[PATCH net-next] mptcp: fix NULL ptr dereference in MP_JOIN error path

[Paolo Abeni]

When token lookup on MP_JOIN 3rd ack fails, the server
socket closes with a reset the incoming child. Such socket
has the 'is_mptcp' flag set, but no msk socket associated
- due to the failed lookup.

While crafting the reset packet mptcp_established_options_mp()
will try to dereference the child's master socket, causing
a NULL ptr dereference.

This change addresses the issue with explicit fallback to
TCP in such error path.

Fixes: 729cd6436f35 ("mptcp: cope better with MP_JOIN failure")
Signed-off-by: Paolo Abeni <pab...@redhat.com>
---
 net/mptcp/subflow.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index f3c06b8af92d..493b98a0825c 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -413,6 +413,20 @@ static void subflow_ulp_fallback(struct sock *sk,
        tcp_sk(sk)->is_mptcp = 0;
 }
 
+static void subflow_drop_ctx(struct sock *ssk)
+{
+       struct mptcp_subflow_context *ctx = mptcp_subflow_ctx(ssk);
+
+       if (!ctx)
+               return;
+
+       subflow_ulp_fallback(ssk, ctx);
+       if (ctx->conn)
+               sock_put(ctx->conn);
+
+       kfree_rcu(ctx, rcu);
+}
+
 static struct sock *subflow_syn_recv_sock(const struct sock *sk,
                                          struct sk_buff *skb,
                                          struct request_sock *req,
@@ -485,10 +499,7 @@ static struct sock *subflow_syn_recv_sock(const struct 
sock *sk,
                        if (fallback_is_fatal)
                                goto dispose_child;
 
-                       if (ctx) {
-                               subflow_ulp_fallback(child, ctx);
-                               kfree_rcu(ctx, rcu);
-                       }
+                       subflow_drop_ctx(child);
                        goto out;
                }
 
@@ -537,6 +548,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock 
*sk,
        return child;
 
 dispose_child:
+       subflow_drop_ctx(child);
        tcp_rsk(req)->drop_req = true;
        tcp_send_active_reset(child, GFP_ATOMIC);
        inet_csk_prepare_for_destroy_sock(child);
-- 
2.21.3