[PATCH net v3 3/4] net: mptcp: reject token 0 in mptcp_token_get_sock()

[Hang Nan]

Token 0 is never assigned to a connection: __token_bucket_busy() treats
it as busy.  It is, however, the value mptcp_token_destroy() stores into a
socket that is being torn down, and the value a freshly allocated msk
holds until it is hashed.

Both validations in mptcp_token_get_sock() compare the socket's token
with the requested one, so for a request for token 0 they cannot tell a
match apart from a socket whose token has just been cleared.  With
SLAB_TYPESAFE_BY_RCU the same is true for a socket whose memory has
already been reused by a new msk that has not been hashed yet.

The token comes from the wire: mptcp_parse_option() copies the MP_JOIN
SYN token without validating it.  A lookup for token 0 can therefore
return a dying or half-initialised msk, which subflow_token_join_request()
then hands to mptcp_pm_get_local_id() and, when the subflow uses a
different source port, to mptcp_pm_announced_has_ssk(), which walks
msk->pm.anno_list.  That list is only initialised later, from
mptcp_pm_data_init().

Reject token 0 before walking the chain.

Fixes: 2c5ebd001d4f ("mptcp: refactor token container")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hang Nan <2122295973@xxxxxx>
---
 net/mptcp/token.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/mptcp/token.c b/net/mptcp/token.c
index 3429b43a9f7c..8a603fc8dc05 100644
--- a/net/mptcp/token.c
+++ b/net/mptcp/token.c
@@ -299,6 +299,10 @@ struct mptcp_sock *mptcp_token_get_sock(struct net *net, u32 token)
 	struct mptcp_sock *msk;
 	struct sock *sk;
 
+	/* Token 0 is never assigned, but is what a torn down socket holds. */
+	if (!token)
+		return NULL;
+
 	rcu_read_lock();
 	bucket = token_bucket(token);
 
-- 
2.47.3