Return early from parsing DHCP response if End is encoutered

Up until now DHCP response was parsed by reading the option
and the length first. The length field obviously does not
apply to the End option, so if muvm got a response that ended exactly
at the End byte, muvm would crash because of an out-of-bound read.

Signed-off-by: Nikodem Rabuliński <[email protected]>

Author: nrabulinski

crates/muvm/src/guest/net.rs

@@ -171,6 +171,12 @@ fn do_dhcp(rtnl: &NlRouter) -> Result<()> {
 
         while p < len {
             let o = msg[p];
+
+            if o == 0xff {
+                // Option 255: End (of options)
+                break;
+            }
+
             let l: u8 = msg[p + 1];
             p += 2; // Length doesn't include code and length field itself
 
@@ -195,9 +201,6 @@ fn do_dhcp(rtnl: &NlRouter) -> Result<()> {
 
                 // We don't know yet if IPv6 is available: don't go below 1280 B
                 mtu = mtu.clamp(1280, 65520);
-            } else if o == 0xff {
-                // Option 255: End (of options)
-                break;
             }
 
             p += l as usize;