[PATCH 3.12 087/103] mactap: Fix checksum errors for non-gso packets in bridge mode

From: Vlad Yasevich <redacted>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

[ Upstream commit cbdb04279ccaefcc702c8757757eea8ed76e50cf ]

The following is a problematic configuration:

 VM1: virtio-net device connected to macvtap0@eth0
 VM2: e1000 device connect to macvtap1@eth0

The problem is is that virtio-net supports checksum offloading
and thus sends the packets to the host with CHECKSUM_PARTIAL set.
On the other hand, e1000 does not support any acceleration.

For small TCP packets (and this includes the 3-way handshake),
e1000 ends up receiving packets that only have a partial checksum
set.  This causes TCP to fail checksum validation and to drop
packets.  As a result tcp connections can not be established.

Commit 3e4f8b787370978733ca6cae452720a4f0c296b8
	macvtap: Perform GSO on forwarding path.
fixes this issue for large packets wthat will end up undergoing GSO.
This commit adds a check for the non-GSO case and attempts to
compute the checksum for partially checksummed packets in the
non-GSO case.

CC: Daniel Lezcano <redacted>
CC: Patrick McHardy <redacted>
CC: Andrian Nord <redacted>
CC: Eric Dumazet <redacted>
CC: Michael S. Tsirkin <mst@redhat.com>
CC: Jason Wang <redacted>
Signed-off-by: Vlad Yasevich <redacted>
Acked-by: Jason Wang <redacted>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <redacted>
---
 drivers/net/macvtap.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index 5895e4dbbf2a..d0f165f2877b 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c

@@ -315,6 +315,15 @@ static int macvtap_forward(struct net_device *dev, struct sk_buff *skb)
 			segs = nskb;
 		}
 	} else {
+		/* If we receive a partial checksum and the tap side
+		 * doesn't support checksum offload, compute the checksum.
+		 * Note: it doesn't matter which checksum feature to
+		 *        check, we either support them all or none.
+		 */
+		if (skb->ip_summed == CHECKSUM_PARTIAL &&
+		    !(features & NETIF_F_ALL_CSUM) &&
+		    skb_checksum_help(skb))
+			goto drop;
 		skb_queue_tail(&q->sk.sk_receive_queue, skb);
 	}
 

-- 
1.9.3