Re: [PATCHv5] exec: Fix a deadlock in ptrace

[Bernd Edlinger]

On 3/3/20 9:08 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> writes:
> 
>> On 3/3/20 4:18 PM, Eric W. Biederman wrote:
>>> Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> writes:
>>>> diff --git a/tools/testing/selftests/ptrace/vmaccess.c b/tools/testing/selftests/ptrace/vmaccess.c
>>>> new file mode 100644
>>>> index 0000000..6d8a048
>>>> --- /dev/null
>>>> +++ b/tools/testing/selftests/ptrace/vmaccess.c
>>>> @@ -0,0 +1,66 @@
>>>> +// SPDX-License-Identifier: GPL-2.0+
>>>> +/*
>>>> + * Copyright (c) 2020 Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
>>>> + * All rights reserved.
>>>> + *
>>>> + * Check whether /proc/$pid/mem can be accessed without causing deadlocks
>>>> + * when de_thread is blocked with ->cred_guard_mutex held.
>>>> + */
>>>> +
>>>> +#include "../kselftest_harness.h"
>>>> +#include <stdio.h>
>>>> +#include <fcntl.h>
>>>> +#include <pthread.h>
>>>> +#include <signal.h>
>>>> +#include <unistd.h>
>>>> +#include <sys/ptrace.h>
>>>> +
>>>> +static void *thread(void *arg)
>>>> +{
>>>> +	ptrace(PTRACE_TRACEME, 0, 0L, 0L);
>>>> +	return NULL;
>>>> +}
>>>> +
>>>> +TEST(vmaccess)
>>>> +{
>>>> +	int f, pid = fork();
>>>> +	char mm[64];
>>>> +
>>>> +	if (!pid) {
>>>> +		pthread_t pt;
>>>> +
>>>> +		pthread_create(&pt, NULL, thread, NULL);
>>>> +		pthread_join(pt, NULL);
>>>> +		execlp("true", "true", NULL);
>>>> +	}
>>>> +
>>>> +	sleep(1);
>>>> +	sprintf(mm, "/proc/%d/mem", pid);
>>>> +	f = open(mm, O_RDONLY);
>>>> +	ASSERT_LE(0, f);
>>>> +	close(f);
>>>> +	f = kill(pid, SIGCONT);
>>>> +	ASSERT_EQ(0, f);
>>>> +}
>>>> +
>>>> +TEST(attach)
>>>> +{
>>>> +	int f, pid = fork();
>>>> +
>>>> +	if (!pid) {
>>>> +		pthread_t pt;
>>>> +
>>>> +		pthread_create(&pt, NULL, thread, NULL);
>>>> +		pthread_join(pt, NULL);
>>>> +		execlp("true", "true", NULL);
>>>> +	}
>>>> +
>>>> +	sleep(1);
>>>> +	f = ptrace(PTRACE_ATTACH, pid, 0L, 0L);
>>>
>>> To be meaningful this code needs to learn to loop when
>>> ptrace returns -EAGAIN.
>>>
>>> Because that is pretty much what any self respecting user space
>>> process will do.
>>>
>>> At which point I am not certain we can say that the behavior has
>>> sufficiently improved not to be a deadlock.
>>>
>>
>> In this special dead-duck test it won't work, but it would
>> still be lots more transparent what is going on, since previously
>> you had two zombie process, and no way to even output debug
>> messages, which also all self respecting user space processes
>> should do.
> 
> Agreed it is more transparent.  So if you are going to deadlock
> it is better.
> 
> My previous proposal (which I admit is more work to implement) would
> actually allow succeeding in this case and so it would not be subject to
> a dead lock (even via -EGAIN) at this point.
> 
>> So yes, I can at least give a good example and re-try it several
>> times together with wait4 which a tracer is expected to do.
> 
> Thank you,
> 
> Eric
> 

Okay, I think it can be done with minimal API changes,
but it needs two mutexes, one that guards the execve,
and one that guards only the credentials.

If no traced sibling thread exists, the mutexes are used this way:
lock(exec_guard_mutex)
cred_locked_in_execve = true;
de_thread()
lock(cred_guard_mutex)
unlock(cred_guard_mutex)
cred_locked_in_execve = false;
unlock(exec_guard_mutex)

so effectively no API change at all.

If a traced sibling thread exists, the mutexes are used differently:
lock(exec_guard_mutex)
cred_locked_in_execve = true;
unlock(exec_guard_mutex)
de_thread()
lock(cred_guard_mutex)
unlock(cred_guard_mutex)
lock(exec_guard_mutex)
cred_locked_in_execve = false;
unlock(exec_guard_mutex)

Only the case changes that would deadlock anyway.


Bernd.