net: mqtt: Improve PUBLISH message length validation

Identify when received PUBLISH message is malformed and overall packet
length received is smaller than parsed variable header lenght.
Add unit test to cover this case.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

subsys/net/lib/mqtt/mqtt_decoder.c

@@ -256,6 +256,13 @@ int publish_decode(u8_t flags, u32_t var_length, struct buf_ctx *buf,
 		var_header_length += sizeof(u16_t);
 	}
 
+	if (var_length < var_header_length) {
+		MQTT_ERR("Corrupted PUBLISH message, header length (%u) larger "
+			 "than total length (%u)", var_header_length,
+			 var_length);
+		return -EINVAL;
+	}
+
 	param->message.payload.data = NULL;
 	param->message.payload.len = var_length - var_header_length;
 

tests/net/lib/mqtt_packet/src/mqtt_packet.c

@@ -109,6 +109,15 @@ static int eval_msg_connect(struct mqtt_test *mqtt_test);
  */
 static int eval_msg_publish(struct mqtt_test *mqtt_test);
 
+/**
+ * @brief eval_msg_corrupted_publish Evaluate the given mqtt_test against the
+ *				     corrupted publish message.
+ * @param [in] mqtt_test	     MQTT test structure
+ * @return			     TC_PASS on success
+ * @return			     TC_FAIL on error
+ */
+static int eval_msg_corrupted_publish(struct mqtt_test *mqtt_test);
+
 /**
  * @brief eval_msg_subscribe	Evaluate the given mqtt_test against the
  *				subscribe packing/unpacking routines.
@@ -452,6 +461,14 @@ static ZTEST_DMEM struct mqtt_publish_param msg_publish4 = {
 	.message.payload.len = 2,
 };
 
+static ZTEST_DMEM
+u8_t publish_corrupted[] = {0x30, 0x07, 0x00, 0x07, 0x73, 0x65, 0x6e, 0x73,
+			    0x6f, 0x72, 0x73, 0x00, 0x01, 0x4f, 0x4b};
+static ZTEST_DMEM struct buf_ctx publish_corrupted_buf = {
+	.cur = publish_corrupted,
+	.end = publish_corrupted + sizeof(publish_corrupted)
+};
+
 /*
  * MQTT SUBSCRIBE msg:
  * pkt_id: 1, topic: sensors, qos: 0
@@ -622,6 +639,9 @@ struct mqtt_test mqtt_tests[] = {
 	 .ctx = &msg_publish4, .eval_fcn = eval_msg_publish,
 	 .expected = publish4, .expected_len = sizeof(publish4)},
 
+	{.test_name = "PUBLISH, corrupted message length (smaller than topic)",
+	 .ctx = &publish_corrupted_buf, .eval_fcn = eval_msg_corrupted_publish},
+
 	{.test_name = "SUBSCRIBE, one topic, qos = 0",
 	 .ctx = &msg_subscribe1, .eval_fcn = eval_msg_subscribe,
 	 .expected = subscribe1, .expected_len = sizeof(subscribe1)},
@@ -829,6 +849,23 @@ static int eval_msg_publish(struct mqtt_test *mqtt_test)
 	return TC_PASS;
 }
 
+static int eval_msg_corrupted_publish(struct mqtt_test *mqtt_test)
+{
+	struct buf_ctx *buf = (struct buf_ctx *)mqtt_test->ctx;
+	int rc;
+	u8_t type_and_flags;
+	u32_t length;
+	struct mqtt_publish_param dec_param;
+
+	rc = fixed_header_decode(buf, &type_and_flags, &length);
+	zassert_equal(rc, 0, "fixed_header_decode failed");
+
+	rc = publish_decode(type_and_flags, length, buf, &dec_param);
+	zassert_equal(rc, -EINVAL, "publish_decode should fail");
+
+	return TC_PASS;
+}
+
 static int eval_msg_subscribe(struct mqtt_test *mqtt_test)
 {
 	struct mqtt_subscription_list *param =