[PATCH 4.14 091/239] signal: Always ignore SIGKILL and SIGSTOP sent to the global init

From: Greg Kroah-Hartman
Date: Tue Nov 19 2019 - 00:47:57 EST

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 bb801156628ee..c9b203875001e 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -77,6 +77,10 @@ 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;