On 06/24, Kees Cook wrote:

+static inline void seccomp_assign_mode(struct task_struct *task,
+				       unsigned long seccomp_mode)
+{
+	BUG_ON(!spin_is_locked(&task->sighand->siglock));
+
+	task->seccomp.mode = seccomp_mode;
+	set_tsk_thread_flag(task, TIF_SECCOMP);
+}

OK, but unless task == current this can race with secure_computing().
I think this needs smp_mb__before_atomic() and secure_computing() needs
rmb() after test_bit(TIF_SECCOMP).

Otherwise, can't __secure_computing() hit BUG() if it sees the old
mode == SECCOMP_MODE_DISABLED ?

Or seccomp_run_filters() can see ->filters == NULL and WARN(),
smp_load_acquire() only serializes that LOAD with the subsequent memory
operations.

Oleg.