Re: [patch 6/9] signal: Fix racy access to __task_cred in kill_pid_info_as_uid()

[Oleg Nesterov <oleg@redhat.com>]

On 12/10, Thomas Gleixner wrote:
>
> kill_pid_info_as_uid() accesses __task_cred() without being in a RCU
> read side critical section. tasklist_lock is not protecting that when
> CONFIG_TREE_PREEMPT_RCU=y.
>
> Convert the whole tasklist_lock section to rcu and use
> lock_task_sighand to prevent the exit race.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Oleg Nesterov <oleg@redhat.com>
> ---
>  kernel/signal.c |   17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)

Acked-by: Oleg Nesterov <oleg@redhat.com>

> Index: linux-2.6-tip/kernel/signal.c
> ===================================================================
> --- linux-2.6-tip.orig/kernel/signal.c
> +++ linux-2.6-tip/kernel/signal.c
> @@ -1175,11 +1175,12 @@ int kill_pid_info_as_uid(int sig, struct
>  	int ret = -EINVAL;
>  	struct task_struct *p;
>  	const struct cred *pcred;
> +	unsigned long flags;
>  
>  	if (!valid_signal(sig))
>  		return ret;
>  
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	p = pid_task(pid, PIDTYPE_PID);
>  	if (!p) {
>  		ret = -ESRCH;
> @@ -1196,14 +1197,16 @@ int kill_pid_info_as_uid(int sig, struct
>  	ret = security_task_kill(p, info, sig, secid);
>  	if (ret)
>  		goto out_unlock;
> -	if (sig && p->sighand) {
> -		unsigned long flags;
> -		spin_lock_irqsave(&p->sighand->siglock, flags);
> -		ret = __send_signal(sig, info, p, 1, 0);
> -		spin_unlock_irqrestore(&p->sighand->siglock, flags);
> +
> +	if (sig) {
> +		if (lock_task_sighand(p, &flags)) {
> +			ret = __send_signal(sig, info, p, 1, 0);
> +			unlock_task_sighand(p, &flags);
> +		} else
> +			ret = -ESRCH;
>  	}
>  out_unlock:
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(kill_pid_info_as_uid);
> 
>