[PATCH AUTOSEL 6.11 001/244] l2tp: prevent possible tunnel refcount underflow

Sasha Levin
2024-09-25T11:36:45Z

From: James Chapman <jchapman@katalix.com>

[ Upstream commit 24256415d18695b46da06c93135f5b51c548b950 ]

When a session is created, it sets a backpointer to its tunnel. When
the session refcount drops to 0, l2tp_session_free drops the tunnel
refcount if session->tunnel is non-NULL. However, session->tunnel is
set in l2tp_session_create, before the tunnel refcount is incremented
by l2tp_session_register, which leaves a small window where
session->tunnel is non-NULL when the tunnel refcount hasn't been
bumped.

Moving the assignment to l2tp_session_register is trivial but
l2tp_session_create calls l2tp_session_set_header_len which uses
session->tunnel to get the tunnel's encap. Add an encap arg to
l2tp_session_set_header_len to avoid using session->tunnel.

If l2tpv3 sessions have colliding IDs, it is possible for
l2tp_v3_session_get to race with l2tp_session_register and fetch a
session which doesn't yet have session->tunnel set. Add a check for
this case.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/l2tp/l2tp_core.c | 24 +++++++++++++++++-------
net/l2tp/l2tp_core.h | 3 ++-
net/l2tp/l2tp_netlink.c | 4 +++-
net/l2tp/l2tp_ppp.c | 3 ++-
4 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 2e86f520f7994..a9cbcbc9d016d 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -254,7 +254,14 @@ struct l2tp_session *l2tp_v3_session_get(const struct net *net, struct sock *sk,

hash_for_each_possible_rcu(pn->l2tp_v3_session_htable, session,
hlist, key) {
- if (session->tunnel->sock == sk &&
+ /* session->tunnel may be NULL if another thread is in
+ * l2tp_session_register and has added an item to
+ * l2tp_v3_session_htable but hasn't yet added the
+ * session to its tunnel's session_list.
+ */
+ struct l2tp_tunnel *tunnel = READ_ONCE(session->tunnel);
+
+ if (tunnel && tunnel->sock == sk &&
refcount_inc_not_zero(&session->ref_count)) {
rcu_read_unlock_bh();
return session;
@@ -482,6 +489,7 @@ int l2tp_session_register(struct l2tp_session *session,
}

l2tp_tunnel_inc_refcount(tunnel);
+ WRITE_ONCE(session->tunnel, tunnel);
list_add(&session->list, &tunnel->session_list);

if (tunnel->version == L2TP_HDR_VER_3) {
@@ -797,7 +805,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
if (!session->lns_mode && !session->send_seq) {
trace_session_seqnum_lns_enable(session);
session->send_seq = 1;
- l2tp_session_set_header_len(session, tunnel->version);
+ l2tp_session_set_header_len(session, tunnel->version,
+ tunnel->encap);
}
} else {
/* No sequence numbers.
@@ -818,7 +827,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
if (!session->lns_mode && session->send_seq) {
trace_session_seqnum_lns_disable(session);
session->send_seq = 0;
- l2tp_session_set_header_len(session, tunnel->version);
+ l2tp_session_set_header_len(session, tunnel->version,
+ tunnel->encap);
} else if (session->send_seq) {
pr_debug_ratelimited("%s: recv data has no seq numbers when required. Discarding.\n",
session->name);
@@ -1663,7 +1673,8 @@ EXPORT_SYMBOL_GPL(l2tp_session_delete);
/* We come here whenever a session's send_seq, cookie_len or
* l2specific_type parameters are set.
*/
-void l2tp_session_set_header_len(struct l2tp_session *session, int version)
+void l2tp_session_set_header_len(struct l2tp_session *session, int version,
+ enum l2tp_encap_type encap)
{
if (version == L2TP_HDR_VER_2) {
session->hdr_len = 6;
@@ -1672,7 +1683,7 @@ void l2tp_session_set_header_len(struct l2tp_session *session, int version)
} else {
session->hdr_len = 4 + session->cookie_len;
session->hdr_len += l2tp_get_l2specific_len(session);
- if (session->tunnel->encap == L2TP_ENCAPTYPE_UDP)
+ if (encap == L2TP_ENCAPTYPE_UDP)
session->hdr_len += 4;
}
}
@@ -1686,7 +1697,6 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
session = kzalloc(sizeof(*session) + priv_size, GFP_KERNEL);
if (session) {
session->magic = L2TP_SESSION_MAGIC;
- session->tunnel = tunnel;

session->session_id = session_id;
session->peer_session_id = peer_session_id;
@@ -1724,7 +1734,7 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
memcpy(&session->peer_cookie[0], &cfg->peer_cookie[0], cfg->peer_cookie_len);
}

- l2tp_session_set_header_len(session, tunnel->version);
+ l2tp_session_set_header_len(session, tunnel->version, tunnel->encap);

refcount_set(&session->ref_count, 1);

diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 8ac81bc1bc6fa..6c25c196cc222 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -260,7 +260,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb);

/* Transmit path helpers for sending packets over the tunnel socket. */
-void l2tp_session_set_header_len(struct l2tp_session *session, int version);
+void l2tp_session_set_header_len(struct l2tp_session *session, int version,
+ enum l2tp_encap_type encap);
int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb);

/* Pseudowire management.
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index d105030520f95..fc43ecbd128cc 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -692,8 +692,10 @@ static int l2tp_nl_cmd_session_modify(struct sk_buff *skb, struct genl_info *inf
session->recv_seq = nla_get_u8(info->attrs[L2TP_ATTR_RECV_SEQ]);

if (info->attrs[L2TP_ATTR_SEND_SEQ]) {
+ struct l2tp_tunnel *tunnel = session->tunnel;
+
session->send_seq = nla_get_u8(info->attrs[L2TP_ATTR_SEND_SEQ]);
- l2tp_session_set_header_len(session, session->tunnel->version);
+ l2tp_session_set_header_len(session, tunnel->version, tunnel->encap);
}

if (info->attrs[L2TP_ATTR_LNS_MODE])
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 3596290047b28..4f25c1212cacb 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -1205,7 +1205,8 @@ static int pppol2tp_session_setsockopt(struct sock *sk,
po->chan.hdrlen = val ? PPPOL2TP_L2TP_HDR_SIZE_SEQ :
PPPOL2TP_L2TP_HDR_SIZE_NOSEQ;
}
- l2tp_session_set_header_len(session, session->tunnel->version);
+ l2tp_session_set_header_len(session, session->tunnel->version,
+ session->tunnel->encap);
break;

case PPPOL2TP_SO_LNSMODE:
--
2.43.0