On 06/24, Kees Cook wrote:
>
> +static void seccomp_sync_threads(void)
> +{
> + struct task_struct *thread, *caller;
> +
> + BUG_ON(!spin_is_locked(¤t->sighand->siglock));
> +
> + /* Synchronize all threads. */
> + caller = current;
> + for_each_thread(caller, thread) {
> + /* Get a task reference for the new leaf node. */
> + get_seccomp_filter(caller);
> + /*
> + * Drop the task reference to the shared ancestor since
> + * current's path will hold a reference. (This also
> + * allows a put before the assignment.)
> + */
> + put_seccomp_filter(thread);
> + thread->seccomp.filter = caller->seccomp.filter;
> + /* Opt the other thread into seccomp if needed.
> + * As threads are considered to be trust-realm
> + * equivalent (see ptrace_may_access), it is safe to
> + * allow one thread to transition the other.
> + */
> + if (thread->seccomp.mode == SECCOMP_MODE_DISABLED) {
> + /*
> + * Don't let an unprivileged task work around
> + * the no_new_privs restriction by creating
> + * a thread that sets it up, enters seccomp,
> + * then dies.
> + */
> + if (task_no_new_privs(caller))
> + task_set_no_new_privs(thread);
> +
> + seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);
> + }
> + }
> +}
OK, personally I think this all make sense. I even think that perhaps
SECCOMP_FILTER_FLAG_TSYNC should allow filter == NULL, a thread might
want to "sync" without adding the new filter, but this is minor/offtopic.
But. Doesn't this change add the new security hole?
Obviously, we should not allow to install a filter and then (say) exec
a suid binary, that is why we have no_new_privs/LSM_UNSAFE_NO_NEW_PRIVS.
But what if "thread->seccomp.filter = caller->seccomp.filter" races with
any user of task_no_new_privs() ? Say, suppose this thread has already
passed check_unsafe_exec/etc and it is going to exec the suid binary?
Oleg.
