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

Bernd Edlinger
2020-03-02T16:06:05Z

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.

> 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.
>
> Eric
>