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>

2 files changed, 7 insertions, 0 deletions

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index ee485df73ccd..d8c2a89a76e1 100644
--- a/net/l2tp/l2tp_core.c
+++ b/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);
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index a305e0c5925a..70a12df40a5f 100644
--- a/net/l2tp/l2tp_core.h
+++ b/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 */