linux-kernel - Re: [PATCH v3 08/11] ptrace: Admit ptrace

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Date:   Thu, 5 May 2022 16:57:22 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, linux-ia64@...r.kernel.org
Subject: Re: [PATCH v3 08/11] ptrace: Admit ptrace_stop can generate spuriuos
 SIGTRAPs

On 05/04, Eric W. Biederman wrote:
>
> -static int ptrace_stop(int exit_code, int why, int clear_code,
> -			unsigned long message, kernel_siginfo_t *info)
> +static int ptrace_stop(int exit_code, int why, unsigned long message,
> +		       kernel_siginfo_t *info)
>  	__releases(&current->sighand->siglock)
>  	__acquires(&current->sighand->siglock)
>  {
> @@ -2259,54 +2259,33 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
>  
>  	spin_unlock_irq(&current->sighand->siglock);
>  	read_lock(&tasklist_lock);
> -	if (likely(current->ptrace)) {
> -		/*
> -		 * Notify parents of the stop.
> -		 *
> -		 * While ptraced, there are two parents - the ptracer and
> -		 * the real_parent of the group_leader.  The ptracer should
> -		 * know about every stop while the real parent is only
> -		 * interested in the completion of group stop.  The states
> -		 * for the two don't interact with each other.  Notify
> -		 * separately unless they're gonna be duplicates.
> -		 */
> +	/*
> +	 * Notify parents of the stop.
> +	 *
> +	 * While ptraced, there are two parents - the ptracer and
> +	 * the real_parent of the group_leader.  The ptracer should
> +	 * know about every stop while the real parent is only
> +	 * interested in the completion of group stop.  The states
> +	 * for the two don't interact with each other.  Notify
> +	 * separately unless they're gonna be duplicates.
> +	 */
> +	if (current->ptrace)
>  		do_notify_parent_cldstop(current, true, why);
> -		if (gstop_done && ptrace_reparented(current))
> -			do_notify_parent_cldstop(current, false, why);
> +	if (gstop_done && (!current->ptrace || ptrace_reparented(current)))
> +		do_notify_parent_cldstop(current, false, why);
>  
> -		/*
> -		 * Don't want to allow preemption here, because
> -		 * sys_ptrace() needs this task to be inactive.
> -		 *
> -		 * XXX: implement read_unlock_no_resched().
> -		 */
> -		preempt_disable();
> -		read_unlock(&tasklist_lock);
> -		cgroup_enter_frozen();
> -		preempt_enable_no_resched();
> -		freezable_schedule();
> -		cgroup_leave_frozen(true);
> -	} else {
> -		/*
> -		 * By the time we got the lock, our tracer went away.
> -		 * Don't drop the lock yet, another tracer may come.
> -		 *
> -		 * If @gstop_done, the ptracer went away between group stop
> -		 * completion and here.  During detach, it would have set
> -		 * JOBCTL_STOP_PENDING on us and we'll re-enter
> -		 * TASK_STOPPED in do_signal_stop() on return, so notifying
> -		 * the real parent of the group stop completion is enough.
> -		 */
> -		if (gstop_done)
> -			do_notify_parent_cldstop(current, false, why);
> -
> -		/* tasklist protects us from ptrace_freeze_traced() */
> -		__set_current_state(TASK_RUNNING);
> -		read_code = false;
> -		if (clear_code)
> -			exit_code = 0;
> -		read_unlock(&tasklist_lock);
> -	}
> +	/*
> +	 * Don't want to allow preemption here, because
> +	 * sys_ptrace() needs this task to be inactive.
> +	 *
> +	 * XXX: implement read_unlock_no_resched().
> +	 */
> +	preempt_disable();
> +	read_unlock(&tasklist_lock);
> +	cgroup_enter_frozen();
> +	preempt_enable_no_resched();
> +	freezable_schedule();

I must have missed something.

So the tracee calls ptrace_notify() but debugger goes away before the
ptrace_notify() takes siglock. After that the no longer traced task
will sleep in TASK_TRACED ?

Looks like ptrace_stop() needs to check current->ptrace before it does
set_special_state(TASK_TRACED) with siglock held? Then we can rely on
ptrace_unlink() which will wake the tracee up even if debugger exits.

No?

Oleg.