PKINIT (draft9) null ptr deref [CVE-2012-1016]

Don't check for an agility KDF identifier in the non-draft9 reply
structure when we're building a draft9 reply, because it'll be NULL.

The KDC plugin for PKINIT can dereference a null pointer when handling
a draft9 request, leading to a crash of the KDC process.  An attacker
would need to have a valid PKINIT certificate, or an unauthenticated
attacker could execute the attack if anonymous PKINIT is enabled.

CVSSv2 vector: AV:N/AC:M/Au:N/C:N/I:N/A:P/E:P/RL:O/RC:C

[tlyu@mit.edu: reformat comment and edit log message]

(cherry picked from commit cd5ff93)

ticket: 7506
version_fixed: 1.11
status: resolved

src/plugins/preauth/pkinit/pkinit_srv.c

@@ -1026,8 +1026,9 @@ pkinit_server_return_padata(krb5_context context,
          rep9->choice == choice_pa_pk_as_rep_draft9_dhSignedData) ||
         (rep != NULL && rep->choice == choice_pa_pk_as_rep_dhInfo)) {
 
-        /* If mutually supported KDFs were found, use the alg agility KDF */
-        if (rep->u.dh_Info.kdfID) {
+	/* If we're not doing draft 9, and mutually supported KDFs were found,
+	 * use the algorithm agility KDF. */
+        if (rep != NULL && rep->u.dh_Info.kdfID) {
             secret.data = (char *)server_key;
             secret.length = server_key_len;