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

[Eric W. Biederman]

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