[PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

[Kees Cook]

On Tue, Jun 24, 2014 at 11:37 AM, Oleg Nesterov <oleg at redhat.com> wrote:
> On 06/24, Kees Cook wrote:
>>
>> On Tue, Jun 24, 2014 at 10:27 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) {
>> >> +             pid_t failed;
>> >> +
>> >> +             if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
>> >> +                 (thread->seccomp.mode == SECCOMP_MODE_FILTER &&
>> >> +                  is_ancestor(thread->seccomp.filter,
>> >> +                              caller->seccomp.filter)))
>> >> +                     continue;
>> >> +
>> >> +             /* Return the first thread that cannot be synchronized. */
>> >> +             failed = task_pid_vnr(thread);
>> >> +             /* If the pid cannot be resolved, then return -ESRCH */
>> >> +             if (failed == 0)
>> >> +                     failed = -ESRCH;
>> >
>> > forgot to mention, task_pid_vnr() can't fail. sighand->siglock is held,
>> > for_each_thread() can't see a thread which has passed unhash_process().
>>
>> Certainly good to know, but I'd be much more comfortable leaving this
>> check as-is. Having "failed" return with "0" would be very very bad
>> (userspace would think the filter had been successfully applied, etc).
>> I'd rather stay highly defensive here.
>
> OK, agreed. Although in this case I'd suggest
>
>                 if (WARN_ON(failed == 0))
>                         failed = -ESRCH;
>
> but I won't insist.

Excellent idea, yes! I'll include this as well.

-Kees

-- 
Kees Cook
Chrome OS Security