linux-kernel - Re: [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped

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Date:   Sun, 02 Apr 2017 13:53:11 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <asarai@...e.com>,
        Andy Lutomirski <luto@...capital.net>,
        Attila Fazekas <afazekas@...hat.com>,
        Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...nel.org>,
        Ulrich Obergfell <uobergfe@...hat.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped

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

> On 04/01, Eric W. Biederman wrote:
>>
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1052,6 +1052,7 @@ static int de_thread(struct task_struct *tsk)
>>  	struct signal_struct *sig = tsk->signal;
>>  	struct sighand_struct *oldsighand = tsk->sighand;
>>  	spinlock_t *lock = &oldsighand->siglock;
>> +	bool may_hang;
>>
>>  	if (thread_group_empty(tsk))
>>  		goto no_thread_group;
>> @@ -1069,9 +1070,10 @@ static int de_thread(struct task_struct *tsk)
>>  		return -EAGAIN;
>>  	}
>>
>> +	may_hang = atomic_read(&oldsighand->count) != 1;
>>  	sig->group_exit_task = tsk;
>> -	sig->notify_count = zap_other_threads(tsk);
>> -	if (!thread_group_leader(tsk))
>> +	sig->notify_count = zap_other_threads(tsk, may_hang ? 1 : -1);
>
> Eric, this is amazing. So with this patch exec does different things depening
> on whether sighand is shared with another CLONE_SIGHAND task or not. To me
> this doesn't look sane in any case.

It is a 99% solution that makes it possible to talk about and review
letting the exec continue after the subthreads are killed but not
reaped.

Sigh I should have made may_hang say:

may_hang = (atomic_read(&oldsignand->count) != 1) && (sig->nr_threads > 1)

Which covers all know ways userspace actually uses these clone flags.

> And btw zap_other_threads(may_hang == 0) is racy. Either you need tasklist or
> exit_notify() should set tsk->exit_state under siglock, otherwise zap() can
> return the wrong count.

zap_other_thread(tsk, 0) only gets called in the case where we don't
care about the return value.  It does not get called from fs/exec.c

> Finally. This patch creates the nice security hole. Let me modify my test-case
> again:
>
> 	void *thread(void *arg)
> 	{
> 		ptrace(PTRACE_TRACEME, 0,0,0);
> 		return NULL;
> 	}
>
> 	int main(void)
> 	{
> 		int pid = fork();
>
> 		if (!pid) {
> 			pthread_t pt;
> 			pthread_create(&pt, NULL, thread, NULL);
> 			pthread_join(pt, NULL);
> 			execlp(path-to-setuid-binary, args);
> 		}
>
> 		sleep(1);
>
> 		// Now we can send the signals to setiuid app
> 		kill(pid+1, ANYSIGNAL);
>
> 		return 0;
> 	}

That is a substantive objection, and something that definitely needs
to get fixed.   Can you think of anything else?

Eric