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

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8fdb05de0e2d Merge tag 'net-6.19-rc9' of git://git.kernel...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14fd8a52580000
kernel config: https://syzkaller.appspot.com/x/.config?x=8e27f4588a0f2183
dashboard link: https://syzkaller.appspot.com/bug?extid=2d65c5a857fc4cc393d2
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/ec4849f6af8e/disk-8fdb05de.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/79c8d2fd2b94/vmlinux-8fdb05de.xz
kernel image: https://storage.googleapis.com/syzbot-assets/ddd0f3e3e081/bzImage-8fdb05de.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2d65c5...@syzkaller.appspotmail.com

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

write to 0xffff88811a218c58 of 8 bytes by task 26782 on cpu 0:
__rb_erase_augmented include/linux/rbtree_augmented.h:317 [inline]
rb_erase+0x267/0x680 lib/rbtree.c:443
ns_tree_node_del kernel/nstree.c:147 [inline]
__ns_tree_remove+0x5e/0x220 kernel/nstree.c:244
mnt_ns_tree_remove fs/namespace.c:156 [inline]
free_mnt_ns+0x8e/0xb0 fs/namespace.c:4053
namespace_unlock+0x393/0x4c0 fs/namespace.c:1707
class_namespace_excl_destructor fs/namespace.c:96 [inline]
put_mnt_ns+0x101/0x140 fs/namespace.c:6039
nsproxy_free+0x20/0x3a0 kernel/nsproxy.c:65
deactivate_nsproxy kernel/nsproxy.c:79 [inline]
put_nsproxy include/linux/nsproxy.h:110 [inline]
switch_task_namespaces+0x9a/0xe0 kernel/nsproxy.c:252
exit_nsproxy_namespaces+0x17/0x20 kernel/nsproxy.c:257
do_exit+0x461/0x1590 kernel/exit.c:970
do_group_exit+0xfe/0x140 kernel/exit.c:1112
get_signal+0xe4f/0xf60 kernel/signal.c:3034
arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
exit_to_user_mode_loop+0x6a/0x6f0 kernel/entry/common.c:75
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
syscall_exit_to_user_mode_work include/linux/entry-common.h:159 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:194 [inline]
do_syscall_64+0x1d3/0x2a0 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811a218c58 of 8 bytes by task 26777 on cpu 1:
mnt_ns_tree_remove fs/namespace.c:155 [inline]
free_mnt_ns+0x5f/0xb0 fs/namespace.c:4053
namespace_unlock+0x393/0x4c0 fs/namespace.c:1707
class_namespace_excl_destructor fs/namespace.c:96 [inline]
put_mnt_ns+0x101/0x140 fs/namespace.c:6039
nsproxy_free+0x20/0x3a0 kernel/nsproxy.c:65
deactivate_nsproxy kernel/nsproxy.c:79 [inline]
put_nsproxy include/linux/nsproxy.h:110 [inline]
switch_task_namespaces+0x9a/0xe0 kernel/nsproxy.c:252
exit_nsproxy_namespaces+0x17/0x20 kernel/nsproxy.c:257
do_exit+0x461/0x1590 kernel/exit.c:970
do_group_exit+0xfe/0x140 kernel/exit.c:1112
get_signal+0xe4f/0xf60 kernel/signal.c:3034
arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
exit_to_user_mode_loop+0x6a/0x6f0 kernel/entry/common.c:75
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
syscall_exit_to_user_mode_work include/linux/entry-common.h:159 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:194 [inline]
do_syscall_64+0x1d3/0x2a0 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff88811b0a7e58 -> 0xffff88811acf5a59

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 26777 Comm: syz.4.6991 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
==================================================================


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