ppp: Buffer overflow in radius plugin

From: https://bugs.debian.org/cgi-bin/bugreport.cgi?msg=5;bug=782450

Upstream-Status: Backport
CVE: CVE-2015-3310

On systems with more than 65535 processes running, pppd aborts when
sending a "start" accounting message to the RADIUS server because of a
buffer overflow in rc_mksid.

The process id is used in rc_mksid to generate a pseudo-unique string,
assuming that the hex representation of the pid will be at most 4
characters (FFFF). __sprintf_chk(), used when compiling with
optimization levels greater than 0 and FORTIFY_SOURCE, detects the
buffer overflow and makes pppd crash.

The following patch fixes the problem.

--- ppp-2.4.6.orig/pppd/plugins/radius/util.c
+++ ppp-2.4.6/pppd/plugins/radius/util.c
@@ -77,7 +77,7 @@ rc_mksid (void)
   static unsigned short int cnt = 0;
   sprintf (buf, "%08lX%04X%02hX",
 	   (unsigned long int) time (NULL),
-	   (unsigned int) getpid (),
+	   (unsigned int) getpid () % 65535,
 	   cnt & 0xFF);
   cnt++;
   return buf;