When sending an MP_JOIN + SYN + ACK, it is possible to mark the subflow
as 'backup' by setting the flag with the same name. Before this patch,
the backup was set if the other peer set it in its MP_JOIN + SYN
request.

It is not correct: the backup flag should be set in the MPJ+SYN+ACK only
if the host asks for it, and not mirroring what was done by the other
peer. It is then required to have a dedicated bit for each direction,
similar to what is done in the subflow context.

Fixes: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/options.c  | 2 +-
 net/mptcp/protocol.h | 1 +
 net/mptcp/subflow.c  | 1 +
 3 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 8e8dcfbc2993..8a68382a4fe9 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -909,7 +909,7 @@ bool mptcp_synack_options(const struct request_sock *req, unsigned int *size,
 		return true;
 	} else if (subflow_req->mp_join) {
 		opts->suboptions = OPTION_MPTCP_MPJ_SYNACK;
-		opts->backup = subflow_req->backup;
+		opts->backup = subflow_req->request_bkup;
 		opts->join_id = subflow_req->local_id;
 		opts->thmac = subflow_req->thmac;
 		opts->nonce = subflow_req->local_nonce;
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index b11a4e50d52b..b8b25124e7de 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -448,6 +448,7 @@ struct mptcp_subflow_request_sock {
 	u16	mp_capable : 1,
 		mp_join : 1,
 		backup : 1,
+		request_bkup : 1,
 		csum_reqd : 1,
 		allow_join_id0 : 1;
 	u8	local_id;
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 39e2cbdf3801..a3778aee4e77 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -2005,6 +2005,7 @@ static void subflow_ulp_clone(const struct request_sock *req,
 		new_ctx->fully_established = 1;
 		new_ctx->remote_key_valid = 1;
 		new_ctx->backup = subflow_req->backup;
+		new_ctx->request_bkup = subflow_req->request_bkup;
 		WRITE_ONCE(new_ctx->remote_id, subflow_req->remote_id);
 		new_ctx->token = subflow_req->token;
 		new_ctx->thmac = subflow_req->thmac;

-- 
2.45.2