Re: [PATCH RFC] net: Validate size of non-TSO packets in validate_xmit_skb().

[Eric Dumazet <eric.dumazet@gmail.com>]

On 02/26/2019 02:56 AM, Michael Chan wrote:
> There have been reports of oversize UDP packets being sent to the
> driver to be transmitted, causing error conditions.  The issue is
> likely caused by the dst of the SKB switching between 'lo' with
> 64K MTU and the hardware device with a smaller MTU.  Patches are
> being proposed by Mahesh Bandewar <maheshb@google.com> to fix the
> issue.
> 
> Separately, we should add a length check in validate_xmit_skb()
> to drop these oversize packets before they reach the driver.

Why ?

We keep adding checks in the 'fast path' and make slower and slower after each release.

We need to fix the root cause really.

> This patch only validates non-TSO packets.  Complete validation
> of segmented TSO packet size will probably be too slow.
> 
> Signed-off-by: Michael Chan <michael.chan@broadcom.com>
> ---
>  net/core/dev.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 5d03889..50c5174 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3373,6 +3373,13 @@ static struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device
>  		}
>  	}
>  
> +	if (!skb_is_gso(skb) &&
> +	    skb->len > (dev->mtu + dev->hard_header_len + VLAN_HLEN)) {
> +		net_warn_ratelimited("%s(): Dropping %d bytes oversize skb.\n",
> +				     __func__, skb->len);
> +		goto out_kfree_skb;
> +	}
> +
>  	skb = validate_xmit_xfrm(skb, features, again);
>  
>  	return skb;
>