netdev - Re: [PATCH] macvlan: verify MTU before lowerdev xmit

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Date:   Tue, 14 Nov 2017 09:03:11 -0800
From:   Shannon Nelson <shannon.nelson@...cle.com>
To:     Daniel Axtens <dja@...ens.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] macvlan: verify MTU before lowerdev xmit

On 11/14/2017 2:32 AM, Daniel Axtens wrote:
> 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@...cle.com>
> Signed-off-by: Daniel Axtens <dja@...ens.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;
> +

Good catch.  I remember your comment on this last week, but hadn't had a 
chance to look it up yet.

As for the other paths, I see that the call to dev_forward_skb() already 
has this protection in it, but does the call to dev_queue_xmit_accel() 
in macvlan_start_xmit() need similar protection?

sln


>   	skb->dev = vlan->lowerdev;
>   	return dev_queue_xmit(skb);
>   }
>