[PATCH 6/8] nat: Send 16 bytes of rand to the BSC and remember it

[Holger Hans Peter Freyther]

From: Holger Hans Peter Freyther <holger at moiji-mobile.com>

Generate 16 byte of random data to be used for A3A8 by
the BSC in the response. We can't know which BSC it is
at this point and I don't want to send another message
once the token has been received so always send the data
with an undefined code. The old BSCs don't parse the
message and will happily ignore the RAND.

/dev/urandom can give short reads on Linux so loop
around it until the bytes have been read from the kernel.
---
 openbsc/include/openbsc/bsc_nat.h  |  1 +
 openbsc/src/osmo-bsc_nat/bsc_nat.c | 40 +++++++++++++++++++++++++++++++++++---
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index 1035937..c313e52 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -84,6 +84,7 @@ struct bsc_connection {
 
 	/* do we know anything about this BSC? */
 	int authenticated;
+	uint8_t last_rand[16];
 
 	/* the fd we use to communicate */
 	struct osmo_wqueue write_queue;
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat.c b/openbsc/src/osmo-bsc_nat/bsc_nat.c
index 82562ba..254ea42 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat.c
@@ -188,9 +188,9 @@ static void send_id_ack(struct bsc_connection *bsc)
 	bsc_send_data(bsc, id_ack, sizeof(id_ack), IPAC_PROTO_IPACCESS);
 }
 
-static void send_id_req(struct bsc_connection *bsc)
+static void send_id_req(struct bsc_nat *nat, struct bsc_connection *bsc)
 {
-	static const uint8_t id_req[] = {
+	static const uint8_t s_id_req[] = {
 		IPAC_MSGT_ID_GET,
 		0x01, IPAC_IDTAG_UNIT,
 		0x01, IPAC_IDTAG_MACADDR,
@@ -202,7 +202,41 @@ static void send_id_req(struct bsc_connection *bsc)
 		0x01, IPAC_IDTAG_SERNR,
 	};
 
+	int toread, rounds;
+	uint8_t *mrand, *randoff;
+	uint8_t id_req[sizeof(s_id_req) + (2+16)];
+	uint8_t *buf = &id_req[sizeof(s_id_req)];
+
+	/* copy the static data */
+	memcpy(id_req, s_id_req, sizeof(s_id_req));
+
+	/* put the RAND with length, tag, value */
+	buf = v_put(buf, 0x11);
+	buf = v_put(buf, 0x23);
+	mrand = bsc->last_rand;
+	randoff = mrand;
+	memset(randoff, 0, 16);
+
+	for (toread = 16, rounds = 0; rounds < 5 && toread > 0; ++rounds) {
+		int rc = read(nat->random_fd, randoff, toread);
+		if (rc <= 0)
+			goto failed_random;
+		toread -= rc;
+		randoff += rc;
+	}
+
+	if (toread != 0)
+		goto failed_random;
+	memcpy(buf, mrand, 16);
+	buf += 16;
+
 	bsc_send_data(bsc, id_req, sizeof(id_req), IPAC_PROTO_IPACCESS);
+	return;
+
+failed_random:
+	/* the timeout will trigger and close this connection */
+	LOGP(DNAT, LOGL_ERROR, "Failed to read from urandom.\n");
+	return;
 }
 
 static struct msgb *nat_create_rlsd(struct nat_sccp_connection *conn)
@@ -1362,7 +1396,7 @@ static int ipaccess_listen_bsc_cb(struct osmo_fd *bfd, unsigned int what)
 	bsc->last_id = 0;
 
 	send_id_ack(bsc);
-	send_id_req(bsc);
+	send_id_req(nat, bsc);
 	send_mgcp_reset(bsc);
 
 	/*
-- 
2.3.5