[PATCH] macvlan: verify MTU before lowerdev xmit

[Daniel Axtens <dja@axtens.net>]

If a macvlan device which is not in bridge mode receives a packet,
it is sent straight to the lowerdev without checking against the
device's MTU. This also happens for multicast traffic.

Add an is_skb_forwardable() check against the lowerdev before
sending the packet out through it. I think this is the simplest
and best way to do it, and is consistent with the use of
dev_forward_skb() in the bridge path.

This is easy to replicate:
 - create a VM with a macvtap connection in private mode
 - set the lowerdev MTU to something low in the host (e.g. 1480)
 - do not set the MTU lower in the guest (e.g. keep at 1500)
 - netperf to a different host with the same high MTU
 - observe that currently, the driver will forward too-big packets
 - observe that with this patch the packets are dropped

Cc: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>

---

After hearing Shannon's lightning talk on macvlan at netdev I
figured I'd strike while the iron is hot and get this out of my
patch queue where it has been languishing.
---
 drivers/net/macvlan.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index a178c5efd33e..8adcad6798c5 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -534,6 +534,10 @@ static int macvlan_queue_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 xmit_world:
+	/* verify MTU */
+	if (!is_skb_forwardable(vlan->lowerdev, skb))
+		return NET_XMIT_DROP;
+
 	skb->dev = vlan->lowerdev;
 	return dev_queue_xmit(skb);
 }
-- 
2.11.0