[PATCHv2] exec: Fix a deadlock in ptrace

Bernd Edlinger bernd.edlinger at hotmail.de
Mon Mar 2 17:01:15 UTC 2020

On 3/2/20 5:43 PM, Jann Horn wrote:
> On Mon, Mar 2, 2020 at 5:19 PM Eric W. Biederman <ebiederm at xmission.com> wrote:
>> Bernd Edlinger <bernd.edlinger at hotmail.de> writes:
>>> On 3/2/20 4:57 PM, Eric W. Biederman wrote:
>>>> Bernd Edlinger <bernd.edlinger at hotmail.de> writes:
>>>>> I tried this with s/EACCESS/EACCES/.
>>>>> The test case in this patch is not fixed, but strace does not freeze,
>>>>> at least with my setup where it did freeze repeatable.
>>>> Thanks, That is what I was aiming at.
>>>> So we have one method we can pursue to fix this in practice.
>>>>> That is
>>>>> obviously because it bypasses the cred_guard_mutex.  But all other
>>>>> process that access this file still freeze, and cannot be
>>>>> interrupted except with kill -9.
>>>>> However that smells like a denial of service, that this
>>>>> simple test case which can be executed by guest, creates a /proc/$pid/mem
>>>>> that freezes any process, even root, when it looks at it.
>>>>> I mean: "ln -s README /proc/$pid/mem" would be a nice bomb.
>>>> Yes.  Your the test case in your patch a variant of the original
>>>> problem.
>>>> I have been staring at this trying to understand the fundamentals of the
>>>> original deeper problem.
>>>> The current scope of cred_guard_mutex in exec is because being ptraced
>>>> causes suid exec to act differently.  So we need to know early if we are
>>>> ptraced.
>>> It has a second use, that it prevents two threads entering execve,
>>> which would probably result in disaster.
>> Exec can fail with an error code up until de_thread.  de_thread causes
>> exec to fail with the error code -EAGAIN for the second thread to get
>> into de_thread.
>> So no.  The cred_guard_mutex is not needed for that case at all.
>>>> If that case did not exist we could reduce the scope of the
>>>> cred_guard_mutex in exec to where your patch puts the cred_change_mutex.
>>>> I am starting to think reworking how we deal with ptrace and exec is the
>>>> way to solve this problem.
>> I am 99% convinced that the fix is to move cred_guard_mutex down.
> "move cred_guard_mutex down" as in "take it once we've already set up
> the new process, past the point of no return"?
>> Then right after we take cred_guard_mutex do:
>>         if (ptraced) {
>>                 use_original_creds();
>>         }
>> And call it a day.
>> The details suck but I am 99% certain that would solve everyones
>> problems, and not be too bad to audit either.
> Ah, hmm, that sounds like it'll work fine at least when no LSMs are involved.
> SELinux normally doesn't do the execution-degrading thing, it just
> blocks the execution completely - see their selinux_bprm_set_creds()
> hook. So I think they'd still need to set some state on the task that
> says "we're currently in the middle of an execution where the target
> task will run in context X", and then check against that in the
> ptrace_may_access hook. Or I suppose they could just kill the task
> near the end of execve, although that'd be kinda ugly.

We have current->in_execve for that, right?
I think when the cred_guard_mutex is taken only in the critical section,
then PTRACE_ATTACH could take the guard_mutex, and look at current->in_execve,
and just return -EAGAIN in that case, right, everybody happy :)


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