Fix OOB writes in contrib/leasequesry/leasequery.c

Thanks to Michalis Vasileiadis for spotting this.

print_mac() writes each MAC byte to its output buffer with unbounded
sprintf and is called from main() with pkt.header.hlen straight out
of a BOOTREPLY. hlen is an attacker-controlled uint8_t (up to 255),
each byte expands to up to 3 chars, and the destination is a 500-byte
stack buffer. A malicious leasequery server that echoes the client's
transaction ID and replies with DHCPLEASEACTIVE and an oversized
hlen overflows that stack buffer.

The patch caps len inside print_mac at DHCP_CHADDR_MAX (16), which is
the actual size of chaddr in the BOOTP header and an upper bound on
any legitimate hardware-address length. The other in-file caller of
print_mac already clamps its length argument to 14 before the call,
so this change is local to the vulnerable path.

contrib/leasequery/leasequery.c

@@ -331,9 +331,15 @@ static char *print_mac(char *buff, unsigned char *mac, int len)
   if (len == 0)
     sprintf(p, "<null>");
   else
-    for (i = 0; i < len; i++)
-      p += sprintf(p, "%.2x%s", mac[i], (i == len - 1) ? "" : ":");
-  
+    {
+      /* Server-controlled lengths (e.g. BOOTREPLY hlen) must not be trusted
+	 to fit the destination buffer; cap at the actual chaddr field size. */
+      if (len > DHCP_CHADDR_MAX)
+	len = DHCP_CHADDR_MAX;
+      for (i = 0; i < len; i++)
+	p += sprintf(p, "%.2x%s", mac[i], (i == len - 1) ? "" : ":");
+    }
+
   return buff;
 }