[v3,1/8] exec: introduce cred_guard_light

ebiederm@xmission.com (Eric W. Biederman) writes:

> Oleg Nesterov <oleg@redhat.com> writes:
>
>> On 11/02, Jann Horn wrote:
>>>
>>> On Wed, Nov 02, 2016 at 07:18:06PM +0100, Oleg Nesterov wrote:
>>> > On 10/30, Jann Horn wrote:
>>> > >
>>> > > This is a new per-threadgroup lock that can often be taken instead of
>>> > > cred_guard_mutex and has less deadlock potential. I'm doing this because
>>> > > Oleg Nesterov mentioned the potential for deadlocks, in particular if a
>>> > > debugged task is stuck in execve, trying to get rid of a ptrace-stopped
>>> > > thread, and the debugger attempts to inspect procfs files of the debugged
>>> > > task.
>>> >
>>> > Yes, but let me repeat that we need to fix this anyway. So I don't really
>>> > understand why should we add yet another mutex.
>>>
>>> execve() only takes the new mutex immediately after de_thread(), so this
>>> problem shouldn't occur there.
>>
>> Yes, I see.
>>
>>> Basically, I think that I'm not making the
>>> problem worse with my patches this way.
>>
>> In a sense that it doesn't add the new deadlocks, I agree. But it adds
>> yet another per-process mutex while we already have the similar one,
>>
>>> I believe that it should be possible to convert most existing users of the
>>> cred_guard_mutex to the new cred_guard_light - exceptions to that that I
>>> see are:
>>>
>>>  - PTRACE_ATTACH
>>
>> This is the main problem afaics. So "strace -f" can hang if it races
>> with mt-exec. And we need to fix this. I constantly forget about this
>> problem, but I tried many times to find a reasonable solution, still
>> can't.
>>
>> IMO, it would be nice to rework the lsm hooks, so that we could take
>> cred_guard_mutex after de_thread() (like your cred_guard_light) or
>> at least drop it earlier, but unlikely this is possible...
>>
>> So the only plan I currently have is change de_thread() to wait until
>> other threads pass exit_notify() or even exit_signals(), but I don't
>> like this.
>>
>>>  - SECCOMP_FILTER_FLAG_TSYNC (sets NO_NEW_PRIVS on remote task)
>>
>> I forgot about this one... Need to re-check but at first glance this
>> is not a real problem.
>>
>>> Beyond that, conceptually, the new cred_guard_light could also be turned
>>> into a read-write mutex
>>
>> Not sure I understand how this can help... doesn't matter.
>>
>> My point is, imo you should not add the new mutex. Just use the old
>> one in (say) 4/8 (which I do not personally like as you know ;), this
>> won't add the new problem.
>>
>>
>>> It seems to me like SECCOMP_FILTER_FLAG_TSYNC doesn't really have
>>> deadlocking issues.
>>
>> Yes, agreed.
>>
>>> PTRACE_ATTACH isn't that clear to me; if a debugger
>>> tries to attach to a newly spawned thread while another ptraced thread is
>>> dying because of de_thread() in a third thread, that might still cause
>>> the debugger to deadlock, right?
>>
>> This is the trivial test-case I wrote when the problem was initially
>> reported. And damn, I always knew that cred_guard_mutex needs fixes,
>> but somehow I completely forgot that it is used by PTRACE_ATTACH when
>> I was going to try to remove from fs/proc a long ago.
>>
>> 	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("echo", "echo", "passed", NULL);
>> 		}
>>
>> 		sleep(1);
>> 		// or anything else which needs ->cred_guard_mutex,
>> 		// say open(/proc/$pid/mem)
>> 		ptrace(PTRACE_ATTACH, pid, 0,0);
>> 		kill(pid, SIGCONT);
>>
>> 		return 0;
>> 	}
>>
>> The problem is trivial. The execing thread waits until its sub-thread
>> goes away, it should be reaped by the tracer, the tracer waits for
>> cred_guard_mutex.
>
> There is a bug here but I don't believe it has anything to do with
> the cred_guard_mutex.
>
> If we reach zap_other_threads fundamentally the tracer should not
> be able to block the traced thread from exiting.  Those are the
> semantics described in the comments in the code.
>
> I have poked things a little and have a half fix for that but
> the fix appears to be the wrong, but enlightening.
>
> AKA the following prevents the hang of your test case.
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 75761acc77cf..a6f83450500e 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1200,7 +1200,7 @@ int zap_other_threads(struct task_struct *p)
>  		if (t->exit_state)
>  			continue;
>  		sigaddset(&t->pending.signal, SIGKILL);
> -		signal_wake_up(t, 1);
> +		signal_wake_up_state(t, TASK_WAKEKILL | __TASK_TRACED);
>  	}
>  
>  	return count;
>
> It looks like somewhere on the exit path the traced thread is blocking
> without setting TASK_WAKEKILL.

Apologies there was a testing mistake and that patch does not actually
help anything.

The following mostly correct patch modifies zap_other_threads in
the case of a de_thread to not wait for zombies to be reaped.  The only
case that cares is ptrace (as threads are self reaping).  So I don't
think this will cause any problems except removing the strace -f race.

Not waiting for zombies to be reaped in de_thread keeps the kernel from
holding the cred_guard_mutex while waiting for userspace.  Which should
mean we don't have to move it.

Not waiting for zombies to be reaped should also speed of mt-exec.  So I
think this is a benefit all around.

Eric
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[v3,1/8] exec: introduce cred_guard_light

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