l2tp: fix race between l2tp_session_delete() and l2tp_tunnel_closeall()

There are several ways to remove L2TP sessions:

  * deleting a session explicitly using the netlink interface (with
    L2TP_CMD_SESSION_DELETE),
  * deleting the session's parent tunnel (either by closing the
    tunnel's file descriptor or using the netlink interface),
  * closing the PPPOL2TP file descriptor of a PPP pseudo-wire.

In some cases, when these methods are used concurrently on the same
session, the session can be removed twice, leading to use-after-free
bugs.

This patch adds a 'dead' flag, used by l2tp_session_delete() and
l2tp_tunnel_closeall() to prevent them from stepping on each other's
toes.

The session deletion path used when closing a PPPOL2TP file descriptor
doesn't need to be adapted. It already has to ensure that a session
remains valid for the lifetime of its PPPOL2TP file descriptor.
So it takes an extra reference on the session in the ->session_close()
callback (pppol2tp_session_close()), which is eventually dropped
in the ->sk_destruct() callback of the PPPOL2TP socket
(pppol2tp_session_destruct()).
Still, __l2tp_session_unhash() and l2tp_session_queue_purge() can be
called twice and even concurrently for a given session, but thanks to
proper locking and re-initialisation of list fields, this is not an
issue.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
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 */