[PATCH 3/3] smpp_smsc: Fix 32-bit signed integer overflow in read return value

[Daniel Willmann]

In case the length is 2^31+4 or larger we compute a negative value to
read in rdlen (since it is a signed int). This results in read failing
with EFAULT.

This patch changes the type of rdlen and rc to ssize_t (the return value
of read) and guards against the read length being larger than
(SSIZE_MAX).

To reproduce the issue run:
echo -e "\x80\x00\x00\x05\x00" |socat stdin tcp:localhost:2775
---
 openbsc/src/libmsc/smpp_smsc.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/openbsc/src/libmsc/smpp_smsc.c b/openbsc/src/libmsc/smpp_smsc.c
index a3dc311..23acfc2 100644
--- a/openbsc/src/libmsc/smpp_smsc.c
+++ b/openbsc/src/libmsc/smpp_smsc.c
@@ -23,6 +23,7 @@
 #include <string.h>
 #include <stdint.h>
 #include <errno.h>
+#include <limits.h>
 
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -770,8 +771,7 @@ static int esme_link_read_cb(struct osmo_fd *ofd)
 	uint8_t *lenptr = (uint8_t *) &len;
 	uint8_t *cur;
 	struct msgb *msg;
-	int rdlen;
-	int rc;
+	ssize_t rdlen, rc;
 
 	switch (esme->read_state) {
 	case READ_ST_IN_LEN:
@@ -781,7 +781,7 @@ static int esme_link_read_cb(struct osmo_fd *ofd)
 			/* EINTR is a non-fatal error, just try again */
 			if (errno == EINTR)
 				return 0;
-			LOGP(DSMPP, LOGL_ERROR, "[%s] read returned %d (%s)\n",
+			LOGP(DSMPP, LOGL_ERROR, "[%s] read returned %zd (%s)\n",
 			     esme->system_id, rc, strerror(errno));
 			goto dead_socket;
 		} else if (rc == 0) {
@@ -790,8 +790,8 @@ static int esme_link_read_cb(struct osmo_fd *ofd)
 			esme->read_idx += rc;
 		if (esme->read_idx >= sizeof(uint32_t)) {
 			esme->read_len = ntohl(len);
-			if (esme->read_len <= 4) {
-				LOGP(DSMPP, LOGL_ERROR, "[%s] read length too small %d\n",
+			if (esme->read_len <= 4 || esme->read_len > SSIZE_MAX) {
+				LOGP(DSMPP, LOGL_ERROR, "[%s] length invalid %d\n",
 						esme->system_id, esme->read_len);
 				goto dead_socket;
 			}
@@ -811,7 +811,7 @@ static int esme_link_read_cb(struct osmo_fd *ofd)
 		rdlen = esme->read_len - esme->read_idx;
 		rc = read(ofd->fd, msg->tail, OSMO_MIN(rdlen, msgb_tailroom(msg)));
 		if (rc < 0) {
-			LOGP(DSMPP, LOGL_ERROR, "[%s] read returned %d (%s)\n",
+			LOGP(DSMPP, LOGL_ERROR, "[%s] read returned %zd (%s)\n",
 				esme->system_id, rc, strerror(errno));
 			goto dead_socket;
 		} else if (rc == 0) {
-- 
1.8.4.2