Re: [PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

[Kees Cook <keescook@xxxxxxxxxxxx>]

On Tue, Jun 24, 2014 at 10:27 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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.

-Kees

-- 
Kees Cook
Chrome OS Security