[PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

[Kees Cook]

On Tue, Jun 24, 2014 at 10:08 AM, Oleg Nesterov <oleg at redhat.com> wrote:
> On 06/23, Kees Cook wrote:
>>
>> +static pid_t seccomp_can_sync_threads(void)
>> +{
>> +     struct task_struct *thread, *caller;
>> +
>> +     BUG_ON(write_can_lock(&tasklist_lock));
>> +     BUG_ON(!spin_is_locked(&current->sighand->siglock));
>> +
>> +     if (current->seccomp.mode != SECCOMP_MODE_FILTER)
>> +             return -EACCES;
>> +
>> +     /* Validate all threads being eligible for synchronization. */
>> +     thread = caller = current;
>> +     for_each_thread(caller, thread) {
>
> You only need to initialize "caller" for for_each_thread(). Same for
> seccomp_sync_threads().

Thanks, I'll fix this up.

>> @@ -586,6 +701,17 @@ static long seccomp_set_mode_filter(unsigned int flags,
>>       if (IS_ERR(prepared))
>>               return PTR_ERR(prepared);
>>
>> +     /*
>> +      * If we're doing thread sync, we must hold tasklist_lock
>> +      * to make sure seccomp filter changes are stable on threads
>> +      * entering or leaving the task list. And we must take it
>> +      * before the sighand lock to avoid deadlocking.
>> +      */
>> +     if (flags & SECCOMP_FILTER_FLAG_TSYNC)
>> +             write_lock_irqsave(&tasklist_lock, taskflags);
>> +     else
>> +             __acquire(&tasklist_lock); /* keep sparse happy */
>> +
>
> Why? ->siglock should be enough, it seems.
>
> It obviously does not protect the global process list, but *sync_threads()
> only care about current's thread group list, no?

I think I was concerned about the exit case, but reading through those
paths again, I can't find a race. Calls to put_seccomp_filter() should
already be safe. Let me see what happens if I drop the tasklist_lock
usage...

-Kees


-- 
Kees Cook
Chrome OS Security