Commit message: Avoid divide-by-zero in raw encoding (OSX RealVNC)

OS X RealVNC server crashes out Remmina because the server can provoke
bytesPerLine to be zero. Assume this is coding for zero lines.

The condition could be checked before the calculation of bytesPerLine.
I don’t understand the preconditions of this code to say one way or the
other.
CVE: CVE-2018-20021
CWE: CWE-835
Vulnerability description: LibVNC before commit c3115350eb8bb635d0fdb4dbbb0d0541f38ed19c contains a CWE-835: Infinite loop vulnerability in VNC client code. Vulnerability allows attacker to consume excessive amount of resources like CPU and RAM

Project: libvncserver
File: rfbproto.c

HandleRFBServerMessage(rfbClient* client)
{
  rfbServerToClientMsg msg;

  if (client->serverPort==-1)
    client->vncRec->readTimestamp = TRUE;
  if (!ReadFromRFBServer(client, (char *)&msg, 1))
    return FALSE;

  switch (msg.type) {

  case rfbSetColourMapEntries:
  {
    /* TODO:
    int i;
    uint16_t rgb[3];
    XColor xc;

    if (!ReadFromRFBServer(client, ((char *)&msg) + 1,
			   sz_rfbSetColourMapEntriesMsg - 1))
      return FALSE;

    msg.scme.firstColour = rfbClientSwap16IfLE(msg.scme.firstColour);
    msg.scme.nColours = rfbClientSwap16IfLE(msg.scme.nColours);

    for (i = 0; i < msg.scme.nColours; i++) {
      if (!ReadFromRFBServer(client, (char *)rgb, 6))
	return FALSE;
      xc.pixel = msg.scme.firstColour + i;
      xc.red = rfbClientSwap16IfLE(rgb[0]);
      xc.green = rfbClientSwap16IfLE(rgb[1]);
      xc.blue = rfbClientSwap16IfLE(rgb[2]);
      xc.flags = DoRed|DoGreen|DoBlue;
      XStoreColor(dpy, cmap, &xc);
    }
    */

    break;
  }

  case rfbFramebufferUpdate:
  {
    rfbFramebufferUpdateRectHeader rect;
    int linesToRead;
    int bytesPerLine;
    int i;

    if (!ReadFromRFBServer(client, ((char *)&msg.fu) + 1,
			   sz_rfbFramebufferUpdateMsg - 1))
      return FALSE;

    msg.fu.nRects = rfbClientSwap16IfLE(msg.fu.nRects);

    for (i = 0; i < msg.fu.nRects; i++) {
      if (!ReadFromRFBServer(client, (char *)&rect, sz_rfbFramebufferUpdateRectHeader))
	return FALSE;

      rect.encoding = rfbClientSwap32IfLE(rect.encoding);
      if (rect.encoding == rfbEncodingLastRect)
	break;

      rect.r.x = rfbClientSwap16IfLE(rect.r.x);
      rect.r.y = rfbClientSwap16IfLE(rect.r.y);
      rect.r.w = rfbClientSwap16IfLE(rect.r.w);
      rect.r.h = rfbClientSwap16IfLE(rect.r.h);


      if (rect.encoding == rfbEncodingXCursor ||
	  rect.encoding == rfbEncodingRichCursor) {

	if (!HandleCursorShape(client,
			       rect.r.x, rect.r.y, rect.r.w, rect.r.h,
			       rect.encoding)) {
	  return FALSE;
	}
	continue;
      }

      if (rect.encoding == rfbEncodingPointerPos) {
	if (!client->HandleCursorPos(client,rect.r.x, rect.r.y)) {
	  return FALSE;
	}
	continue;
      }
      
      if (rect.encoding == rfbEncodingKeyboardLedState) {
          /* OK! We have received a keyboard state message!!! */
          client->KeyboardLedStateEnabled = 1;
          if (client->HandleKeyboardLedState!=NULL)
              client->HandleKeyboardLedState(client, rect.r.x, 0);
          /* stash it for the future */
          client->CurrentKeyboardLedState = rect.r.x;
          continue;
      }

