ksmbd: fix use-after-free in kerberos authentication

Setting sess->user = NULL was introduced to fix the dangling pointer
created by ksmbd_free_user. However, it is possible another thread could
be operating on the session and make use of sess->user after it has been
passed to ksmbd_free_user but before sess->user is set to NULL.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Heelan <seanheelan@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: SeanHeelan

fs/smb/server/auth.c

@@ -550,7 +550,19 @@ int ksmbd_krb5_authenticate(struct ksmbd_session *sess, char *in_blob,
 		retval = -ENOMEM;
 		goto out;
 	}
-	sess->user = user;
+
+	if (!sess->user) {
+		/* First successful authentication */
+		sess->user = user;
+	} else {
+		if (!ksmbd_compare_user(sess->user, user)) {
+			ksmbd_debug(AUTH, "different user tried to reuse session\n");
+			retval = -EPERM;
+			ksmbd_free_user(user);
+			goto out;
+		}
+		ksmbd_free_user(user);
+	}
 
 	memcpy(sess->sess_key, resp->payload, resp->session_key_len);
 	memcpy(out_blob, resp->payload + resp->session_key_len,

fs/smb/server/smb2pdu.c

@@ -1607,11 +1607,6 @@ static int krb5_authenticate(struct ksmbd_work *work,
 	if (prev_sess_id && prev_sess_id != sess->id)
 		destroy_previous_session(conn, sess->user, prev_sess_id);
 
-	if (sess->state == SMB2_SESSION_VALID) {
-		ksmbd_free_user(sess->user);
-		sess->user = NULL;
-	}
-
 	retval = ksmbd_krb5_authenticate(sess, in_blob, in_len,
 					 out_blob, &out_len);
 	if (retval) {