seccomp: allow mode setting across threads

This changes the mode setting helper to allow threads to change the
seccomp mode from another thread. We must maintain barriers to keep
TIF_SECCOMP synchronized with the rest of the seccomp state.

Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Oleg Nesterov <[e-mail]>
Reviewed-by: Andy Lutomirski <[e-mail]>

kernel/seccomp.c

···
  */
 static u32 seccomp_run_filters(int syscall)
 {
-	struct seccomp_filter *f;
+	struct seccomp_filter *f = ACCESS_ONCE(current->seccomp.filter);
 	struct seccomp_data sd;
 	u32 ret = SECCOMP_RET_ALLOW;
 
 	/* Ensure unexpected behavior doesn't result in failing open. */
-	if (WARN_ON(current->seccomp.filter == NULL))
+	if (unlikely(WARN_ON(f == NULL)))
 		return SECCOMP_RET_KILL;
+
+	/* Make sure cross-thread synced filter points somewhere sane. */
+	smp_read_barrier_depends();
 
 	populate_seccomp_data(&sd);
 
···
 	 * All filters in the list are evaluated and the lowest BPF return
 	 * value always takes priority (ignoring the DATA).
 	 */
-	for (f = current->seccomp.filter; f; f = f->prev) {
+	for (; f; f = f->prev) {
 		u32 cur_ret = SK_RUN_FILTER(f->prog, (void *)&sd);
 
 		if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
···
 	return true;
 }
 
-static inline void seccomp_assign_mode(unsigned long seccomp_mode)
+static inline void seccomp_assign_mode(struct task_struct *task,
+				       unsigned long seccomp_mode)
 {
-	BUG_ON(!spin_is_locked(&current->sighand->siglock));
+	BUG_ON(!spin_is_locked(&task->sighand->siglock));
 
-	current->seccomp.mode = seccomp_mode;
-	set_tsk_thread_flag(current, TIF_SECCOMP);
+	task->seccomp.mode = seccomp_mode;
+	/*
+	 * Make sure TIF_SECCOMP cannot be set before the mode (and
+	 * filter) is set.
+	 */
+	smp_mb__before_atomic();
+	set_tsk_thread_flag(task, TIF_SECCOMP);
 }
 
 #ifdef CONFIG_SECCOMP_FILTER
···
 
 int __secure_computing(int this_syscall)
 {
-	int mode = current->seccomp.mode;
 	int exit_sig = 0;
 	int *syscall;
 	u32 ret;
 
-	switch (mode) {
+	/*
+	 * Make sure that any changes to mode from another thread have
+	 * been seen after TIF_SECCOMP was seen.
+	 */
+	rmb();
+
+	switch (current->seccomp.mode) {
 	case SECCOMP_MODE_STRICT:
 		syscall = mode1_syscalls;
 #ifdef CONFIG_COMPAT
···
 #ifdef TIF_NOTSC
 	disable_TSC();
 #endif
-	seccomp_assign_mode(seccomp_mode);
+	seccomp_assign_mode(current, seccomp_mode);
 	ret = 0;
 
 out:
···
 	/* Do not free the successfully attached filter. */
 	prepared = NULL;
 
-	seccomp_assign_mode(seccomp_mode);
+	seccomp_assign_mode(current, seccomp_mode);
 out:
 	spin_unlock_irq(&current->sighand->siglock);
 	seccomp_filter_free(prepared);