linux-kernel - Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT

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Date:   Tue, 26 Apr 2022 19:24:03 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.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>,
        linux-kernel@...r.kernel.org, tj@...nel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs
 PREEMPT_RT

Oleg Nesterov <oleg@...hat.com> writes:

> On 04/21, Peter Zijlstra wrote:
>>
>> @@ -2225,7 +2238,7 @@ static int ptrace_stop(int exit_code, in
>>  	 * schedule() will not sleep if there is a pending signal that
>>  	 * can awaken the task.
>>  	 */
>> -	current->jobctl |= JOBCTL_TRACED;
>> +	current->jobctl |= JOBCTL_TRACED | JOBCTL_TRACED_QUIESCE;
>>  	set_special_state(TASK_TRACED);
>
> OK, this looks wrong. I actually mean the previous patch which sets
> JOBCTL_TRACED.
>
> The problem is that the tracee can be already killed, so that
> fatal_signal_pending(current) is true. In this case we can't rely on
> signal_wake_up_state() which should clear JOBCTL_TRACED, or the
> callers of ptrace_signal_wake_up/etc which clear this flag by hand.
>
> In this case schedule() won't block and ptrace_stop() will leak
> JOBCTL_TRACED. Unless I missed something.
>
> We could check fatal_signal_pending() and damn! this is what I think
> ptrace_stop() should have done from the very beginning. But for now
> I'd suggest to simply clear this flag before return, along with
> DELAY_WAKEKILL and LISTENING.

Oh.  That is an interesting case for JOBCTL_TRACED.  The
scheduler refuses to stop if signal_pending_state(TASK_TRACED, p)
returns true.

The ptrace_stop code used to handle this explicitly and in commit
7d613f9f72ec ("signal: Remove the bogus sigkill_pending in ptrace_stop")
I actually removed the test.  As the test was somewhat wrong and
redundant, and in slightly the wrong location.

But doing:

	/* Don't stop if the task is dying */
	if (unlikely(__fatal_signal_pending(current)))
		return exit_code;

Should work.

>
>>  	current->jobctl &= ~JOBCTL_LISTENING;
>> +	current->jobctl &= ~JOBCTL_DELAY_WAKEKILL;
>
> 	current->jobctl &=
> 		~(~JOBCTL_TRACED | JOBCTL_DELAY_WAKEKILL | JOBCTL_LISTENING);

I presume you meant:

	current->jobctl &=
 		~(JOBCTL_TRACED | JOBCTL_DELAY_WAKEKILL | JOBCTL_LISTENING);

I don't think we want to do that.  For the case you are worried about it
is a valid fix.

In general this is the wrong approach as we want the waker to clear
JOBCTL_TRACED.  If the waker does not it is possible that
ptrace_freeze_traced might attempt to freeze a process whose state
is not appropriate for attach, because the code is past the call
to schedule().

In fact I think clearing JOBCTL_TRACED at the end of ptrace_stop
will allow ptrace_freeze_traced to come in while siglock is dropped,
expect the process to stop, and have the process not stop.  Of
course wait_task_inactive coming first that might not be a problem.

This is a minor problem with the patchset I just posted.  I thought the
only reason wait_task_inactive could fail was if ptrace_stop() hit the
!current->ptrace case.  Thinking about any it any SIGKILL coming in
before tracee stops in schedule will trigger this, so it is not as
safe as I thought to not pass a state into wait_task_inactive.

It is time for me to shut down today.  I will sleep on that and
see what I can see tomorrow.

Eric