      if (rect.encoding == rfbEncodingNewFBSize) {
	client->width = rect.r.w;
	client->height = rect.r.h;
	client->updateRect.x = client->updateRect.y = 0;
	client->updateRect.w = client->width;
	client->updateRect.h = client->height;
	if (!client->MallocFrameBuffer(client))
	  return FALSE;
	SendFramebufferUpdateRequest(client, 0, 0, rect.r.w, rect.r.h, FALSE);
	rfbClientLog("Got new framebuffer size: %dx%d\n", rect.r.w, rect.r.h);
	continue;
      }

      /* rect.r.w=byte count */
      if (rect.encoding == rfbEncodingSupportedMessages) {
          int loop;
          if (!ReadFromRFBServer(client, (char *)&client->supportedMessages, sz_rfbSupportedMessages))
              return FALSE;

          /* msgs is two sets of bit flags of supported messages client2server[] and server2client[] */
          /* currently ignored by this library */

          rfbClientLog("client2server supported messages (bit flags)\n");
          for (loop=0;loop<32;loop+=8)
            rfbClientLog("%02X: %04x %04x %04x %04x - %04x %04x %04x %04x\n", loop,
                client->supportedMessages.client2server[loop],   client->supportedMessages.client2server[loop+1],
                client->supportedMessages.client2server[loop+2], client->supportedMessages.client2server[loop+3],
                client->supportedMessages.client2server[loop+4], client->supportedMessages.client2server[loop+5],
                client->supportedMessages.client2server[loop+6], client->supportedMessages.client2server[loop+7]);

          rfbClientLog("server2client supported messages (bit flags)\n");
          for (loop=0;loop<32;loop+=8)
            rfbClientLog("%02X: %04x %04x %04x %04x - %04x %04x %04x %04x\n", loop,
                client->supportedMessages.server2client[loop],   client->supportedMessages.server2client[loop+1],
                client->supportedMessages.server2client[loop+2], client->supportedMessages.server2client[loop+3],
                client->supportedMessages.server2client[loop+4], client->supportedMessages.server2client[loop+5],
                client->supportedMessages.server2client[loop+6], client->supportedMessages.server2client[loop+7]);
          continue;
      }

      /* rect.r.w=byte count, rect.r.h=# of encodings */
      if (rect.encoding == rfbEncodingSupportedEncodings) {
          char *buffer;
          buffer = malloc(rect.r.w);
          if (!ReadFromRFBServer(client, buffer, rect.r.w))
          {
              free(buffer);
              return FALSE;
          }

          /* buffer now contains rect.r.h # of uint32_t encodings that the server supports */
          /* currently ignored by this library */
          free(buffer);
          continue;
      }

      /* rect.r.w=byte count */
      if (rect.encoding == rfbEncodingServerIdentity) {
          char *buffer;
          buffer = malloc(rect.r.w+1);
          if (!ReadFromRFBServer(client, buffer, rect.r.w))
          {
              free(buffer);
              return FALSE;
          }
          buffer[rect.r.w]=0; /* null terminate, just in case */
          rfbClientLog("Connected to Server \"%s\"\n", buffer);
          free(buffer);
          continue;
      }

      /* rfbEncodingUltraZip is a collection of subrects.   x = # of subrects, and h is always 0 */
      if (rect.encoding != rfbEncodingUltraZip)
      {
        if ((rect.r.x + rect.r.w > client->width) ||
	    (rect.r.y + rect.r.h > client->height))
	    {
	      rfbClientLog("Rect too large: %dx%d at (%d, %d)\n",
	  	  rect.r.w, rect.r.h, rect.r.x, rect.r.y);
	      return FALSE;
            }

