fix inter-BSC-in handover encryption

In the field we saw Handover Requests without any Chosen Encryption
Algorithm IE, and osmo-bsc completely failed on those. This made me
understand my mistake from when I wrote this handover code.

So far, from a BSSMAP Handover Request, we (I) used only the Chosen
Encryption Algorithm IE to pick the encryption to use on the target
lchan. That is very wrong.

Instead, figure out the intersection of permitted algorithms MSC & BSC,
and pick the best of those. Which means, actually, completely ignore the
Chosen Encryption Algorithm IE.

In the message, the permitted algorithms are passed as a bitmask. The
current code using gsm0808_dec_encrypt_info() passes this on as an
array. In order to select_best_cipher(), I could convert that array back
to a bitmask. Instead pass the bitmask on from message decoding
alongside the struct gsm0808_encrypt_info in req->ei_as_bitmask.

In handover_end(), change the condition so that we can also pass
HO_RESULT_FAIL_RR_HO_FAIL to emit a Handover Failure.

Related: SYS#5839
Change-Id: Iffedc981b60d309ed2e5decd5efedee07a757b53

3 files changed, 26 insertions, 8 deletions

diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 7657a5f6a..a1aa2df67 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -250,6 +250,8 @@ struct handover_in_req {
 	struct gsm0808_channel_type ct;
 	struct gsm0808_speech_codec_list scl;
 	struct gsm0808_encrypt_info ei;
+	/* The same information as in 'ei' but as the handy bitmask as on the wire. */
+	uint8_t ei_as_bitmask;
 	bool kc128_present;
 	uint8_t kc128[16];
 	struct gsm_classmark classmark;
@@ -1435,4 +1437,6 @@ extern const struct gsm_interf_meas_params interf_meas_params_def;
 
 enum rsl_cmod_spd chan_mode_to_rsl_cmod_spd(enum gsm48_chan_mode chan_mode);
 
+int select_best_cipher(uint8_t msc_mask, uint8_t bsc_mask);
+
 #endif /* _GSM_DATA_H */
diff --git a/src/osmo-bsc/handover_fsm.c b/src/osmo-bsc/handover_fsm.c
index 37e74176d..487135b89 100644
--- a/src/osmo-bsc/handover_fsm.c
+++ b/src/osmo-bsc/handover_fsm.c
@@ -490,6 +490,7 @@ static bool parse_ho_request(struct gsm_subscriber_connection *conn, const struc
 		LOG_HO(conn, LOGL_ERROR, "Failed to parse Encryption Information IE\n");
 		return false;
 	}
+	req->ei_as_bitmask = *e->val;
 
 	if ((e = TLVP_GET(tp, GSM0808_IE_KC_128))) {
 		if (e->len != 16) {
@@ -630,6 +631,7 @@ void handover_start_inter_bsc_in(struct gsm_subscriber_connection *conn,
 	int match_idx;
 	struct osmo_fsm_inst *fi;
 	struct channel_mode_and_rate ch_mode_rate = {};
+	int chosen_a5_n;
 
 	handover_fsm_alloc(conn);
 
@@ -717,16 +719,28 @@ void handover_start_inter_bsc_in(struct gsm_subscriber_connection *conn,
 		.msc_assigned_cic = req->msc_assigned_cic,
 	};
 
-	if (req->chosen_encr_alg) {
-		info.encr.alg_id = req->chosen_encr_alg;
-		if (info.encr.alg_id > 1 && !req->ei.key_len) {
-			ho_fail(HO_RESULT_ERROR, "Chosen Encryption Algorithm (Serving) reflects A5/%u"
-				" but there is no key (Encryption Information)", info.encr.alg_id - 1);
+	/* Figure out the encryption algorithm */
+	chosen_a5_n = select_best_cipher(req->ei_as_bitmask, bsc_gsmnet->a5_encryption_mask);
+	if (chosen_a5_n < 0) {
+		ho_fail(HO_RESULT_FAIL_RR_HO_FAIL,
+			"There is no A5 encryption mode that both BSC and MSC permit: MSC 0x%x & BSC 0x%x = 0\n",
+			req->ei_as_bitmask, bsc_gsmnet->a5_encryption_mask);
+		return;
+	}
+	if (chosen_a5_n > 0 && !req->ei.key_len) {
+		/* There is no key. Is A5/0 permitted? */
+		if ((req->ei_as_bitmask & bsc_gsmnet->a5_encryption_mask & 0x1) == 0x1) {
+			chosen_a5_n = 0;
+		} else {
+			ho_fail(HO_RESULT_ERROR,
+				"Encryption is required, but there is no key (Encryption Information)");
 			return;
 		}
 	}
 
-	if (req->ei.key_len) {
+	/* Put encryption info in the chan activation info */
+	info.encr.alg_id = ALG_A5_NR_TO_RSL(chosen_a5_n);
+	if (chosen_a5_n > 0) {
 		if (req->ei.key_len > sizeof(info.encr.key)) {
 			ho_fail(HO_RESULT_ERROR, "Encryption Information IE key length is too large: %u\n",
 				req->ei.key_len);
@@ -965,7 +979,7 @@ void handover_end(struct gsm_subscriber_connection *conn, enum handover_result r
 				result = bsc_tx_bssmap_ho_complete(conn, ho->new_lchan);
 		}
 		/* Not 'else': above checks may still result in HO_RESULT_ERROR. */
-		if (result == HO_RESULT_ERROR) {
+		if (result != HO_RESULT_OK) {
 			/* Return a BSSMAP Handover Failure, as described in 3GPP TS 48.008 3.1.5.2.2
 			 * "Handover Resource Allocation Failure" */
 			bsc_tx_bssmap_ho_failure(conn);
diff --git a/src/osmo-bsc/osmo_bsc_bssap.c b/src/osmo-bsc/osmo_bsc_bssap.c
index cf8254c01..9d7b89fa3 100644
--- a/src/osmo-bsc/osmo_bsc_bssap.c
+++ b/src/osmo-bsc/osmo_bsc_bssap.c
@@ -394,7 +394,7 @@ int bsc_paging_start(struct bsc_paging_params *params)
 }
 
 /* select the best cipher permitted by the intersection of both masks */
-static int select_best_cipher(uint8_t msc_mask, uint8_t bsc_mask)
+int select_best_cipher(uint8_t msc_mask, uint8_t bsc_mask)
 {
 	/* A5/7 ... A5/3: We assume higher is better,
 	 * but: A5/1 is better than A5/2, which is better than A5/0 */