[PATCH AUTOSEL 4.4 25/44] signal: Always ignore SIGKILL and SIGSTOP sent to the global init

[Sasha Levin]

From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

[ Upstream commit 86989c41b5ea08776c450cb759592532314a4ed6 ]

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@xxxxxxxxxxxxx>
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/signal.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/signal.c b/kernel/signal.c
index 072fd152ab01e..3095b2309876d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -71,6 +71,10 @@ static int 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 1;
-- 
2.20.1