[PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

On Tue, Jun 24, 2014 at 10:27 AM, Oleg Nesterov [off-list ref] wrote:

On 06/23, Kees Cook wrote:

quoted

+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