l2tp: prevent l2tp_tunnel_delete racing with userspace close

If a tunnel socket is created by userspace, l2tp hooks the socket destructor
in order to clean up resources if userspace closes the socket or crashes.  It
also caches a pointer to the struct sock for use in the data path and in the
netlink interface.

While it is safe to use the cached sock pointer in the data path, where the
skb references keep the socket alive, it is not safe to use it elsewhere as
such access introduces a race with userspace closing the socket.  In
particular, l2tp_tunnel_delete is prone to oopsing if a multithreaded
userspace application closes a socket at the same time as sending a netlink
delete command for the tunnel.

This patch fixes this oops by forcing l2tp_tunnel_delete to explicitly look up
a tunnel socket held by userspace using sockfd_lookup().

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 1 deletions

diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 56d583e083a7..e62204cad4fe 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -188,7 +188,8 @@ struct l2tp_tunnel {
 	int (*recv_payload_hook)(struct sk_buff *skb);
 	void (*old_sk_destruct)(struct sock *);
 	struct sock		*sock;		/* Parent socket */
-	int			fd;
+	int			fd;		/* Parent fd, if tunnel socket
+						 * was created by userspace */
 
 	uint8_t			priv[0];	/* private data */
 };
@@ -228,6 +229,8 @@ out:
 	return tunnel;
 }
 
+extern struct sock *l2tp_tunnel_sock_lookup(struct l2tp_tunnel *tunnel);
+extern void l2tp_tunnel_sock_put(struct sock *sk);
 extern struct l2tp_session *l2tp_session_find(struct net *net, struct l2tp_tunnel *tunnel, u32 session_id);
 extern struct l2tp_session *l2tp_session_find_nth(struct l2tp_tunnel *tunnel, int nth);
 extern struct l2tp_session *l2tp_session_find_by_ifname(struct net *net, char *ifname);