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>

4 files changed, 17 insertions, 25 deletions

diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 4593d48df953..a305e0c5925a 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -201,7 +201,9 @@ struct l2tp_tunnel {
 };
 
 struct l2tp_nl_cmd_ops {
-        int (*session_create)(struct net *net, u32 tunnel_id, u32 session_id, u32 peer_session_id, struct l2tp_session_cfg *cfg);
+        int (*session_create)(struct net *net, struct l2tp_tunnel *tunnel,
+                              u32 session_id, u32 peer_session_id,
+                              struct l2tp_session_cfg *cfg);
         int (*session_delete)(struct l2tp_session *session);
 };
 
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;
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index 57427d430f10..7135f4645d3a 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -643,10 +643,10 @@ static int l2tp_nl_cmd_session_create(struct sk_buff *skb, struct genl_info *inf
                 break;
         }
 
-        ret = -EPROTONOSUPPORT;
-        if (l2tp_nl_cmd_ops[cfg.pw_type]->session_create)
-                ret = (*l2tp_nl_cmd_ops[cfg.pw_type]->session_create)(net, tunnel_id,
-                        session_id, peer_session_id, &cfg);
+        ret = l2tp_nl_cmd_ops[cfg.pw_type]->session_create(net, tunnel,
+                                                           session_id,
+                                                           peer_session_id,
+                                                           &cfg);
 
         if (ret >= 0) {
                 session = l2tp_session_get(net, tunnel, session_id, false);
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index f0edb7209079..50e3ee9a9d61 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -788,25 +788,20 @@ end:
 
 #ifdef CONFIG_L2TP_V3
 
-/* Called when creating sessions via the netlink interface.
- */
-static int pppol2tp_session_create(struct net *net, u32 tunnel_id, u32 session_id, u32 peer_session_id, struct l2tp_session_cfg *cfg)
+/* Called when creating sessions via the netlink interface. */
+static int pppol2tp_session_create(struct net *net, struct l2tp_tunnel *tunnel,
+                                   u32 session_id, u32 peer_session_id,
+                                   struct l2tp_session_cfg *cfg)
 {
         int error;
-        struct l2tp_tunnel *tunnel;
         struct l2tp_session *session;
         struct pppol2tp_session *ps;
 
-        tunnel = l2tp_tunnel_find(net, tunnel_id);
-
-        /* Error if we can't find the tunnel */
-        error = -ENOENT;
-        if (tunnel == NULL)
-                goto out;
-
         /* Error if tunnel socket is not prepped */
-        if (tunnel->sock == NULL)
+        if (!tunnel->sock) {
+                error = -ENOENT;
                 goto out;
+        }
 
         /* Default MTU values. */
         if (cfg->mtu == 0)