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

Bernd Edlinger
2020-03-02T15:56:16Z

On 3/2/20 1:28 PM, Oleg Nesterov wrote:
> On 03/01, Bernd Edlinger wrote:
>>
>> This fixes a deadlock in the tracer when tracing a multi-threaded
>> application that calls execve while more than one thread are running.
>
> Heh. Yes, known problem. See my attempt to fix it:
> https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
>
>> @@ -1224,7 +1224,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
>> struct mm_struct *mm;
>> int err;
>>
>> - err = mutex_lock_killable(&task->signal->cred_guard_mutex);
>> + err = mutex_lock_killable(&task->signal->cred_change_mutex);
>
> So if I understand correctly your patch doesn't fix other problems
> with debugger waiting for cred_guard_mutex.
>

No, but I see this just as a first step.

> I too do not think this can justify the new mutex in signal_struct...
>

I think for the vm_access the semantic of this mutex is clear, that it
prevents the credentials to change while it is held by vm_access,
and probably other places can take advantage of this mutex as well.

While on the other hand, the cred_guard_mutex is needed to avoid two
threads calling execve at the same time. So that is needed as well.

What remains is probably making PTHREAD_ATTACH detect that the process
is currently in execve, and make that call fail in that situation.
I have not thought in depth about that problem, but it will probably
just need the right mutex to access current->in_execve.

That's at least how I see it.

Thanks
Bernd.