[syzbot] [fs?] KCSAN: data-race in __lookup_mnt / __se_sys_pivot_root (6)

[syzbot] [fs?] KCSAN: data-race in __lookup_mnt / __se_sys_pivot_root (6)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    a7f2e10ecd8f Merge tag 'hwmon-fixes-for-v6.14-rc8/6.14' of..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=166a383f980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f33d372c4021745
dashboard link: https://syzkaller.appspot.com/bug?extid=de8b27abd23eac60e15f
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/614aabc71b48/disk-a7f2e10e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d47dd90a010a/vmlinux-a7f2e10e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/418d8cf8782b/bzImage-a7f2e10e.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+de8b27abd23eac60e15f@syzkaller.appspotmail.com

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

write to 0xffff888118782d98 of 8 bytes by task 20163 on cpu 0:
 unhash_mnt fs/namespace.c:1030 [inline]
 __do_sys_pivot_root fs/namespace.c:4456 [inline]
 __se_sys_pivot_root+0x850/0x1090 fs/namespace.c:4388
 __x64_sys_pivot_root+0x31/0x40 fs/namespace.c:4388
 x64_sys_call+0x1abf/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:156
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888118782d98 of 8 bytes by task 20164 on cpu 1:
 __lookup_mnt+0xa0/0xf0 fs/namespace.c:839
 __follow_mount_rcu fs/namei.c:1592 [inline]
 handle_mounts fs/namei.c:1622 [inline]
 step_into+0x426/0x820 fs/namei.c:1952
 walk_component fs/namei.c:2120 [inline]
 link_path_walk+0x50e/0x830 fs/namei.c:2479
 path_lookupat+0x72/0x2b0 fs/namei.c:2635
 filename_lookup+0x150/0x340 fs/namei.c:2665
 user_path_at+0x3c/0x120 fs/namei.c:3072
 __do_sys_pivot_root fs/namespace.c:4404 [inline]
 __se_sys_pivot_root+0x10e/0x1090 fs/namespace.c:4388
 __x64_sys_pivot_root+0x31/0x40 fs/namespace.c:4388
 x64_sys_call+0x1abf/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:156
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff888106a31d80 -> 0xffff8881004dccc0

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 20164 Comm: syz.0.5594 Tainted: G        W          6.14.0-rc7-syzkaller-00074-ga7f2e10ecd8f #0
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
==================================================================


---
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Re: [syzbot] [fs?] KCSAN: data-race in __lookup_mnt / __se_sys_pivot_root (6)

[Christian Brauner]

On Wed, Mar 19, 2025 at 08:28:27PM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    a7f2e10ecd8f Merge tag 'hwmon-fixes-for-v6.14-rc8/6.14' of..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=166a383f980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f33d372c4021745
> dashboard link: https://syzkaller.appspot.com/bug?extid=de8b27abd23eac60e15f
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/614aabc71b48/disk-a7f2e10e.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/d47dd90a010a/vmlinux-a7f2e10e.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/418d8cf8782b/bzImage-a7f2e10e.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+de8b27abd23eac60e15f@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in __lookup_mnt / __se_sys_pivot_root
> 
> write to 0xffff888118782d98 of 8 bytes by task 20163 on cpu 0:
>  unhash_mnt fs/namespace.c:1030 [inline]
>  __do_sys_pivot_root fs/namespace.c:4456 [inline]
>  __se_sys_pivot_root+0x850/0x1090 fs/namespace.c:4388
>  __x64_sys_pivot_root+0x31/0x40 fs/namespace.c:4388
>  x64_sys_call+0x1abf/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:156
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> read to 0xffff888118782d98 of 8 bytes by task 20164 on cpu 1:
>  __lookup_mnt+0xa0/0xf0 fs/namespace.c:839
>  __follow_mount_rcu fs/namei.c:1592 [inline]
>  handle_mounts fs/namei.c:1622 [inline]
>  step_into+0x426/0x820 fs/namei.c:1952
>  walk_component fs/namei.c:2120 [inline]
>  link_path_walk+0x50e/0x830 fs/namei.c:2479
>  path_lookupat+0x72/0x2b0 fs/namei.c:2635
>  filename_lookup+0x150/0x340 fs/namei.c:2665
>  user_path_at+0x3c/0x120 fs/namei.c:3072
>  __do_sys_pivot_root fs/namespace.c:4404 [inline]
>  __se_sys_pivot_root+0x10e/0x1090 fs/namespace.c:4388
>  __x64_sys_pivot_root+0x31/0x40 fs/namespace.c:4388
>  x64_sys_call+0x1abf/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:156
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> value changed: 0xffff888106a31d80 -> 0xffff8881004dccc0

The race is perfectly benign. The change will be detected once the
sequence counter will be read in __follow_mount_rcu() which will cause a
drop-out of RCU into REF mode.