[PATCH net] mptcp: fix bogus sendmsg() return code under pressure

[PATCH net] mptcp: fix bogus sendmsg() return code under pressure

[Paolo Abeni]

In case of memory pressure, mptcp_sendmsg() may call
sk_stream_wait_memory() after succesfully xmitting some
bytes. If the latter fails we currently return to the
user-space the error code, ignoring the succeful xmit.

Address the issue always checking for the xmitted bytes
before mptcp_sendmsg() completes.

Fixes: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c0abe738e7d3..a761d3c613bb 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -880,7 +880,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	mptcp_set_timeout(sk, ssk);
 	if (copied) {
-		ret = copied;
 		tcp_push(ssk, msg->msg_flags, mss_now, tcp_sk(ssk)->nonagle,
 			 size_goal);
 
@@ -893,7 +892,7 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	release_sock(ssk);
 out:
 	release_sock(sk);
-	return ret;
+	return copied ? : ret;
 }
 
 static void mptcp_wait_data(struct sock *sk, long *timeo)
-- 
2.26.2

Re: [PATCH net] mptcp: fix bogus sendmsg() return code under pressure

[David Miller]

From: Paolo Abeni <pabeni@redhat.com>
Date: Mon,  3 Aug 2020 18:40:39 +0200

> In case of memory pressure, mptcp_sendmsg() may call
> sk_stream_wait_memory() after succesfully xmitting some
> bytes. If the latter fails we currently return to the
> user-space the error code, ignoring the succeful xmit.
> 
> Address the issue always checking for the xmitted bytes
> before mptcp_sendmsg() completes.
> 
> Fixes: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
> Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Applied and queued up for v5.7+ -stable, thank you.