krb5 commit: Prevent null dereference with keyboard master key

[Greg Hudson]

https://github.com/krb5/krb5/commit/29c504504f0c56c861d968ba2498590bf34714cd
commit 29c504504f0c56c861d968ba2498590bf34714cd
Author: Greg Hudson <ghudson at mit.edu>
Date:   Tue Jul 18 12:29:12 2017 -0400

    Prevent null dereference with keyboard master key
    
    If krb5_db_fetch_mkey() prompts for a master key and needs to
    determine the kvno, check that the master entry contains any key data
    before dereferencing the first element.  Reported by Joshua Schaeffer.
    
    ticket: 8600 (new)
    target_version: 1.15-next
    target_version: 1.14-next
    tags: pullup

 src/lib/kdb/kdb5.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/lib/kdb/kdb5.c b/src/lib/kdb/kdb5.c
index ad637b6..da53322 100644
--- a/src/lib/kdb/kdb5.c
+++ b/src/lib/kdb/kdb5.c
@@ -1215,11 +1215,12 @@ krb5_db_fetch_mkey(krb5_context context, krb5_principal mname,
             krb5_db_entry *master_entry;
 
             rc = krb5_db_get_principal(context, mname, 0, &master_entry);
-            if (rc == 0) {
+            if (rc == 0 && master_entry->n_key_data > 0)
                 *kvno = (krb5_kvno) master_entry->key_data->key_data_kvno;
-                krb5_db_free_principal(context, master_entry);
-            } else
+            else
                 *kvno = 1;
+            if (rc == 0)
+                krb5_db_free_principal(context, master_entry);
         }
 
         if (!salt)