[PATCH net 3/4] mptcp: fix NULL pointer dereference on fastopen early fallback

[Matthieu Baerts]

From: Paolo Abeni <pabeni@xxxxxxxxxx>

In case of early fallback to TCP, subflow_syn_recv_sock() deletes
the subflow context before returning the newly allocated sock to
the caller.

The fastopen path does not cope with the above unconditionally
dereferencing the subflow context.

Fixes: 36b122baf6a8 ("mptcp: add subflow_v(4,6)_send_synack()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Reviewed-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
Signed-off-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
---
 net/mptcp/fastopen.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
index d237d142171c..bceaab8dd8e4 100644
--- a/net/mptcp/fastopen.c
+++ b/net/mptcp/fastopen.c
@@ -9,11 +9,18 @@
 void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subflow,
 					      struct request_sock *req)
 {
-	struct sock *ssk = subflow->tcp_sock;
-	struct sock *sk = subflow->conn;
+	struct sock *sk, *ssk;
 	struct sk_buff *skb;
 	struct tcp_sock *tp;
 
+	/* on early fallback the subflow context is deleted by
+	 * subflow_syn_recv_sock()
+	 */
+	if (!subflow)
+		return;
+
+	ssk = subflow->tcp_sock;
+	sk = subflow->conn;
 	tp = tcp_sk(ssk);
 
 	subflow->is_mptfo = 1;

-- 
2.39.2