[PATCH -mm v2] do_notify_parent: sanitize the valid_signal() checks

[Oleg Nesterov <oleg@redhat.com>]

Now that kernel_clone() checks valid_signal(args->exit_signal), the "sig"
argument of do_notify_parent() must always be valid or we have a bug.

However, do_notify_parent() only checks that sig != -1 at the start, then
it does another valid_signal() check before __send_signal_locked().

This is confusing. Change do_notify_parent() to WARN and return early if
valid_signal(sig) is false.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 86aad7badb9a..683ef92f7234 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2171,7 +2171,8 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
 	bool autoreap = false;
 	u64 utime, stime;
 
-	WARN_ON_ONCE(sig == -1);
+	if (WARN_ON_ONCE(!valid_signal(sig)))
+		return false;
 
 	/* do_notify_parent_cldstop should have been called instead.  */
 	WARN_ON_ONCE(task_is_stopped_or_traced(tsk));
@@ -2252,7 +2253,7 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
 	 * Send with __send_signal as si_pid and si_uid are in the
 	 * parent's namespaces.
 	 */
-	if (valid_signal(sig) && sig)
+	if (sig)
 		__send_signal_locked(sig, &info, tsk->parent, PIDTYPE_TGID, false);
 	__wake_up_parent(tsk, tsk->parent);
 	spin_unlock_irqrestore(&psig->siglock, flags);
-- 
2.52.0