Patch "signal: protect SIGNAL_UNKILLABLE from unintentional clearing." has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

signal: protect SIGNAL_UNKILLABLE from unintentional clearing.

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
signal-protect-signal_unkillable-from-unintentional-clearing.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jamie Iles <[email protected]>
Date: Tue, 10 Jan 2017 16:57:54 -0800
Subject: signal: protect SIGNAL_UNKILLABLE from unintentional clearing.

From: Jamie Iles <[email protected]>

[ Upstream commit 2d39b3cd34e6d323720d4c61bd714f5ae202c022 ]

Since commit 00cd5c37afd5 ("ptrace: permit ptracing of /sbin/init") we can now trace init processes. init is initially protected with SIGNAL_UNKILLABLE which will prevent fatal signals such as SIGSTOP, but there are a number of paths during tracing where SIGNAL_UNKILLABLE can be implicitly cleared.

This can result in init becoming stoppable/killable after tracing. For example, running:

while true; do kill -STOP 1; done &
strace -p 1

and then stopping strace and the kill loop will result in init being left in state TASK_STOPPED. Sending SIGCONT to init will resume it, but init will now respond to future SIGSTOP signals rather than ignoring them.

Make sure that when setting SIGNAL_STOP_CONTINUED/SIGNAL_STOP_STOPPED that we don't clear SIGNAL_UNKILLABLE.

---
 include/linux/sched.h |   10 ++++++++++
 kernel/signal.c       |    4 ++--
 2 files changed, 12 insertions(+), 2 deletions(-)

@@ -830,6 +830,16 @@ struct signal_struct {
 
 #define SIGNAL_UNKILLABLE	0x00000040 /* for init: ignore fatal signals */
 
+#define SIGNAL_STOP_MASK (SIGNAL_CLD_MASK | SIGNAL_STOP_STOPPED | \
+			  SIGNAL_STOP_CONTINUED)
+
+static inline void signal_set_stop_flags(struct signal_struct *sig,
+					 unsigned int flags)
+{
+	WARN_ON(sig->flags & (SIGNAL_GROUP_EXIT|SIGNAL_GROUP_COREDUMP));
+	sig->flags = (sig->flags & ~SIGNAL_STOP_MASK) | flags;
+}
+
 /* If true, all threads except ->group_exit_task have pending SIGKILL */
 static inline int signal_group_exit(const struct signal_struct *sig)
 {

@@ -346,7 +346,7 @@ static bool task_participate_group_stop(
 	 * fresh group stop.  Read comment in do_signal_stop() for details.
 	 */
 	if (!sig->group_stop_count && !(sig->flags & SIGNAL_STOP_STOPPED)) {
-		sig->flags = SIGNAL_STOP_STOPPED;
+		signal_set_stop_flags(sig, SIGNAL_STOP_STOPPED);
 		return true;
 	}
 	return false;

@@ -845,7 +845,7 @@ static bool prepare_signal(int sig, stru
 			 * will take ->siglock, notice SIGNAL_CLD_MASK, and
 			 * notify its parent. See get_signal_to_deliver().
 			 */
-			signal->flags = why | SIGNAL_STOP_CONTINUED;
+			signal_set_stop_flags(signal, why | SIGNAL_STOP_CONTINUED);
 			signal->group_stop_count = 0;
 			signal->group_exit_code = 0;
 		}

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/signal-protect-signal_unkillable-from-unintentional-clearing.patch