osmo_bsc_bssap: check bssmap length field

At the moment the length field of the bssmap header is not parsed.
Instead the length is computed out of the known header length and the
number of bytes received. This is prone to error, lets make sure that
extranous data at the end of a message is ignored by parsing the bssmap
length correctly.

Change-Id: Idef2e783d2377a2ad1f697ea4d26491a32b3e549
Related: OS#3806

1 files changed, 31 insertions, 1 deletions

diff --git a/src/osmo-bsc/osmo_bsc_bssap.c b/src/osmo-bsc/osmo_bsc_bssap.c
index 85aab22d2..65618fdb3 100644
--- a/src/osmo-bsc/osmo_bsc_bssap.c
+++ b/src/osmo-bsc/osmo_bsc_bssap.c
@@ -1081,6 +1081,36 @@ int bsc_handle_udt(struct bsc_msc_data *msc,
 	return 0;
 }
 
+/* Extract and verify the length information from the BSSMAP header. */
+static unsigned int bssmap_msg_len(struct msgb *msg, unsigned int length,
+				   const struct gsm_subscriber_connection *conn)
+{
+	unsigned int expected_len;
+	unsigned int calculated_len;
+	struct bssmap_header *bssmap_header;
+
+	bssmap_header = (struct bssmap_header *)msg->l3h;
+
+	calculated_len = length - sizeof(struct bssmap_header);
+	expected_len = bssmap_header->length;
+
+	/* In case of contradictory length information, decide for the
+	 * shorter length */
+	if (calculated_len > expected_len) {
+		LOGPFSML(conn->fi, LOGL_NOTICE,
+			 "BSSMAP message contains extra data, expected %u bytes, got %u bytes, truncated\n",
+			 expected_len, calculated_len);
+		return expected_len;
+	} else if (calculated_len < expected_len) {
+		LOGPFSML(conn->fi, LOGL_NOTICE,
+			 "Short BSSMAP message, expected %u bytes, got %u bytes\n",
+			 expected_len, calculated_len);
+		return calculated_len;
+	}
+
+	return expected_len;
+}
+
 int bsc_handle_dt(struct gsm_subscriber_connection *conn,
 		  struct msgb *msg, unsigned int len)
 {
@@ -1093,7 +1123,7 @@ int bsc_handle_dt(struct gsm_subscriber_connection *conn,
 	switch (msg->l3h[0]) {
 	case BSSAP_MSG_BSS_MANAGEMENT:
 		msg->l4h = &msg->l3h[sizeof(struct bssmap_header)];
-		bssmap_rcvmsg_dt1(conn, msg, len - sizeof(struct bssmap_header));
+		bssmap_rcvmsg_dt1(conn, msg, bssmap_msg_len(msg, len, conn));
 		break;
 	case BSSAP_MSG_DTAP:
 		dtap_rcvmsg(conn, msg, len);