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

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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>
This commit is contained in:
Guillaume Nault 2017-09-22 15:39:24 +02:00 • committed by David S. Miller
parent cdd10c9627
commit b228a94066
2 changed files with 7 additions and 0 deletions
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6
net/l2tp/l2tp_core.c
View file

@ -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);

1
net/l2tp/l2tp_core.h
View file

@ -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 */