l2tp: pass tunnel pointer to ->session_create()

Using l2tp_tunnel_find() in pppol2tp_session_create() and
l2tp_eth_create() is racy, because no reference is held on the
returned session. These functions are only used to implement the
->session_create callback which is run by l2tp_nl_cmd_session_create().
Therefore searching for the parent tunnel isn't necessary because
l2tp_nl_cmd_session_create() already has a pointer to it and holds a
reference.

This patch modifies ->session_create()'s prototype to directly pass the
the parent tunnel as parameter, thus avoiding searching for it in
pppol2tp_session_create() and l2tp_eth_create().

Since we have to touch the ->session_create() call in
l2tp_nl_cmd_session_create(), let's also remove the useless conditional:
we know that ->session_create isn't NULL at this point because it's
already been checked earlier in this same function.

Finally, one might be tempted to think that the removed
l2tp_tunnel_find() calls were harmless because they would return the
same tunnel as the one held by l2tp_nl_cmd_session_create() anyway.
But that tunnel might be removed and a new one created with same tunnel
Id before the l2tp_tunnel_find() call. In this case l2tp_tunnel_find()
would return the new tunnel which wouldn't be protected by the
reference held by l2tp_nl_cmd_session_create().

Fixes: 309795f4bec2 ("l2tp: Add netlink control API for L2TP")
Fixes: d9e31d17ceba ("l2tp: Add L2TP ethernet pseudowire support")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 8 deletions

diff --git a/net/l2tp/l2tp_eth.c b/net/l2tp/l2tp_eth.c
index 4de2ec94b08c..87da9ef61860 100644
--- a/net/l2tp/l2tp_eth.c
+++ b/net/l2tp/l2tp_eth.c
@@ -262,24 +262,19 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
 	dev->needed_headroom += session->hdr_len;
 }
 
-static int l2tp_eth_create(struct net *net, u32 tunnel_id, u32 session_id, u32 peer_session_id, struct l2tp_session_cfg *cfg)
+static int l2tp_eth_create(struct net *net, struct l2tp_tunnel *tunnel,
+			   u32 session_id, u32 peer_session_id,
+			   struct l2tp_session_cfg *cfg)
 {
 	unsigned char name_assign_type;
 	struct net_device *dev;
 	char name[IFNAMSIZ];
-	struct l2tp_tunnel *tunnel;
 	struct l2tp_session *session;
 	struct l2tp_eth *priv;
 	struct l2tp_eth_sess *spriv;
 	int rc;
 	struct l2tp_eth_net *pn;
 
-	tunnel = l2tp_tunnel_find(net, tunnel_id);
-	if (!tunnel) {
-		rc = -ENODEV;
-		goto out;
-	}
-
 	if (cfg->ifname) {
 		strlcpy(name, cfg->ifname, IFNAMSIZ);
 		name_assign_type = NET_NAME_USER;