Subject: [PATCH 14/15] mptcp: sendmsg: reset iter on error

From: Florian Westphal <[email-masked]>
Date: Fri, 14 Aug 2020 15:56:34 +0200

[ Upstream commit 35759383133f64d90eba120a0d3efe8f71241650 ]

Once we've copied data from the iterator we need to revert in case we
end up not sending any data.

This bug doesn't trigger with normal 'poll' based tests, because
we only feed a small chunk of data to kernel after poll indicated
POLLOUT.  With blocking IO and large writes this triggers. Receiver
ends up with less data than it should get.

Fixes: 72511aab95c94d ("mptcp: avoid blocking in tcp_sendpages")
Signed-off-by: Florian Westphal <[email-masked]>
Reviewed-by: Mat Martineau <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/mptcp/protocol.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c0abe738e7d3..ac8880514d5d 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -605,8 +605,10 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
 		if (!psize)
 			return -EINVAL;
 
-		if (!sk_wmem_schedule(sk, psize + dfrag->overhead))
+		if (!sk_wmem_schedule(sk, psize + dfrag->overhead)) {
+			iov_iter_revert(&msg->msg_iter, psize);
 			return -ENOMEM;
+		}
 	} else {
 		offset = dfrag->offset;
 		psize = min_t(size_t, dfrag->data_len, avail_size);
@@ -617,8 +619,10 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
 	 */
 	ret = do_tcp_sendpages(ssk, page, offset, psize,
 			       msg->msg_flags | MSG_SENDPAGE_NOTLAST | MSG_DONTWAIT);
-	if (ret <= 0)
+	if (ret <= 0) {
+		iov_iter_revert(&msg->msg_iter, psize);
 		return ret;
+	}
 
 	frag_truesize += ret;
 	if (!retransmission) {
-- 
2.26.2