CVE-2010-3849: Linux kernel PF_ECONET NULL Pointer Dereference

This is the next vulnerability being killed by Nelson Elhage. While reading econet_sendmsg() (check out CVE-2010-3848) is almost impossible to avoid noticing this comment:

        /*
         *      Get and verify the address.
         */

        mutex_lock(&econet_mutex);

        if (saddr == NULL) {
                struct econet_sock *eo = ec_sk(sk);

                addr.station = eo->station;
                addr.net     = eo->net;
                port         = eo->port;
                cb           = eo->cb;
        } else {
                if (msg->msg_namelen < sizeof(struct sockaddr_ec)) {
                        mutex_unlock(&econet_mutex);
                        return -EINVAL;
                }
                addr.station = saddr->addr.station;
                addr.net = saddr->addr.net;
                port = saddr->port;
                cb = saddr->cb;
        }
   ...
        if (dev->type == ARPHRD_ECONET) {
                /* Real hardware Econet.  We're not worthy etc. */
#ifdef CONFIG_ECONET_NATIVE
   ...
                /* BUG: saddr may be NULL */
                eb->cookie = saddr->cookie;
                eb->sec = *saddr;
                eb->sent = ec_tx_done;

You can read that there is an initial check of ‘saddr’ being NULL that initializes ‘addr’ appropriately. However, In case of a hardware ECONET type, it will initialize ‘eb’ structure’s members without any checks. There is even a comment to help us out. To fix this, the NULL checking code was changed.

        mutex_lock(&econet_mutex);
 
-       if (saddr == NULL) {
-               struct econet_sock *eo = ec_sk(sk);
-
-               addr.station = eo->station;
-               addr.net     = eo->net;
-               port         = eo->port;
-               cb           = eo->cb;
-       } else {
-               if (msg->msg_namelen < sizeof(struct sockaddr_ec)) {
-                       mutex_unlock(&econet_mutex);
-                       return -EINVAL;
-               }
-               addr.station = saddr->addr.station;
-               addr.net = saddr->addr.net;
-               port = saddr->port;
-               cb = saddr->cb;
-       }
+        if (saddr == NULL || msg->msg_namelen < sizeof(struct sockaddr_ec)) {
+                mutex_unlock(&econet_mutex);
+                return -EINVAL;
+        }
+        addr.station = saddr->addr.station;
+        addr.net = saddr->addr.net;
+        port = saddr->port;
+        cb = saddr->cb;
 
        /* Look for a device with the right network number. */

This way, if either ‘saddr’ is NULL or ‘msg->msg_namelen’ is less than the size of ‘sockaddr_ec’ structure it will immediately return with ‘-EINVAL’ (Invalid Argument) before reaching the code that triggers the NULL pointer dereference. If this is not the case, it will initialize the appropriate structures using ‘saddr’.