shadow-4.2.1: Fix CVE-2017-2616

[No upstream tracking] -- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=855943

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.

Upstream-Status: Backport [https://github.com/shadow-maint/shadow/commit/08fd4b69e84364677a10e519ccb25b71710ee686]
CVE: CVE-2017-2616
bug: 855943
Signed-off-by: Andrej Valek <andrej.valek@siemens.com>

diff --git a/src/su.c b/src/su.c
index 3704217..1efcd61 100644
--- a/src/su.c
+++ b/src/su.c
@@ -363,20 +363,35 @@ 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);
 		(void) kill (-pid_child, caught);
 
 		(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);
 	}