        /* UltraVNC with scaling, will send rectangles with a zero W or H
         *
        if ((rect.encoding != rfbEncodingTight) && 
            (rect.r.h * rect.r.w == 0))
        {
	  rfbClientLog("Zero size rect - ignoring (encoding=%d (0x%08x) %dx, %dy, %dw, %dh)\n", rect.encoding, rect.encoding, rect.r.x, rect.r.y, rect.r.w, rect.r.h);
	  continue;
        }
        */
        
        /* If RichCursor encoding is used, we should prevent collisions
	   between framebuffer updates and cursor drawing operations. */
        client->SoftCursorLockArea(client, rect.r.x, rect.r.y, rect.r.w, rect.r.h);
      }

      switch (rect.encoding) {

      case rfbEncodingRaw: {
	int y=rect.r.y, h=rect.r.h;

	bytesPerLine = rect.r.w * client->format.bitsPerPixel / 8;
	linesToRead = RFB_BUFFER_SIZE / bytesPerLine;

	while (h > 0) {
	  if (linesToRead > h)
	    linesToRead = h;

	  if (!ReadFromRFBServer(client, client->buffer,bytesPerLine * linesToRead))
	    return FALSE;

	  CopyRectangle(client, (uint8_t *)client->buffer,
			   rect.r.x, y, rect.r.w,linesToRead);

	  h -= linesToRead;
	  y += linesToRead;

	}
      } break;

      case rfbEncodingCopyRect:
      {
	rfbCopyRect cr;

	if (!ReadFromRFBServer(client, (char *)&cr, sz_rfbCopyRect))
	  return FALSE;

	cr.srcX = rfbClientSwap16IfLE(cr.srcX);
	cr.srcY = rfbClientSwap16IfLE(cr.srcY);

	/* If RichCursor encoding is used, we should extend our
	   "cursor lock area" (previously set to destination
	   rectangle) to the source rectangle as well. */
	client->SoftCursorLockArea(client,
				   cr.srcX, cr.srcY, rect.r.w, rect.r.h);

        if (client->GotCopyRect != NULL) {
          client->GotCopyRect(client, cr.srcX, cr.srcY, rect.r.w, rect.r.h,
              rect.r.x, rect.r.y);
        } else
		CopyRectangleFromRectangle(client,
				   cr.srcX, cr.srcY, rect.r.w, rect.r.h,
				   rect.r.x, rect.r.y);

	break;
      }

      case rfbEncodingRRE:
      {
	switch (client->format.bitsPerPixel) {
	case 8:
	  if (!HandleRRE8(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 16:
	  if (!HandleRRE16(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 32:
	  if (!HandleRRE32(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	}
	break;
      }

      case rfbEncodingCoRRE:
      {
	switch (client->format.bitsPerPixel) {
	case 8:
	  if (!HandleCoRRE8(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 16:
	  if (!HandleCoRRE16(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 32:
	  if (!HandleCoRRE32(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	}
	break;
      }

      case rfbEncodingHextile:
      {
	switch (client->format.bitsPerPixel) {
	case 8:
	  if (!HandleHextile8(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 16:
	  if (!HandleHextile16(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 32:
	  if (!HandleHextile32(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	}
	break;
      }

      case rfbEncodingUltra:
      {
        switch (client->format.bitsPerPixel) {
        case 8:
          if (!HandleUltra8(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
            return FALSE;
          break;
        case 16:
          if (!HandleUltra16(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
            return FALSE;
          break;
        case 32:
          if (!HandleUltra32(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
            return FALSE;
          break;
        }
        break;
      }
      case rfbEncodingUltraZip:
      {
        switch (client->format.bitsPerPixel) {
        case 8:
          if (!HandleUltraZip8(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
            return FALSE;
          break;
        case 16:
          if (!HandleUltraZip16(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
            return FALSE;
          break;
        case 32:
          if (!HandleUltraZip32(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
            return FALSE;
          break;
        }
        break;
      }

#ifdef LIBVNCSERVER_HAVE_LIBZ
      case rfbEncodingZlib:
      {
	switch (client->format.bitsPerPixel) {
	case 8:
	  if (!HandleZlib8(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 16:
	  if (!HandleZlib16(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 32:
	  if (!HandleZlib32(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	}
	break;
     }

