Re: [PATCHv2] exec: Fix a deadlock in ptrace

[Eric W. Biederman]

Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> writes:

> On 3/2/20 5:17 PM, Eric W. Biederman wrote:
>> Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> writes:
>> 
>>> On 3/2/20 4:57 PM, Eric W. Biederman wrote:
>>>> Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> 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.
>> 
>
> Okay, but that will reset current->in_execve, right?

Absolutely.

The error handling kicks in and exec_binprm fails with a negative
return code.  Then __do_excve_file cleans up and clears
current->in_execve.

Eric