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

Bernd Edlinger
2020-03-02T17:13:35Z

On 3/2/20 5:17 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>
>> On 3/2/20 4:57 PM, Eric W. Biederman wrote:
>>> Bernd Edlinger <bernd.edlinger@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.
>

Okay, but that will reset current->in_execve, right?

>>> 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.
>
> 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.
>
> Eric
>