Bluetooth: SMP: Re-pairing cannot lower the security level of the bond

Make sure that a new pairing procedure with an existing bond does not
result in a security with weaker security properties.

Signed-off-by: Joakim Andersson <[email protected]>

Author: joerchan

subsys/bluetooth/host/smp.c

@@ -294,10 +294,20 @@ static u8_t get_io_capa(void)
 	}
 }
 
+#if !defined(CONFIG_BT_SMP_SC_PAIR_ONLY)
+static u8_t legacy_get_pair_method(struct bt_smp *smp, u8_t remote_io);
+#endif
+
 static u8_t get_pair_method(struct bt_smp *smp, u8_t remote_io)
 {
 	struct bt_smp_pairing *req, *rsp;
 
+#if !defined(CONFIG_BT_SMP_SC_PAIR_ONLY)
+	if (!atomic_test_bit(smp->flags, SMP_FLAG_SC)) {
+		return legacy_get_pair_method(smp, remote_io);
+	}
+#endif
+
 	if (remote_io > BT_SMP_IO_KEYBOARD_DISPLAY)
 		return JUST_WORKS;
 
@@ -541,6 +551,39 @@ static u8_t get_encryption_key_size(struct bt_smp *smp)
 	return MIN(req->max_key_size, rsp->max_key_size);
 }
 
+/* Check that if a new pairing procedure with an existing bond will not lower
+ * the established security level of the bond.
+ */
+bool update_keys_check(struct bt_smp *smp)
+{
+	struct bt_conn *conn = smp->chan.chan.conn;
+
+	if (!conn->le.keys) {
+		conn->le.keys = bt_keys_get_addr(conn->id, &conn->le.dst);
+	}
+
+	if (!conn->le.keys ||
+	    !(conn->le.keys->keys & (BT_KEYS_LTK_P256 | BT_KEYS_LTK))) {
+		return true;
+	}
+
+	if (conn->le.keys->enc_size > get_encryption_key_size(smp)) {
+		return false;
+	}
+
+	if ((conn->le.keys->keys & BT_KEYS_LTK_P256) &&
+	    !atomic_test_bit(smp->flags, SMP_FLAG_SC)) {
+		return false;
+	}
+
+	if ((conn->le.keys->flags & BT_KEYS_AUTHENTICATED) &&
+	     smp->method == JUST_WORKS) {
+		return false;
+	}
+
+	return true;
+}
+
 #if defined(CONFIG_BT_PRIVACY) || defined(CONFIG_BT_SIGNING) || \
 	!defined(CONFIG_BT_SMP_SC_PAIR_ONLY)
 /* For TX callbacks */
@@ -2001,14 +2044,12 @@ static u8_t legacy_send_pairing_confirm(struct bt_smp *smp)
 }
 
 #if defined(CONFIG_BT_PERIPHERAL)
-static u8_t legacy_pairing_req(struct bt_smp *smp, u8_t remote_io)
+static u8_t legacy_pairing_req(struct bt_smp *smp)
 {
 	u8_t ret;
 
 	BT_DBG("");
 
-	smp->method = legacy_get_pair_method(smp, remote_io);
-
 	/* ask for consent if pairing is not due to sending SecReq*/
 	if ((DISPLAY_FIXED(smp) || smp->method == JUST_WORKS) &&
 	    !atomic_test_bit(smp->flags, SMP_FLAG_SEC_REQ) &&
@@ -2207,14 +2248,12 @@ static u8_t smp_master_ident(struct bt_smp *smp, struct net_buf *buf)
 }
 
 #if defined(CONFIG_BT_CENTRAL)
-static u8_t legacy_pairing_rsp(struct bt_smp *smp, u8_t remote_io)
+static u8_t legacy_pairing_rsp(struct bt_smp *smp)
 {
 	u8_t ret;
 
 	BT_DBG("");
 
-	smp->method = legacy_get_pair_method(smp, remote_io);
-
 	/* ask for consent if this is due to received SecReq */
 	if ((DISPLAY_FIXED(smp) || smp->method == JUST_WORKS) &&
 	    atomic_test_bit(smp->flags, SMP_FLAG_SEC_REQ) &&
@@ -2442,16 +2481,20 @@ static u8_t smp_pairing_req(struct bt_smp *smp, struct net_buf *buf)
 
 	atomic_set_bit(smp->flags, SMP_FLAG_PAIRING);
 
+	smp->method = get_pair_method(smp, req->io_capability);
+
+	if (!update_keys_check(smp)) {
+		return BT_SMP_ERR_AUTH_REQUIREMENTS;
+	}
+
 	if (!atomic_test_bit(smp->flags, SMP_FLAG_SC)) {
 #if defined(CONFIG_BT_SMP_SC_PAIR_ONLY)
 		return BT_SMP_ERR_AUTH_REQUIREMENTS;
 #else
-		return legacy_pairing_req(smp, req->io_capability);
+		return legacy_pairing_req(smp);
 #endif /* CONFIG_BT_SMP_SC_PAIR_ONLY */
 	}
 
-	smp->method = get_pair_method(smp, req->io_capability);
-
 	if ((IS_ENABLED(CONFIG_BT_SMP_SC_ONLY) ||
 	     conn->required_sec_level == BT_SECURITY_FIPS) &&
 		smp->method == JUST_WORKS) {
@@ -2604,16 +2647,20 @@ static u8_t smp_pairing_rsp(struct bt_smp *smp, struct net_buf *buf)
 		atomic_set_bit(smp->flags, SMP_FLAG_BOND);
 	}
 
+	smp->method = get_pair_method(smp, rsp->io_capability);
+
+	if (!update_keys_check(smp)) {
+		return BT_SMP_ERR_AUTH_REQUIREMENTS;
+	}
+
 	if (!atomic_test_bit(smp->flags, SMP_FLAG_SC)) {
 #if defined(CONFIG_BT_SMP_SC_PAIR_ONLY)
 		return BT_SMP_ERR_AUTH_REQUIREMENTS;
 #else
-		return legacy_pairing_rsp(smp, rsp->io_capability);
+		return legacy_pairing_rsp(smp);
 #endif /* CONFIG_BT_SMP_SC_PAIR_ONLY */
 	}
 
-	smp->method = get_pair_method(smp, rsp->io_capability);
-
 	if ((IS_ENABLED(CONFIG_BT_SMP_SC_ONLY) ||
 	     conn->required_sec_level == BT_SECURITY_FIPS) &&
 	     smp->method == JUST_WORKS) {
@@ -3368,6 +3415,14 @@ static u8_t smp_public_key(struct bt_smp *smp, struct net_buf *buf)
 	if (memcmp(smp->pkey, sc_debug_public_key, 64) == 0) {
 		BT_INFO("Remote is using Debug Public key");
 		atomic_set_bit(smp->flags, SMP_FLAG_SC_DEBUG_KEY);
+
+		/* Don't allow a bond established without debug key to be
+		 * updated using LTK generated from debug key.
+		 */
+		if (smp->chan.chan.conn->le.keys &&
+		    !(smp->chan.chan.conn->le.keys->flags & BT_KEYS_DEBUG)) {
+			return BT_SMP_ERR_AUTH_REQUIREMENTS;
+		}
 	}
 
 	if (IS_ENABLED(CONFIG_BT_CENTRAL) &&