mptcp: fix fallback for MP_JOIN subflows

Additional/MP_JOIN subflows that do not pass some initial handshake
tests currently causes fallback to TCP. That is an RFC violation:
we should instead reset the subflow and leave the the msk untouched.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/91
Fixes: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 9d7fa93..b2f018d 100644 (file)
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -626,6 +626,12 @@ bool mptcp_established_options(struct sock *sk, struct sk_buff *skb,
        if (unlikely(mptcp_check_fallback(sk)))
                return false;
 
+       /* prevent adding of any MPTCP related options on reset packet
+        * until we support MP_TCPRST/MP_FASTCLOSE
+        */
+       if (unlikely(skb && TCP_SKB_CB(skb)->tcp_flags & TCPHDR_RST))
+               return false;
+
        if (mptcp_established_options_mp(sk, skb, &opt_size, remaining, opts))
                ret = true;
        else if (mptcp_established_options_dss(sk, skb, &opt_size, remaining,
@@ -676,7 +682,7 @@ bool mptcp_synack_options(const struct request_sock *req, unsigned int *size,
        return false;
 }
 
-static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
+static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
                                    struct mptcp_subflow_context *subflow,
                                    struct sk_buff *skb,
                                    struct mptcp_options_received *mp_opt)
@@ -693,15 +699,20 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
                    TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq &&
                    subflow->mp_join && mp_opt->mp_join &&
                    READ_ONCE(msk->pm.server_side))
-                       tcp_send_ack(sk);
+                       tcp_send_ack(ssk);
                goto fully_established;
        }
 
-       /* we should process OoO packets before the first subflow is fully
-        * established, but not expected for MP_JOIN subflows
+       /* we must process OoO packets before the first subflow is fully
+        * established. OoO packets are instead a protocol violation
+        * for MP_JOIN subflows as the peer must not send any data
+        * before receiving the forth ack - cfr. RFC 8684 section 3.2.
         */
-       if (TCP_SKB_CB(skb)->seq != subflow->ssn_offset + 1)
+       if (TCP_SKB_CB(skb)->seq != subflow->ssn_offset + 1) {
+               if (subflow->mp_join)
+                       goto reset;
                return subflow->mp_capable;
+       }
 
        if (mp_opt->dss && mp_opt->use_ack) {
                /* subflows are fully established as soon as we get any
@@ -713,9 +724,12 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
        }
 
        /* If the first established packet does not contain MP_CAPABLE + data
-        * then fallback to TCP
+        * then fallback to TCP. Fallback scenarios requires a reset for
+        * MP_JOIN subflows.
         */
        if (!mp_opt->mp_capable) {
+               if (subflow->mp_join)
+                       goto reset;
                subflow->mp_capable = 0;
                pr_fallback(msk);
                __mptcp_do_fallback(msk);
@@ -732,12 +746,16 @@ fully_established:
 
        subflow->pm_notified = 1;
        if (subflow->mp_join) {
-               clear_3rdack_retransmission(sk);
+               clear_3rdack_retransmission(ssk);
                mptcp_pm_subflow_established(msk, subflow);
        } else {
                mptcp_pm_fully_established(msk);
        }
        return true;
+
+reset:
+       mptcp_subflow_reset(ssk);
+       return false;
 }
 
 static u64 expand_ack(u64 old_ack, u64 cur_ack, bool use_64bit)

diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 285dd8b..a26b335 100644 (file)
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -348,6 +348,7 @@ void mptcp_subflow_fully_established(struct mptcp_subflow_context *subflow,
                                     struct mptcp_options_received *mp_opt);
 bool mptcp_subflow_data_available(struct sock *sk);
 void __init mptcp_subflow_init(void);
+void mptcp_subflow_reset(struct sock *ssk);
 
 /* called with sk socket lock held */
 int __mptcp_subflow_connect(struct sock *sk, int ifindex,

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 91bef7b..298bcfd 100644 (file)
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -270,6 +270,13 @@ static bool subflow_thmac_valid(struct mptcp_subflow_context *subflow)
        return thmac == subflow->thmac;
 }
 
+void mptcp_subflow_reset(struct sock *ssk)
+{
+       tcp_set_state(ssk, TCP_CLOSE);
+       tcp_send_active_reset(ssk, GFP_ATOMIC);
+       tcp_done(ssk);
+}
+
 static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 {
        struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
@@ -342,8 +349,7 @@ fallback:
        return;
 
 do_reset:
-       tcp_send_active_reset(sk, GFP_ATOMIC);
-       tcp_done(sk);
+       mptcp_subflow_reset(sk);
 }
 
 struct request_sock_ops mptcp_subflow_request_sock_ops;