From 11ec2945c7a8f43cdc71c58ccf76846e34150830 Mon Sep 17 00:00:00 2001
From: Richard Mudgett <rmudgett@digium.com>
Date: Thu, 15 Jun 2017 12:33:22 -0500
Subject: [PATCH] chan_pjsip: Fix PJSIP_MEDIA_OFFER dialplan function read.

The construction of the returned string assumed incorrectly that the
supplied buffer would always be initialized as an empty string.  If it is
not an empty string we could overrun the supplied buffer by the length of
the non-empty buffer string plus one.  It is also theoreticaly possible
for the supplied buffer to be overrun by a string terminator during a read
operation even if the supplied buffer is an empty string.

* Fix the assumption that the supplied buffer would already be an empty
string.  The buffer is not guaranteed to contain an empty string by all
possible callers.

* Fix string terminator buffer overrun potential.

Change-Id: If6a0806806527678c8554b1dcb34fd7808aa95c9
---
 channels/pjsip/dialplan_functions.c | 35 +++++++++++++++++------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/channels/pjsip/dialplan_functions.c b/channels/pjsip/dialplan_functions.c
index 7b3434d408..e2c78cd87b 100644
--- a/channels/pjsip/dialplan_functions.c
+++ b/channels/pjsip/dialplan_functions.c
@@ -927,36 +927,40 @@ int pjsip_acf_dial_contacts_read(struct ast_channel *chan, const char *cmd, char
 static int media_offer_read_av(struct ast_sip_session *session, char *buf,
 			       size_t len, enum ast_media_type media_type)
 {
-	int i, size = 0;
+	int idx;
+	size_t accum = 0;
 
-	for (i = 0; i < ast_format_cap_count(session->req_caps); i++) {
-		struct ast_format *fmt = ast_format_cap_get_format(session->req_caps, i);
+	/* Note: buf is not terminated while the string is being built. */
+	for (idx = 0; idx < ast_format_cap_count(session->req_caps); ++idx) {
+		struct ast_format *fmt;
+		size_t size;
 
+		fmt = ast_format_cap_get_format(session->req_caps, idx);
 		if (ast_format_get_type(fmt) != media_type) {
 			ao2_ref(fmt, -1);
 			continue;
 		}
 
-		/* add one since we'll include a comma */
+		/* Add one for a comma or terminator */
 		size = strlen(ast_format_get_name(fmt)) + 1;
 		if (len < size) {
 			ao2_ref(fmt, -1);
 			break;
 		}
+
+		/* Append the format name */
+		strcpy(buf + accum, ast_format_get_name(fmt));/* Safe */
+		ao2_ref(fmt, -1);
+
+		accum += size;
 		len -= size;
 
-		/* no reason to use strncat here since we have already ensured buf has
-                   enough space, so strcat can be safely used */
-		strcat(buf, ast_format_get_name(fmt));
-		strcat(buf, ",");
-
-		ao2_ref(fmt, -1);
+		/* The last comma on the built string will be set to the terminator. */
+		buf[accum - 1] = ',';
 	}
 
-	if (size) {
-		/* remove the extra comma */
-		buf[strlen(buf) - 1] = '\0';
-	}
+	/* Remove the trailing comma or terminate an empty buffer. */
+	buf[accum ? accum - 1 : 0] = '\0';
 	return 0;
 }
 
@@ -996,6 +1000,9 @@ int pjsip_acf_media_offer_read(struct ast_channel *chan, const char *cmd, char *
 		return media_offer_read_av(channel->session, buf, len, AST_MEDIA_TYPE_AUDIO);
 	} else if (!strcmp(data, "video")) {
 		return media_offer_read_av(channel->session, buf, len, AST_MEDIA_TYPE_VIDEO);
+	} else {
+		/* Ensure that the buffer is empty */
+		buf[0] = '\0';
 	}
 
 	return 0;