[PATCH 4.9 01/32] l2tp: ensure sessions are freed after their PPPOL2TP socket

[Greg Kroah-Hartman]

From: Guillaume Nault <g.nault@xxxxxxxxxxxx>

commit cdd10c9627496ad25c87ce6394e29752253c69d3 upstream.

If l2tp_tunnel_delete() or l2tp_tunnel_closeall() deletes a session
right after pppol2tp_release() orphaned its socket, then the 'sock'
variable of the pppol2tp_session_close() callback is NULL. Yet the
session is still used by pppol2tp_release().

Therefore we need to take an extra reference in any case, to prevent
l2tp_tunnel_delete() or l2tp_tunnel_closeall() from freeing the session.

Since the pppol2tp_session_close() callback is only set if the session
is associated to a PPPOL2TP socket and that both l2tp_tunnel_delete()
and l2tp_tunnel_closeall() hold the PPPOL2TP socket before calling
pppol2tp_session_close(), we're sure that pppol2tp_session_close() and
pppol2tp_session_destruct() are paired and called in the right order.
So the reference taken by the former will be released by the later.

Signed-off-by: Guillaume Nault <g.nault@xxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/l2tp/l2tp_ppp.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -437,11 +437,11 @@ static void pppol2tp_session_close(struc
 
 	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