cifs: fix double-fault crash during ntlmssp

The crash occurred because we were calling memzero_explicit() on an
already freed sess_data::iov[1] (ntlmsspblob) in sess_free_buffer().

Fix this by not calling memzero_explicit() on sess_data::iov[1] as
it's already by handled by callers.

Fixes: a4e430c8c8ba ("cifs: replace kfree() with kfree_sensitive() for sensitive data")
Reviewed-by: Enzo Matsumiya <[e-mail]>
Signed-off-by: Paulo Alcantara (SUSE) <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/sess.c

···
 static void
 sess_free_buffer(struct sess_data *sess_data)
 {
-	int i;
+	struct kvec *iov = sess_data->iov;
 
-	/* zero the session data before freeing, as it might contain sensitive info (keys, etc) */
-	for (i = 0; i < 3; i++)
-		if (sess_data->iov[i].iov_base)
-			memzero_explicit(sess_data->iov[i].iov_base, sess_data->iov[i].iov_len);
+	/*
+	 * Zero the session data before freeing, as it might contain sensitive info (keys, etc).
+	 * Note that iov[1] is already freed by caller.
+	 */
+	if (sess_data->buf0_type != CIFS_NO_BUFFER && iov[0].iov_base)
+		memzero_explicit(iov[0].iov_base, iov[0].iov_len);
 
-	free_rsp_buf(sess_data->buf0_type, sess_data->iov[0].iov_base);
+	free_rsp_buf(sess_data->buf0_type, iov[0].iov_base);
 	sess_data->buf0_type = CIFS_NO_BUFFER;
-	kfree(sess_data->iov[2].iov_base);
+	kfree_sensitive(iov[2].iov_base);
 }
 
 static int