SGSN / GMM: Only allow non-roaming IMSIs

As we do not yet use the HLR from the SGSN, we allow all MS to
attach to our GPRS network.  However, if this is running in a public
environment, it could cause service interruption to users of commercial
GPRS networks.

Thus, we now check if the first 5 digits of the IMSI match the MCC/MNC
of the cell that they want to register to.  Thus, any subscribers with
SIM cards from real operators will no longer be accepted.

openbsc/src/gprs/gprs_gmm.c

@@ -671,7 +671,16 @@ static int gsm48_rx_gmm_att_req(struct sgsn_mm_ctx *ctx, struct msgb *msg,
 #if 0
 			return gsm48_tx_gmm_att_rej(msg, GMM_CAUSE_IMSI_UNKNOWN);
 #else
-			/* As a temorary hack, we simply assume that the IMSI exists */
+			/* As a temorary hack, we simply assume that the IMSI exists,
+			 * as long as it is part of 'our' network */
+			char mccmnc[16];
+			snprintf(mccmnc, sizeof(mccmnc), "%03d%02d", ra_id.mcc, ra_id.mnc);
+			if (strncmp(mccmnc, mi_string, 5)) {
+				LOGP(DMM, LOGL_INFO, "Rejecting ATTACH REQUESET IMSI=%s\n",
+				     mi_string);
+				return gsm48_tx_gmm_att_rej_oldmsg(msg,
+								GMM_CAUSE_GPRS_NOTALLOWED);
+			}
 			ctx = sgsn_mm_ctx_alloc(0, &ra_id);
 			if (!ctx)
 				return gsm48_tx_gmm_att_rej_oldmsg(msg, GMM_CAUSE_NET_FAIL);