Subject: Re: [PATCH] net: skbuff: Fix length validation in skb_vlan_pop()

From: Shmulik Ladkani <[email-masked]>
Date: Mon, 19 Sep 2016 09:15:38 +0300

Hi,

On Sun, 18 Sep 2016 13:26:30 -0700, [email-masked] wrote:
> On Sun, Sep 18, 2016 at 3:09 AM, Shmulik Ladkani
> <[email-masked]> wrote:
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 1e329d4112..cc2c004838 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -4537,7 +4537,7 @@ int skb_vlan_pop(struct sk_buff *skb)
> >         } else {
> >                 if (unlikely((skb->protocol != htons(ETH_P_8021Q) &&
> >                               skb->protocol != htons(ETH_P_8021AD)) ||
> > -                            skb->len < VLAN_ETH_HLEN))
> > +                            skb->mac_len < VLAN_ETH_HLEN))
> 
> There is already check in __skb_vlan_pop() to validate skb for a vlan
> header. So it is safe to drop this check entirely.

Seems validation in '__skb_vlan_pop' has slightly different semantics:

	unsigned int offset = skb->data - skb_mac_header(skb);

	__skb_push(skb, offset);
	err = skb_ensure_writable(skb, VLAN_ETH_HLEN);

this pushes 'data' back to mac_header, then makes sure there's sufficient
place in skb to _store_ VLAN_ETH_HLEN bytes (by pulling into linear part
if needed, or erroring if skb is too small).

There's no guarantee the original mac header was sized VLAN_ETH_HLEN.

Interpretation of the following 

		if (unlikely((skb->protocol != htons(ETH_P_8021Q) &&
			      skb->protocol != htons(ETH_P_8021AD)) ||
			     skb->len < VLAN_ETH_HLEN))
			return 0;

in 'skb_vlan_pop' might be read as:
"there's no tag, or protocol says its a tag but it's insufficient to pop,
 so lets do nothing".

Is it superflous?

Thanks,
Shmulik