[Devel] [PATCH vz10 2/2] ve: do not wait for user-mode helpers under ve->op_sem in ve_stop_ns()

[Pavel Tikhomirov]

Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

On 6/12/26 08:57, Konstantin Khorenko wrote:
> ve_stop_ns() switches the VE to VE_STATE_STOPPING and then waits for all
> in-flight per-VE user-mode helpers to drain via wait_khelpers(). After
> the per-VE workqueue / cgroup release_agent infrastructure was removed,
> this wait ended up running under ve->op_sem held for write.
> 
> That is a deadlock: a call_usermodehelper_exec_ve() payload waited for
> by wait_khelpers() may run arbitrary user code that accesses ve.* cgroup
> files, and those take ve->op_sem (for read). ve_stop_ns() would then
> block forever holding op_sem for write while the helper blocks trying to
> acquire it.
> 
> Drop ve->op_sem around wait_khelpers() and re-acquire it afterwards, the
> way the code did before the release_agent removal (it used to drop the
> lock around wait_khelpers() + ve_workqueue_stop()). The VE is already in
> VE_STATE_STOPPING before the lock is dropped, so no new helper can be
> queued and all op_sem entry points bail out on the state check.
> 
> Fixes: 9b103188a9b2 ("ve/kthread: fix race when work can be added to stopped kthread worker")
> Reported-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
> https://virtuozzo.atlassian.net/browse/VSTOR-132310
> Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
> ---
>  kernel/ve/ve.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
> index 65723f28dbad..231c0300e929 100644
> --- a/kernel/ve/ve.c
> +++ b/kernel/ve/ve.c
> @@ -575,7 +575,19 @@ void ve_stop_ns(struct pid_namespace *pid_ns)
>  	ve_set_state(ve, VE_STATE_STOPPING);
>  	synchronize_rcu();
>  
> +	/*
> +	 * Drop the lock before waiting for in-flight user-mode helpers.
> +	 * A call_usermodehelper_exec_ve() payload may access ve.* cgroup
> +	 * files, which take ve->op_sem, so waiting for it via wait_khelpers()
> +	 * while holding op_sem would deadlock. The state is already
> +	 * VE_STATE_STOPPING, so no new helper can be queued; all entry points
> +	 * must check the state before proceeding.
> +	 */
> +	up_write(&ve->op_sem);
> +
>  	wait_khelpers(ve);
> +
> +	down_write(&ve->op_sem);
>  	/*
>  	 * Neither it can be in pseudosuper state
>  	 * anymore, setup it again if needed.

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.