#ifdef LIBVNCSERVER_HAVE_LIBJPEG
      case rfbEncodingTight:
      {
	switch (client->format.bitsPerPixel) {
	case 8:
	  if (!HandleTight8(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 16:
	  if (!HandleTight16(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 32:
	  if (!HandleTight32(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	}
	break;
      }
#endif
      case rfbEncodingZRLE:
	/* Fail safe for ZYWRLE unsupport VNC server. */
	client->appData.qualityLevel = 9;
	/* fall through */
      case rfbEncodingZYWRLE:
      {
	switch (client->format.bitsPerPixel) {
	case 8:
	  if (!HandleZRLE8(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	case 16:
	  if (client->si.format.greenMax > 0x1F) {
	    if (!HandleZRLE16(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	      return FALSE;
	  } else {
	    if (!HandleZRLE15(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	      return FALSE;
	  }
	  break;
	case 32:
	{
	  uint32_t maxColor=(client->format.redMax<<client->format.redShift)|
		(client->format.greenMax<<client->format.greenShift)|
		(client->format.blueMax<<client->format.blueShift);
	  if ((client->format.bigEndian && (maxColor&0xff)==0) ||
	      (!client->format.bigEndian && (maxColor&0xff000000)==0)) {
	    if (!HandleZRLE24(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	      return FALSE;
	  } else if (!client->format.bigEndian && (maxColor&0xff)==0) {
	    if (!HandleZRLE24Up(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	      return FALSE;
	  } else if (client->format.bigEndian && (maxColor&0xff000000)==0) {
	    if (!HandleZRLE24Down(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	      return FALSE;
	  } else if (!HandleZRLE32(client, rect.r.x,rect.r.y,rect.r.w,rect.r.h))
	    return FALSE;
	  break;
	}
	}
	break;
     }

#endif
#ifdef LIBVNCSERVER_CONFIG_LIBVA
      case rfbEncodingH264:
      {
	if (!HandleH264(client, rect.r.x, rect.r.y, rect.r.w, rect.r.h))
	  return FALSE;
	break;
      }
#endif

      default:
	 {
	   rfbBool handled = FALSE;
	   rfbClientProtocolExtension* e;

	   for(e = rfbClientExtensions; !handled && e; e = e->next)
	     if(e->handleEncoding && e->handleEncoding(client, &rect))
	       handled = TRUE;

	   if(!handled) {
	     rfbClientLog("Unknown rect encoding %d\n",
		 (int)rect.encoding);
	     return FALSE;
	   }
	 }
      }

      /* Now we may discard "soft cursor locks". */
      client->SoftCursorUnlockScreen(client);

      client->GotFrameBufferUpdate(client, rect.r.x, rect.r.y, rect.r.w, rect.r.h);
    }

    if (!SendIncrementalFramebufferUpdateRequest(client))
      return FALSE;

    if (client->FinishedFrameBufferUpdate)
      client->FinishedFrameBufferUpdate(client);

    break;
  }

  case rfbBell:
  {
    client->Bell(client);

    break;
  }

  case rfbServerCutText:
  {
    char *buffer;

    if (!ReadFromRFBServer(client, ((char *)&msg) + 1,
			   sz_rfbServerCutTextMsg - 1))
      return FALSE;

    msg.sct.length = rfbClientSwap32IfLE(msg.sct.length);

    buffer = malloc(msg.sct.length+1);

    if (!ReadFromRFBServer(client, buffer, msg.sct.length))
      return FALSE;

    buffer[msg.sct.length] = 0;

    if (client->GotXCutText)
      client->GotXCutText(client, buffer, msg.sct.length);

    free(buffer);

