Merge branch 'l2tp-fix-some-races-in-session-deletion'

Guillaume Nault says:

====================
l2tp: fix some races in session deletion

L2TP provides several interfaces for deleting sessions. Using two of
them concurrently can lead to use-after-free bugs.

Patch #2 uses a flag to prevent double removal of L2TP sessions.
Patch #1 fixes a bug found in the way. Fixing this bug is also
necessary for patch #2 to handle all cases.

This issue is similar to the tunnel deletion bug being worked on by
Sabrina: https://patchwork.ozlabs.org/patch/814173/
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -1314,6 +1314,9 @@ again:
 
 			hlist_del_init(&session->hlist);
 
+			if (test_and_set_bit(0, &session->dead))
+				goto again;
+
 			if (session->ref != NULL)
 				(*session->ref)(session);
 
@@ -1750,6 +1753,9 @@ EXPORT_SYMBOL_GPL(__l2tp_session_unhash);
  */
 int l2tp_session_delete(struct l2tp_session *session)
 {
+	if (test_and_set_bit(0, &session->dead))
+		return 0;
+
 	if (session->ref)
 		(*session->ref)(session);
 	__l2tp_session_unhash(session);

net/l2tp/l2tp_core.h

@@ -76,6 +76,7 @@ struct l2tp_session_cfg {
 struct l2tp_session {
 	int			magic;		/* should be
 						 * L2TP_SESSION_MAGIC */
+	long			dead;
 
 	struct l2tp_tunnel	*tunnel;	/* back pointer to tunnel
 						 * context */

net/l2tp/l2tp_ppp.c

@@ -437,11 +437,11 @@ static void pppol2tp_session_close(struct l2tp_session *session)
 
 	BUG_ON(session->magic != L2TP_SESSION_MAGIC);
 
-	if (sock) {
+	if (sock)
 		inet_shutdown(sock, SEND_SHUTDOWN);
-		/* Don't let the session go away before our socket does */
-		l2tp_session_inc_refcount(session);
-	}
+
+	/* Don't let the session go away before our socket does */
+	l2tp_session_inc_refcount(session);
 }
 
 /* Really kill the session socket. (Called from sock_put() if