signal: Always deliver the kernel's SIGKILL and SIGSTOP to a pid namespace init

Instead of playing whack-a-mole and changing SEND_SIG_PRIV to
SEND_SIG_FORCED throughout the kernel to ensure a pid namespace init
gets signals sent by the kernel, stop allowing a pid namespace init to
ignore SIGKILL or SIGSTOP sent by the kernel.  A pid namespace init is
only supposed to be able to ignore signals sent from itself and
children with SIG_DFL.

Fixes: 921cf9f63089 ("signals: protect cinit from unblocked SIG_DFL signals")
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

kernel/signal.c

@@ -1039,7 +1039,7 @@ static int __send_signal(int sig, struct siginfo *info, struct task_struct *t,
 
 	result = TRACE_SIGNAL_IGNORED;
 	if (!prepare_signal(sig, t,
-			from_ancestor_ns || (info == SEND_SIG_FORCED)))
+			from_ancestor_ns || (info == SEND_SIG_PRIV) || (info == SEND_SIG_FORCED)))
 		goto ret;
 
 	pending = (type != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending;