[moderation] [kernel?] KCSAN: data-race in free_mnt_ns / rb_erase (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 917719c412c4 Merge tag 'selinux-pr-20260507' of git://git...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=169d2a73980000
kernel config: https://syzkaller.appspot.com/x/.config?x=2e40c0f41e01837e
dashboard link: https://syzkaller.appspot.com/bug?extid=59c8f996fd15127395d2
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [linux-...@vger.kernel.org lu...@kernel.org pet...@infradead.org tg...@kernel.org]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/e55e13d50f45/disk-917719c4.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/867492a186ba/vmlinux-917719c4.xz
kernel image: https://storage.googleapis.com/syzbot-assets/61eb606c9dc6/bzImage-917719c4.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+59c8f9...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in free_mnt_ns / rb_erase

write to 0xffff88811c183698 of 8 bytes by task 20276 on cpu 1:
__rb_erase_augmented include/linux/rbtree_augmented.h:244 [inline]
rb_erase+0x21a/0x680 lib/rbtree.c:443
ns_tree_node_del kernel/nstree.c:147 [inline]
__ns_tree_remove+0x61/0x240 kernel/nstree.c:244
mnt_ns_tree_remove fs/namespace.c:150 [inline]
free_mnt_ns+0x94/0xc0 fs/namespace.c:4185
namespace_unlock+0x393/0x4c0 fs/namespace.c:1711
class_namespace_excl_destructor fs/namespace.c:90 [inline]
put_mnt_ns+0x100/0x140 fs/namespace.c:6276
nsproxy_free+0x20/0x3a0 kernel/nsproxy.c:66
deactivate_nsproxy kernel/nsproxy.c:80 [inline]
put_nsproxy include/linux/nsproxy.h:110 [inline]
switch_task_namespaces+0x9a/0xe0 kernel/nsproxy.c:260
exit_nsproxy_namespaces+0x17/0x20 kernel/nsproxy.c:265
do_exit+0x4a9/0x1530 kernel/exit.c:974
do_group_exit+0xfe/0x140 kernel/exit.c:1117
get_signal+0xe30/0xf20 kernel/signal.c:3037
arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:64 [inline]
exit_to_user_mode_loop+0x6a/0x6f0 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:238 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x232/0x3b0 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811c183698 of 8 bytes by task 20295 on cpu 0:
mnt_ns_tree_remove fs/namespace.c:149 [inline]
free_mnt_ns+0x62/0xc0 fs/namespace.c:4185
namespace_unlock+0x393/0x4c0 fs/namespace.c:1711
class_namespace_excl_destructor fs/namespace.c:90 [inline]
put_mnt_ns+0x100/0x140 fs/namespace.c:6276
nsproxy_free+0x20/0x3a0 kernel/nsproxy.c:66
deactivate_nsproxy kernel/nsproxy.c:80 [inline]
put_nsproxy include/linux/nsproxy.h:110 [inline]
switch_task_namespaces+0x9a/0xe0 kernel/nsproxy.c:260
exit_nsproxy_namespaces+0x17/0x20 kernel/nsproxy.c:265
do_exit+0x4a9/0x1530 kernel/exit.c:974
do_group_exit+0xfe/0x140 kernel/exit.c:1117
get_signal+0xe30/0xf20 kernel/signal.c:3037
arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:64 [inline]
exit_to_user_mode_loop+0x6a/0x6f0 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:238 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x232/0x3b0 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff888119e9f698 -> 0xffff88811a2b1899

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 20295 Comm: syz.2.11111 Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup