[PATCH net-next 3/3] mptcp: emit tcp reset when a join request fails

[Florian Westphal]

RFC 8684 says:
 If the token is unknown or the host wants to refuse subflow establishment
 (for example, due to a limit on the number of subflows it will permit),
 the receiver will send back a reset (RST) signal, analogous to an unknown
 port in TCP, containing an MP_TCPRST option (Section 3.6) with an
 "MPTCP specific error" reason code.

mptcp-next doesn't support MP_TCPRST yet, this can be added in another
change.

Signed-off-by: Florian Westphal <f...@strlen.de>
---
 net/mptcp/subflow.c | 47 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 36 insertions(+), 11 deletions(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index c55b8f176746..5a8005746bc8 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -112,9 +112,14 @@ static int __subflow_init_req(struct request_sock *req, 
const struct sock *sk_li
        return 0;
 }
 
-static void subflow_init_req(struct request_sock *req,
-                            const struct sock *sk_listener,
-                            struct sk_buff *skb)
+/* Init mptcp request socket.
+ *
+ * Returns an error code if a JOIN has failed and a TCP reset
+ * should be sent.
+ */
+static int subflow_init_req(struct request_sock *req,
+                           const struct sock *sk_listener,
+                           struct sk_buff *skb)
 {
        struct mptcp_subflow_context *listener = mptcp_subflow_ctx(sk_listener);
        struct mptcp_subflow_request_sock *subflow_req = mptcp_subflow_rsk(req);
@@ -125,7 +130,7 @@ static void subflow_init_req(struct request_sock *req,
 
        ret = __subflow_init_req(req, sk_listener);
        if (ret)
-               return;
+               return 0;
 
        mptcp_get_options(skb, &mp_opt);
 
@@ -133,7 +138,7 @@ static void subflow_init_req(struct request_sock *req,
                SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_MPCAPABLEPASSIVE);
 
                if (mp_opt.mp_join)
-                       return;
+                       return 0;
        } else if (mp_opt.mp_join) {
                SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINSYNRX);
        }
@@ -157,7 +162,7 @@ static void subflow_init_req(struct request_sock *req,
                        } else {
                                subflow_req->mp_capable = 1;
                        }
-                       return;
+                       return 0;
                }
 
                err = mptcp_token_new_request(req);
@@ -175,7 +180,11 @@ static void subflow_init_req(struct request_sock *req,
                subflow_req->remote_nonce = mp_opt.nonce;
                subflow_req->msk = subflow_token_join_request(req, skb);
 
-               if (unlikely(req->syncookie) && subflow_req->msk) {
+               /* Can't fall back to TCP in this case. */
+               if (!subflow_req->msk)
+                       return -EPERM;
+
+               if (unlikely(req->syncookie)) {
                        if (mptcp_can_accept_new_subflow(subflow_req->msk))
                                subflow_init_req_cookie_join_save(subflow_req, 
skb);
                }
@@ -183,6 +192,8 @@ static void subflow_init_req(struct request_sock *req,
                pr_debug("token=%u, remote_nonce=%u msk=%p", subflow_req->token,
                         subflow_req->remote_nonce, subflow_req->msk);
        }
+
+       return 0;
 }
 
 int mptcp_subflow_init_cookie_req(struct request_sock *req,
@@ -234,6 +245,7 @@ static struct dst_entry *subflow_v4_route_req(const struct 
sock *sk,
                                              struct request_sock *req)
 {
        struct dst_entry *dst;
+       int err;
 
        tcp_rsk(req)->is_mptcp = 1;
 
@@ -241,8 +253,14 @@ static struct dst_entry *subflow_v4_route_req(const struct 
sock *sk,
        if (!dst)
                return NULL;
 
-       subflow_init_req(req, sk, skb);
-       return dst;
+       err = subflow_init_req(req, sk, skb);
+       if (err == 0)
+               return dst;
+
+       dst_release(dst);
+       if (!req->syncookie)
+               tcp_request_sock_ops.send_reset(sk, skb);
+       return NULL;
 }
 
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
@@ -252,6 +270,7 @@ static struct dst_entry *subflow_v6_route_req(const struct 
sock *sk,
                                              struct request_sock *req)
 {
        struct dst_entry *dst;
+       int err;
 
        tcp_rsk(req)->is_mptcp = 1;
 
@@ -259,8 +278,14 @@ static struct dst_entry *subflow_v6_route_req(const struct 
sock *sk,
        if (!dst)
                return NULL;
 
-       subflow_init_req(req, sk, skb);
-       return dst;
+       err = subflow_init_req(req, sk, skb);
+       if (err == 0)
+               return dst;
+
+       dst_release(dst);
+       if (!req->syncookie)
+               tcp6_request_sock_ops.send_reset(sk, skb);
+       return NULL;
 }
 #endif
 
-- 
2.26.2