validate advert payload length before parsing

The ADVERT handler copied pub_key, timestamp, and signature from the
payload before checking whether payload_len was large enough to contain
them. With a short payload, the memcpy operations read uninitialized
data from within the payload buffer.

Move the bounds check before any parsing so undersized adverts are
rejected immediately. The minimum required is PUB_KEY_SIZE + 4 +
SIGNATURE_SIZE (100 bytes).

Author: weebl2000

src/Mesh.cpp

@@ -241,16 +241,20 @@ DispatcherAction Mesh::onRecvPacket(Packet* pkt) {
     }
     case PAYLOAD_TYPE_ADVERT: {
       int i = 0;
+      int min_advert_len = PUB_KEY_SIZE + 4 + SIGNATURE_SIZE;
+      if (pkt->payload_len < min_advert_len) {
+        MESH_DEBUG_PRINTLN("%s Mesh::onRecvPacket(): incomplete advertisement packet, payload_len=%d", getLogDateTime(), (int)pkt->payload_len);
+        break;
+      }
+
       Identity id;
       memcpy(id.pub_key, &pkt->payload[i], PUB_KEY_SIZE); i += PUB_KEY_SIZE;
 
       uint32_t timestamp;
       memcpy(&timestamp, &pkt->payload[i], 4); i += 4;
       const uint8_t* signature = &pkt->payload[i]; i += SIGNATURE_SIZE;
 
-      if (i > pkt->payload_len) {
-        MESH_DEBUG_PRINTLN("%s Mesh::onRecvPacket(): incomplete advertisement packet", getLogDateTime());
-      } else if (self_id.matches(id.pub_key)) {
+      if (self_id.matches(id.pub_key)) {
         MESH_DEBUG_PRINTLN("%s Mesh::onRecvPacket(): receiving SELF advert packet", getLogDateTime());
       } else if (!_tables->hasSeen(pkt)) {
         uint8_t* app_data = &pkt->payload[i];