    break;
  }

  case rfbTextChat:
  {
      char *buffer=NULL;
      if (!ReadFromRFBServer(client, ((char *)&msg) + 1,
                             sz_rfbTextChatMsg- 1))
        return FALSE;
      msg.tc.length = rfbClientSwap32IfLE(msg.sct.length);
      switch(msg.tc.length) {
      case rfbTextChatOpen:
          rfbClientLog("Received TextChat Open\n");
          if (client->HandleTextChat!=NULL)
              client->HandleTextChat(client, (int)rfbTextChatOpen, NULL);
          break;
      case rfbTextChatClose:
          rfbClientLog("Received TextChat Close\n");
         if (client->HandleTextChat!=NULL)
              client->HandleTextChat(client, (int)rfbTextChatClose, NULL);
          break;
      case rfbTextChatFinished:
          rfbClientLog("Received TextChat Finished\n");
         if (client->HandleTextChat!=NULL)
              client->HandleTextChat(client, (int)rfbTextChatFinished, NULL);
          break;
      default:
          buffer=malloc(msg.tc.length+1);
          if (!ReadFromRFBServer(client, buffer, msg.tc.length))
          {
              free(buffer);
              return FALSE;
          }
          /* Null Terminate <just in case> */
          buffer[msg.tc.length]=0;
          rfbClientLog("Received TextChat \"%s\"\n", buffer);
          if (client->HandleTextChat!=NULL)
              client->HandleTextChat(client, (int)msg.tc.length, buffer);
          free(buffer);
          break;
      }
      break;
  }

  case rfbXvp:
  {
    if (!ReadFromRFBServer(client, ((char *)&msg) + 1,
                           sz_rfbXvpMsg -1))
      return FALSE;

    SetClient2Server(client, rfbXvp);
    /* technically, we only care what we can *send* to the server
     * but, we set Server2Client Just in case it ever becomes useful
     */
    SetServer2Client(client, rfbXvp);

    if(client->HandleXvpMsg)
      client->HandleXvpMsg(client, msg.xvp.version, msg.xvp.code);

    break;
  }

  case rfbResizeFrameBuffer:
  {
    if (!ReadFromRFBServer(client, ((char *)&msg) + 1,
                           sz_rfbResizeFrameBufferMsg -1))
      return FALSE;
    client->width = rfbClientSwap16IfLE(msg.rsfb.framebufferWidth);
    client->height = rfbClientSwap16IfLE(msg.rsfb.framebufferHeigth);
    client->updateRect.x = client->updateRect.y = 0;
    client->updateRect.w = client->width;
    client->updateRect.h = client->height;
    if (!client->MallocFrameBuffer(client))
      return FALSE;

    SendFramebufferUpdateRequest(client, 0, 0, client->width, client->height, FALSE);
    rfbClientLog("Got new framebuffer size: %dx%d\n", client->width, client->height);
    break;
  }

  case rfbPalmVNCReSizeFrameBuffer:
  {
    if (!ReadFromRFBServer(client, ((char *)&msg) + 1,
                           sz_rfbPalmVNCReSizeFrameBufferMsg -1))
      return FALSE;
    client->width = rfbClientSwap16IfLE(msg.prsfb.buffer_w);
    client->height = rfbClientSwap16IfLE(msg.prsfb.buffer_h);
    client->updateRect.x = client->updateRect.y = 0;
    client->updateRect.w = client->width;
    client->updateRect.h = client->height;
    if (!client->MallocFrameBuffer(client))
      return FALSE;
    SendFramebufferUpdateRequest(client, 0, 0, client->width, client->height, FALSE);
    rfbClientLog("Got new framebuffer size: %dx%d\n", client->width, client->height);
    break;
  }

  default:
    {
      rfbBool handled = FALSE;
      rfbClientProtocolExtension* e;

      for(e = rfbClientExtensions; !handled && e; e = e->next)
	if(e->handleMessage && e->handleMessage(client, &msg))
	  handled = TRUE;

      if(!handled) {
	char buffer[256];
	rfbClientLog("Unknown message type %d from VNC server\n",msg.type);
	ReadFromRFBServer(client, buffer, 256);
	return FALSE;
      }
    }
  }

  return TRUE;
}