Merge branch 'mptcp-reject-invalid-mp_join-requests-right-away'

Florian Westphal says:

====================
mptcp: reject invalid mp_join requests right away

At the moment MPTCP can detect an invalid join request (invalid token,
max number of subflows reached, and so on) right away but cannot reject
the connection until the 3WHS has completed.
Instead the connection will complete and the subflow is reset afterwards.

To send the reset most information is already available, but we don't have
good spot where the reset could be sent:

1. The ->init_req callback is too early and also doesn't allow to return an
   error that could be used to inform the TCP stack that the SYN should be
   dropped.

2. The ->route_req callback lacks the skb needed to send a reset.

3. The ->send_synack callback is the best fit from the available hooks,
   but its called after the request socket has been inserted into the queue
   already. This means we'd have to remove it again right away.

From a technical point of view, the second hook would be best:
 1. Its before insertion into listener queue.
 2. If it returns NULL TCP will drop the packet for us.

Problem is that we'd have to pass the skb to the function just for MPTCP.

Paolo suggested to merge init_req and route_req callbacks instead:
This makes all info available to MPTCP -- a return value of NULL drops the
packet and MPTCP can send the reset if needed.

Because 'route_req' has a 'const struct sock *', this means either removal
of const qualifier, or a bit of code churn to pass 'const' in security land.

This does the latter; I did not find any spots that need write access to struct
sock.

To recap, the two alternatives are:
1. Solve it entirely in MPTCP: use the ->send_synack callback to
   unlink the request socket from the listener & drop it.
2. Avoid 'security' churn by removing the const qualifier.
====================

Link: https://lore.kernel.org/r/20201130153631.21872-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 9 insertions, 9 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index fb3a7750f623..9e8a6c1aa019 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6799,18 +6799,13 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 	/* Note: tcp_v6_init_req() might override ir_iif for link locals */
 	inet_rsk(req)->ir_iif = inet_request_bound_dev_if(sk, skb);
 
-	af_ops->init_req(req, sk, skb);
-
-	if (security_inet_conn_request(sk, skb, req))
+	dst = af_ops->route_req(sk, skb, &fl, req);
+	if (!dst)
 		goto drop_and_free;
 
 	if (tmp_opt.tstamp_ok)
 		tcp_rsk(req)->ts_off = af_ops->init_ts_off(net, skb);
 
-	dst = af_ops->route_req(sk, &fl, req);
-	if (!dst)
-		goto drop_and_free;
-
 	if (!want_cookie && !isn) {
 		/* Kill the following clause, if you dislike this way. */
 		if (!net->ipv4.sysctl_tcp_syncookies &&
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index e4b31e70bd30..af2338294598 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1444,9 +1444,15 @@ static void tcp_v4_init_req(struct request_sock *req,
 }
 
 static struct dst_entry *tcp_v4_route_req(const struct sock *sk,
+					  struct sk_buff *skb,
 					  struct flowi *fl,
-					  const struct request_sock *req)
+					  struct request_sock *req)
 {
+	tcp_v4_init_req(req, sk, skb);
+
+	if (security_inet_conn_request(sk, skb, req))
+		return NULL;
+
 	return inet_csk_route_req(sk, &fl->u.ip4, req);
 }
 
@@ -1466,7 +1472,6 @@ const struct tcp_request_sock_ops tcp_request_sock_ipv4_ops = {
 	.req_md5_lookup	=	tcp_v4_md5_lookup,
 	.calc_md5_hash	=	tcp_v4_md5_hash_skb,
 #endif
-	.init_req	=	tcp_v4_init_req,
 #ifdef CONFIG_SYN_COOKIES
 	.cookie_init_seq =	cookie_v4_init_sequence,
 #endif