rsl: parse RSL_IE_OSMO_OSMUX_CID correctly

This IE has TLV format, even though the only valid form is a single
value octet.  To guard against pathological input with L=0 in this IE,
we have to check the length explicitly with TLVP_PRES_LEN before
accepting TLVP_VAL as if it was TV.

Change-Id: I15fa75b6c30d7fa0bf50424d25fc47a088dada0a
---
M src/common/rsl.c
1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/common/rsl.c b/src/common/rsl.c
index fa5f495..40690f0 100644
--- a/src/common/rsl.c
+++ b/src/common/rsl.c
@@ -2978,7 +2978,8 @@
 	struct tlv_parsed tp;
 	struct gsm_lchan *lchan = msg->lchan;
 	struct gsm_bts *bts = lchan->ts->trx->bts;
-	const uint8_t *payload_type, *speech_mode, *payload_type2, *osmux_cid, *csd_fmt;
+	const uint8_t *payload_type, *speech_mode, *payload_type2, *csd_fmt;
+	const uint8_t *osmux_cid = NULL;
 	uint32_t connect_ip = 0;
 	uint16_t connect_port = 0;
 	int rc, inc_ip_port = 0;
@@ -3029,7 +3030,9 @@
 	if (payload_type2)
 		LOGPC(DRSL, LOGL_DEBUG, "payload_type2=%u ", *payload_type2);
 
-	osmux_cid = TLVP_VAL(&tp, RSL_IE_OSMO_OSMUX_CID);
+	/* this IE has TLV format when TV would have been good enough */
+	if (TLVP_PRES_LEN(&tp, RSL_IE_OSMO_OSMUX_CID, 1))
+		osmux_cid = TLVP_VAL(&tp, RSL_IE_OSMO_OSMUX_CID);
 	if (osmux_cid)
 		LOGPC(DRSL, LOGL_DEBUG, "osmux_cid=%u ", *osmux_cid);
 

