[PATCH 1/1][SRU][X] l2tp: pass tunnel pointer to ->session_create()

[AceLan Kao]

From: Guillaume Nault <g.nault at alphalink.fr>

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().

CVE-2018-9517

Fixes: 309795f4bec2 ("l2tp: Add netlink control API for L2TP")
Fixes: d9e31d17ceba ("l2tp: Add L2TP ethernet pseudowire support")
Signed-off-by: Guillaume Nault <g.nault at alphalink.fr>
Signed-off-by: David S. Miller <davem at davemloft.net>
(backported from commit f026bc29a8e093edfbb2a77700454b285c97e8ad)
Signed-off-by: AceLan Kao <acelan.kao at canonical.com>
---
 net/l2tp/l2tp_core.h    |  4 +++-
 net/l2tp/l2tp_eth.c     | 11 +++--------
 net/l2tp/l2tp_netlink.c |  8 ++++----
 net/l2tp/l2tp_ppp.c     | 19 +++++++------------
 4 files changed, 17 insertions(+), 25 deletions(-)

diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 9cf546846edb..03602df06f4e 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -209,7 +209,9 @@ 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 e253c26f31ac..f116d969416b 100644
--- a/net/l2tp/l2tp_eth.c
+++ b/net/l2tp/l2tp_eth.c
@@ -206,23 +206,18 @@ static void l2tp_eth_show(struct seq_file *m, void *arg)
 }
 #endif
 
-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)
 {
 	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;
-	}
-
 	session = l2tp_session_find(net, tunnel, session_id);
 	if (session) {
 		rc = -EEXIST;
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index fb3248ff8b48..5a68b776a226 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -620,10 +620,10 @@ 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_find(net, tunnel, session_id);
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 2764c4bd072c..783da2550d7b 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -809,25 +809,20 @@ 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;
+	}
 
 	/* Check that this session doesn't already exist */
 	error = -EEXIST;
-- 
2.17.1