dix: integer overflow in GetHosts() [CVE-2014-8092 2/4]

GetHosts() iterates over all the hosts it has in memory, and copies
them to a buffer. The buffer length is calculated by iterating over
all the hosts and adding up all of their combined length. There is a
potential integer overflow, if there are lots and lots of hosts (with
a combined length of > ~4 gig). This should be possible by repeatedly
calling ProcChangeHosts() on 64bit machines with enough memory.

This patch caps the list at 1mb, because multi-megabyte hostname
lists for X access control are insane.

Reported-by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>

os/access.c

@@ -1296,6 +1296,10 @@ GetHosts(void **data, int *pnHosts, int *pLen, BOOL * pEnabled)
     for (host = validhosts; host; host = host->next) {
         nHosts++;
         n += pad_to_int32(host->len) + sizeof(xHostEntry);
+        /* Could check for INT_MAX, but in reality having more than 1mb of
+           hostnames in the access list is ridiculous */
+        if (n >= 1048576)
+            break;
     }
     if (n) {
         *data = ptr = malloc(n);
@@ -1304,6 +1308,8 @@ GetHosts(void **data, int *pnHosts, int *pLen, BOOL * pEnabled)
         }
         for (host = validhosts; host; host = host->next) {
             len = host->len;
+            if ((ptr + sizeof(xHostEntry) + len) > (data + n))
+                break;
             ((xHostEntry *) ptr)->family = host->family;
             ((xHostEntry *) ptr)->length = len;
             ptr += sizeof(xHostEntry);