From bf00ae0c2addf584baf6ad626940d1836f7fa278 Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm@xmission.com>
Date: Thu, 19 Jul 2018 19:47:27 -0500
Subject: [PATCH] signal: Always ignore SIGKILL and SIGSTOP sent to the global
 init

If the first process started (aka /sbin/init) receives a SIGKILL it
will panic the system if it is delivered.  Making the system unusable
and undebugable.  It isn't much better if the first process started
receives SIGSTOP.

So always ignore SIGSTOP and SIGKILL sent to init.

This is done in a separate clause in sig_task_ignored as force_sig_info
can clear SIG_UNKILLABLE and this protection should work even then.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/signal.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/signal.c b/kernel/signal.c
index 5843c541fda9c..b33264bb2064f 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -78,6 +78,10 @@ static bool sig_task_ignored(struct task_struct *t, int sig, bool force)
 
 	handler = sig_handler(t, sig);
 
+	/* SIGKILL and SIGSTOP may not be sent to the global init */
+	if (unlikely(is_global_init(t) && sig_kernel_only(sig)))
+		return true;
+
 	if (unlikely(t->signal->flags & SIGNAL_UNKILLABLE) &&
 	    handler == SIG_DFL && !(force && sig_kernel_only(sig)))
 		return true;
-- 
2.39.5