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

Bernd Edlinger
2020-03-04T21:49:41Z

On 3/4/20 5:33 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>
>> On 3/3/20 9:08 PM, Eric W. Biederman wrote:
>>> 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
>>>
>>
>> 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.
>
>
> Let me propose a slight alternative that I think sets us up for long
> term success.
>
> Leave cred_guard_mutex as is, but declare it undesirable. The
> cred_guard_mutex as designed really is something we should get rid of.
> As it it can sleep over several different userspace accesses. The
> copying of the exec arguments is technically as prone to deadlock as the
> ptrace case.
>
> Add a new mutex with a better name perhaps "exec_change_mutex" that is
> used to guard the changes that exec makes to a process.
>
> Then we gradually shift all the cred_guard_mutex users over to the new
> mutex. AKA one patch per user of cred_guard_mutex. At each patch that
> shifts things over we will have the opportunity to review the code to
> see that there no funny dependencies that were missed.
>
> I will sign up for working on the no_new_privs and ptrace_attach cases
> as I think I can make those happen. Especially no_new_privs.
>
> Getting the easier cases will resolve your issues and put things on a
> better footing.
>
> Eric
>

Okay, however I think we will need two mutexes in the long term.

So currently I have reduced the cred_guard_mutex to protect just
the loading of the executable code in the process vm, since that
is what works for vm_access, (one of the test cases).
And another mutex that protects the whole execve function, that
is need for ptrace, (and seccomp).
But I have only a test case for ptrace.

If I understand that right, I should not recycle cred_guard_mutex
but leave it as is, and create two additional mutexes which will
take over step by step.

Sounds reasonable, indeed.

I will send an update (v6) what I have right now,
but just for information, so you can see how my minimal API-Change
approach works.

Bernd.