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

Eric W. Biederman
2020-03-03T20:11:16Z

Bernd Edlinger <bernd.edlinger@hotmail.de> writes:

> On 3/3/20 4:18 PM, Eric W. Biederman wrote:
>> Bernd Edlinger <bernd.edlinger@hotmail.de> 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@hotmail.de>
>>> + * 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