Re: [PATCH] kernel: release ptraced tasks before zap_pid_ns_processes

[Andrei Vagin]

Oops, Al is here by mistake. Sorry about that.

On Wed, Jan 02, 2019 at 12:59:39PM -0800, Andrei Vagin wrote:
> Currently, exit_ptrace() adds all ptraced tasks in a dead list, than
> zap_pid_ns_processes() waits all tasks in a current pid ns, and only
> then tasks from the dead list are released.
> 
> zap_pid_ns_processes() can stuck on waiting tasks from the dead list. In
> this case, we will have one unkillable process with one or more dead
> children.
> 
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Andrei Vagin <avagin@xxxxxxxxx>
> ---
>  kernel/exit.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 0e21e6d21f35..ccaa6f6549ba 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -664,9 +664,6 @@ static void forget_original_parent(struct task_struct *father,
>  {
>  	struct task_struct *p, *t, *reaper;
>  
> -	if (unlikely(!list_empty(&father->ptraced)))
> -		exit_ptrace(father, dead);
> -
>  	/* Can drop and reacquire tasklist_lock */
>  	reaper = find_child_reaper(father);
>  	if (list_empty(&father->children))
> @@ -705,8 +702,18 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  	LIST_HEAD(dead);
>  
>  	write_lock_irq(&tasklist_lock);
> -	forget_original_parent(tsk, &dead);
> +	if (unlikely(!list_empty(&tsk->ptraced)))
> +		exit_ptrace(tsk, &dead);
> +	write_unlock_irq(&tasklist_lock);
> +
> +	/* Ptraced tasks have to be released before zap_pid_ns_processes(). */
> +	list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
> +		list_del_init(&p->ptrace_entry);
> +		release_task(p);
> +	}
>  
> +	write_lock_irq(&tasklist_lock);
> +	forget_original_parent(tsk, &dead);
>  	if (group_dead)
>  		kill_orphaned_pgrp(tsk->group_leader, NULL);
>  
> -- 
> 2.17.2
>