Fix CVE-2017-2616:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-2616
http://seclists.org/oss-sec/2017/q1/490
http://seclists.org/oss-sec/2017/q1/474

Patch copied from upstream source repository:

https://github.com/shadow-maint/shadow/commit/08fd4b69e84364677a10e519ccb25b71710ee686

From 08fd4b69e84364677a10e519ccb25b71710ee686 Mon Sep 17 00:00:00 2001
From: Tobias Stoeckmann <tobias@stoeckmann.org>
Date: Thu, 23 Feb 2017 09:47:29 -0600
Subject: [PATCH] su: properly clear child PID

If su is compiled with PAM support, it is possible for any local user
to send SIGKILL to other processes with root privileges. There are
only two conditions. First, the user must be able to perform su with
a successful login. This does NOT have to be the root user, even using
su with the same id is enough, e.g. "su $(whoami)". Second, SIGKILL
can only be sent to processes which were executed after the su process.
It is not possible to send SIGKILL to processes which were already
running. I consider this as a security vulnerability, because I was
able to write a proof of concept which unlocked a screen saver of
another user this way.
---
 src/su.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/src/su.c b/src/su.c
index f20d230..d86aa86 100644
--- a/src/su.c
+++ b/src/su.c
@@ -379,11 +379,13 @@ static void prepare_pam_close_session (void)
 				/* wake child when resumed */
 				kill (pid, SIGCONT);
 				stop = false;
+			} else {
+				pid_child = 0;
 			}
 		} while (!stop);
 	}
 
-	if (0 != caught) {
+	if (0 != caught && 0 != pid_child) {
 		(void) fputs ("\n", stderr);
 		(void) fputs (_("Session terminated, terminating shell..."),
 		              stderr);
@@ -393,9 +395,22 @@ static void prepare_pam_close_session (void)
 		snprintf (wait_msg, sizeof wait_msg, _(" ...waiting for child to terminate.\n"));
 
 		(void) signal (SIGALRM, kill_child);
+		(void) signal (SIGCHLD, catch_signals);
 		(void) alarm (2);
 
-		(void) wait (&status);
+		sigemptyset (&ourset);
+		if ((sigaddset (&ourset, SIGALRM) != 0)
+		    || (sigprocmask (SIG_BLOCK, &ourset, NULL) != 0)) {
+			fprintf (stderr, _("%s: signal masking malfunction\n"), Prog);
+			kill_child (0);
+		} else {
+			while (0 == waitpid (pid_child, &status, WNOHANG)) {
+				sigsuspend (&ourset);
+			}
+			pid_child = 0;
+			(void) sigprocmask (SIG_UNBLOCK, &ourset, NULL);
+		}
+
 		(void) fputs (_(" ...terminated.\n"), stderr);
 	}
 
