On 06/24, Kees Cook wrote:
>
> 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(¤t->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.
Oleg.
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