[PATCH net 0/4] l2tp: pppol2tp_connect() fixes

This series fixes a few remaining issues with pppol2tp_connect().

It doesn't try to prevent invalid configurations that have no effect on
kernel's reliability. That would be work for a future patch set.

Patch 2 is the most important as it avoids an invalid pointer
dereference crashing the kernel. It depends on patch 1 for correctly
identifying L2TP session types.

Patches 3 and 4 avoid creating stale tunnels and sessions.

Guillaume Nault (4):
  l2tp: fix pseudo-wire type for sessions created by pppol2tp_connect()
  l2tp: only accept PPP sessions in pppol2tp_connect()
  l2tp: prevent pppol2tp_connect() from creating kernel sockets
  l2tp: clean up stale tunnel or session in pppol2tp_connect's error
    path

 net/l2tp/l2tp_ppp.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

-- 
2.17.1

Define cfg.pw_type so that the new session is created with its .pwtype
field properly set (L2TP_PWTYPE_PPP).

Not setting the pseudo-wire type had several annoying effects:

  * Invalid value returned in the L2TP_ATTR_PW_TYPE attribute when
    dumping sessions with the netlink API.

  * Impossibility to delete the session using the netlink API (because
    l2tp_nl_cmd_session_delete() gets the deletion callback function
    from an array indexed by the session's pseudo-wire type).

Also, there are several cases where we should check a session's
pseudo-wire type. For example, pppol2tp_connect() should refuse to
connect a session that is not PPPoL2TP, but that requires the session's
.pwtype field to be properly set.

Fixes: f7faffa3ff8e ("l2tp: Add L2TPv3 protocol support")
Signed-off-by: Guillaume Nault <redacted>
---
 net/l2tp/l2tp_ppp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index b56cb1df4fc0..270a0a999eaf 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c

@@ -751,6 +751,7 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 		/* Default MTU must allow space for UDP/L2TP/PPP headers */
 		cfg.mtu = 1500 - PPPOL2TP_HEADER_OVERHEAD;
 		cfg.mru = cfg.mtu;
+		cfg.pw_type = L2TP_PWTYPE_PPP;
 
 		session = l2tp_session_create(sizeof(struct pppol2tp_session),
 					      tunnel, session_id,

-- 
2.17.1

l2tp_session_priv() returns a struct pppol2tp_session pointer only for
PPPoL2TP sessions. In particular, if the session is an L2TP_PWTYPE_ETH
pseudo-wire, l2tp_session_priv() returns a pointer to an l2tp_eth_sess
structure, which is much smaller than struct pppol2tp_session. This
leads to invalid memory dereference when trying to lock ps->sk_lock.

Fixes: d9e31d17ceba ("l2tp: Add L2TP ethernet pseudowire support")
Signed-off-by: Guillaume Nault <redacted>
---
 net/l2tp/l2tp_ppp.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 270a0a999eaf..8b3b6947a07d 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c

@@ -734,6 +734,12 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	session = l2tp_session_get(sock_net(sk), tunnel, session_id);
 	if (session) {
 		drop_refcnt = true;
+
+		if (session->pwtype != L2TP_PWTYPE_PPP) {
+			error = -EPROTOTYPE;
+			goto end;
+		}
+
 		ps = l2tp_session_priv(session);
 
 		/* Using a pre-existing session is fine as long as it hasn't

-- 
2.17.1

pppol2tp_connect() may create a tunnel or a session. Remove them in
case of error.

Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Guillaume Nault <redacted>
---
 net/l2tp/l2tp_ppp.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 1b24f76ae210..f429fed06a1e 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c

@@ -612,6 +612,8 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	u32 session_id, peer_session_id;
 	bool drop_refcnt = false;
 	bool drop_tunnel = false;
+	bool new_session = false;
+	bool new_tunnel = false;
 	int ver = 2;
 	int fd;
 

@@ -722,6 +724,7 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 				goto end;
 			}
 			drop_tunnel = true;
+			new_tunnel = true;
 		}
 	} else {
 		/* Error if we can't find the tunnel */

@@ -788,6 +791,7 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 			goto end;
 		}
 		drop_refcnt = true;
+		new_session = true;
 	}
 
 	/* Special case: if source & dest session_id == 0x0000, this

@@ -834,6 +838,12 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 		  session->name);
 
 end:
+	if (error) {
+		if (new_session)
+			l2tp_session_delete(session);
+		if (new_tunnel)
+			l2tp_tunnel_delete(tunnel);
+	}
 	if (drop_refcnt)
 		l2tp_session_dec_refcount(session);
 	if (drop_tunnel)

-- 
2.17.1

If 'fd' is negative, l2tp_tunnel_create() creates a tunnel socket using
the configuration passed in 'tcfg'. Currently, pppol2tp_connect() sets
the relevant fields to zero, tricking l2tp_tunnel_create() into setting
up an unusable kernel socket.

We can't set 'tcfg' with the required fields because there's no way to
get them from the current connect() parameters. So let's restrict
kernel sockets creation to the netlink API, which is the original use
case.

Fixes: 789a4a2c61d8 ("l2tp: Add support for static unmanaged L2TPv3 tunnels")
Signed-off-by: Guillaume Nault <redacted>
---
 net/l2tp/l2tp_ppp.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 8b3b6947a07d..1b24f76ae210 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c

@@ -701,6 +701,15 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 				.encap = L2TP_ENCAPTYPE_UDP,
 				.debug = 0,
 			};
+
+			/* Prevent l2tp_tunnel_register() from trying to set up
+			 * a kernel socket.
+			 */
+			if (fd < 0) {
+				error = -EBADF;
+				goto end;
+			}
+
 			error = l2tp_tunnel_create(sock_net(sk), fd, ver, tunnel_id, peer_tunnel_id, &tcfg, &tunnel);
 			if (error < 0)
 				goto end;

-- 
2.17.1

From: Guillaume Nault <redacted>
Date: Wed, 13 Jun 2018 15:09:16 +0200

This series fixes a few remaining issues with pppol2tp_connect().

It doesn't try to prevent invalid configurations that have no effect on
kernel's reliability. That would be work for a future patch set.

Patch 2 is the most important as it avoids an invalid pointer
dereference crashing the kernel. It depends on patch 1 for correctly
identifying L2TP session types.

Patches 3 and 4 avoid creating stale tunnels and sessions.

Series applied, thank you.