ASCI: Ignore LAPD frames from MS, while the uplink is not active

Do not forward any message that is received on the uplink to LAPD while
the uplink is not active. If the MS did not recognize (fast enough) that
the uplink is free, it may continue to transmit LAPD messages. A
response by LAPD to these messages is not desired and not required. If
LAPD would respond, it would cause stopping transmission of UPLINK FREE
messages. No MS could access the uplink anymore.

Note: UPLINK FREE messages are repeated automatically until a different
      message is transmitted.

Related: OS#5781
Change-Id: I5075115123055b2997481f56ddf473430a1dc9e3
(cherry picked from commit f5c1cd3889bb52b1166066885e2d65b87692a37e)
---
M src/common/l1sap.c
1 file changed, 35 insertions(+), 3 deletions(-)

diff --git a/src/common/l1sap.c b/src/common/l1sap.c
index 9814f86..844fdad 100644
--- a/src/common/l1sap.c
+++ b/src/common/l1sap.c
@@ -1810,9 +1810,20 @@
 	if (lchan->ho.active == HANDOVER_WAIT_FRAME)
 		handover_frame(lchan);
 
-	/* report first valid received frame to VGCS talker process */
-	if (rsl_chan_rt_is_asci(lchan->rsl_chan_rt) && lchan->asci.talker_active == VGCS_TALKER_WAIT_FRAME)
-		vgcs_talker_frame(lchan);
+	if (rsl_chan_rt_is_asci(lchan->rsl_chan_rt)) {
+		/* report first valid received frame to VGCS talker process */
+		if (lchan->asci.talker_active == VGCS_TALKER_WAIT_FRAME)
+			vgcs_talker_frame(lchan);
+		/* Do not forward any message that is received on the uplink to LAPD while
+		 * the uplink is not active. If the MS did not recognize (fast enough) that
+		 * the uplink is free, it may continue to transmit LAPD messages. A
+		 * response by LAPD to these messages is not desired and not required. If
+		 * LAPD would respond, it would cause stopping transmission of UPLINK FREE
+		 * messages. No MS could access the uplink anymore.
+		 */
+		if (lchan->asci.talker_active != VGCS_TALKER_ACTIVE)
+			return 0;
+	}
 
 	if (L1SAP_IS_LINK_SACCH(link_id))
 		le = &lchan->lapdm_ch.lapdm_acch;

