[PATCH 5.9 006/757] mptcp: fix fallback for MP_JOIN subflows

Greg Kroah-Hartman
2020-10-27T15:58:51Z

From: Paolo Abeni <pabeni@redhat.com>

[ Upstream commit d582484726c4c46c8580923e855665fb91e3463e ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/mptcp/options.c | 32 +++++++++++++++++++++++++-------
net/mptcp/protocol.h | 1 +
net/mptcp/subflow.c | 10 ++++++++--
3 files changed, 34 insertions(+), 9 deletions(-)

--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -626,6 +626,12 @@ bool mptcp_established_options(struct so
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 r
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(stru
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(stru
}

/* 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)
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -348,6 +348,7 @@ void mptcp_subflow_fully_established(str
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,
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -270,6 +270,13 @@ static bool subflow_thmac_valid(struct m
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;