Verify decoded kadmin C strings [CVE-2015-8629]

In xdr_nullstring(), check that the decoded string is terminated with
a zero byte and does not contain any internal zero bytes.

CVE-2015-8629:

In all versions of MIT krb5, an authenticated attacker can cause
kadmind to read beyond the end of allocated memory by sending a string
without a terminating zero byte.  Information leakage may be possible
for an attacker with permission to modify the database.

    CVSSv2 Vector: AV:N/AC:H/Au:S/C:P/I:N/A:N/E:POC/RL:OF/RC:C

(cherry picked from commit df17a12)

ticket: 8341
version_fixed: 1.13.4
tags: -pullup
status: resolved

src/lib/kadm5/kadm_rpc_xdr.c

@@ -64,7 +64,14 @@ bool_t xdr_nullstring(XDR *xdrs, char **objp)
 		    return FALSE;
 	       }
 	  }
-	  return (xdr_opaque(xdrs, *objp, size));
+	  if (!xdr_opaque(xdrs, *objp, size))
+		  return FALSE;
+	  /* Check that the unmarshalled bytes are a C string. */
+	  if ((*objp)[size - 1] != '\0')
+		  return FALSE;
+	  if (memchr(*objp, '\0', size - 1) != NULL)
+		  return FALSE;
+	  return TRUE;
 
      case XDR_ENCODE:
 	  if (size != 0)