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

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

[Paolo Abeni]

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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: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: Paolo Abeni <pabeni(a)redhat.com>
---
 net/mptcp/subflow.c | 17 +++++++++++++----
 net/mptcp/token.c   | 10 +++++++---
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index ae7bcdb7f990..55fced4f136d 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -408,6 +408,17 @@ 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);
+	kfree_rcu(ctx, rcu);
+}
+
 static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 					  struct sk_buff *skb,
 					  struct request_sock *req,
@@ -480,10 +491,7 @@ 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;
 		}
 
@@ -532,6 +540,7 @@ 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);
diff --git a/net/mptcp/token.c b/net/mptcp/token.c
index c0327ff5d1b7..d1068b51a45a 100644
--- a/net/mptcp/token.c
+++ b/net/mptcp/token.c
@@ -216,16 +216,20 @@ struct mptcp_sock *mptcp_token_get_sock(u32 token)
 		msk = mptcp_sk(sk);
 		if (READ_ONCE(msk->token) != token)
 			continue;
-		if (!refcount_inc_not_zero(&sk->sk_refcnt))
+		if (!refcount_inc_not_zero(&sk->sk_refcnt)) {
+			pr_warn("token match %x but 0 rfcnt\n", token);
 			goto not_found;
-		if (READ_ONCE(msk->token) != token) {
+		} if (READ_ONCE(msk->token) != token) {
+			pr_warn("token mis-match %x:%x after get\n", READ_ONCE(msk->token), token);
 			sock_put(sk);
 			goto again;
 		}
 		goto found;
 	}
-	if (get_nulls_value(pos) != (token & token_mask))
+	if (get_nulls_value(pos) != (token & token_mask)) {
+		pr_warn("moved nulls %ld id %d\n", get_nulls_value(pos), token & token_mask);
 		goto again;
+	}
 
 not_found:
 	msk = NULL;
-- 
2.21.3