gbproxy: Reset IMSI acquisition and free stored messages in tlli_info

Currently the stored messages are only removed, when IMSI acquisition
has succeeded. In addition, receiving two ATTACH_REQ messages in
sequence (e.g. due to loss of a Identity Req/Resp message) will not
restart the IMSI acquisition procedure.

This patch adds gbproxy_tlli_info_discard_messages() to clean up the
message list and calls it from gbproxy_delete_tlli() fixing a
potential memory leak. It is also called when an Attach Request
message has been received. In that case the imsi_acq_pending flag is
cleared, too. This would (re-)trigger the IMSI acquisition procedure
at each of these messages. If an Ident Response has been lost,
resending the Ident Request with the same N(U) will not work.
Therefore the N(U) gets incremented on each Ident Request generated
by the gbproxy. The first N(U) used is 256 which shouldn't collide
with the V(UT) used by the SGSN given that P-TMSI patching is enabled
(since a new random TLLI is used initially on every new (no
tlli_info) connection and V(U) starts with zero then).

Ticket: OW#1261
Sponsored-by: On-Waves ehf

1 files changed, 35 insertions, 5 deletions

diff --git a/openbsc/src/gprs/gb_proxy.c b/openbsc/src/gprs/gb_proxy.c
index 8b014eafc..fbac298e3 100644
--- a/openbsc/src/gprs/gb_proxy.c
+++ b/openbsc/src/gprs/gb_proxy.c
@@ -253,6 +253,22 @@ uint32_t gbproxy_make_sgsn_tlli(struct gbproxy_peer *peer,
 	return sgsn_tlli;
 }
 
+/* Returns != 0 iff IMSI acquisition was in progress */
+int gbproxy_reset_imsi_acquisition(struct gbproxy_tlli_info* tlli_info)
+{
+	int in_progress = 0;
+	if (!tlli_info)
+		return 0;
+
+	if (tlli_info->imsi_acq_pending)
+		in_progress = 1;
+
+	gbproxy_tlli_info_discard_messages(tlli_info);
+	tlli_info->imsi_acq_pending = 0;
+
+	return in_progress;
+}
+
 /* patch BSSGP message */
 static int gbprox_process_bssgp_ul(struct gbproxy_config *cfg,
 				   struct msgb *msg,
@@ -315,10 +331,21 @@ static int gbprox_process_bssgp_ul(struct gbproxy_config *cfg,
 	gbprox_update_current_raid(parse_ctx.bssgp_raid_enc, peer,
 				   parse_ctx.llc_msg_name);
 
+	gprs_gb_log_parse_context(&parse_ctx, "BSSGP");
+
+	tlli_info = gbproxy_update_tlli_state_ul(peer, now, &parse_ctx);
+
 	if (parse_ctx.g48_hdr) {
 		switch (parse_ctx.g48_hdr->msg_type) {
 		case GSM48_MT_GMM_ATTACH_REQ:
 			rate_ctr_inc(&peer->ctrg->ctr[GBPROX_PEER_CTR_ATTACH_REQS]);
+			if (gbproxy_reset_imsi_acquisition(tlli_info)) {
+				LOGP(DLLC, LOGL_INFO,
+				     "NSEI=%d(BSS) IMSI acquisition was in progress "
+				     "when receiving an ATTACH_REQ.\n",
+				     msgb_nsei(msg));
+				tlli_info->imsi_acq_retries += 1;
+			}
 			break;
 
 		default:
@@ -326,10 +353,6 @@ static int gbprox_process_bssgp_ul(struct gbproxy_config *cfg,
 		}
 	}
 
-	gprs_gb_log_parse_context(&parse_ctx, "BSSGP");
-
-	tlli_info = gbproxy_update_tlli_state_ul(peer, now, &parse_ctx);
-
 	if (tlli_info && cfg->route_to_sgsn2 && gbproxy_check_tlli(peer, tlli_info)) {
 		sgsn_nsei = cfg->nsip_sgsn2_nsei;
 		send_msg_directly = 1;
@@ -415,8 +438,15 @@ static int gbprox_process_bssgp_ul(struct gbproxy_config *cfg,
 		/* Send IDENT REQ */
 		idreq_msg = gsm48_msgb_alloc();
 		gprs_put_identity_req(idreq_msg, GSM_MI_TYPE_IMSI);
+		/* Workaround to avoid N(U) collisions and to enable a restart
+		 * of the IMSI acquisition procedure. This will work unless the
+		 * SGSN has an initial V(UT) within [256-32, 256+n_retries]
+		 * (see GSM 04.64, 8.4.2).
+		 * TODO: Check whether it is sensible to rely on this, it might
+		 * be an issue when P-TMSI patching is _not_ enabled and the SGSN
+		 * doesn't reset V(UT) after a detach. */
 		gprs_push_llc_ui(idreq_msg, 0, GPRS_SAPI_GMM,
-				 /* TODO: use a real N(U) */0);
+				 (tlli_info->imsi_acq_retries + 256) % 512);
 		gprs_push_bssgp_dl_unitdata(idreq_msg, tlli_info->tlli.current);
 		gbprox_relay2peer(idreq_msg, peer, msgb_bvci(msg));
 		msgb_free(idreq_msg);