 handle_dots+0x559/0x790 fs/namei.c:2095
 walk_component fs/namei.c:2132 [inline]
 link_path_walk+0x5f6/0x840 fs/namei.c:2503
 path_lookupat+0x6c/0x2a0 fs/namei.c:2659
 filename_lookup+0x14b/0x340 fs/namei.c:2689
 filename_setxattr+0x59/0x2b0 fs/xattr.c:660
 path_setxattrat+0x28a/0x320 fs/xattr.c:713
 __do_sys_setxattr fs/xattr.c:747 [inline]
 __se_sys_setxattr fs/xattr.c:743 [inline]
 __x64_sys_setxattr+0x6e/0x90 fs/xattr.c:743
 x64_sys_call+0x28e7/0x2e10 arch/x86/include/generated/asm/syscalls_64.h:189
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0x1a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff888116147cc0 -> 0xffff8881065c23c0

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 21704 Comm: syz.6.5865 Not tainted 6.15.0-rc3-syzkaller-00008-ga33b5a08cbbd #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
==================================================================


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