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

From: Daniel Borkmann <[email-masked]>
Date: Mon, 19 Sep 2016 14:22:57 +0200

On 09/19/2016 08:15 AM, Shmulik Ladkani wrote:
> 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).

Yes, but this skb_ensure_writable() is needed for doing the memmove anyway.

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

I'm wondering, what happens when you'd call this on tx path, when you'd
change that to suggested skb->mac_len? Isn't that 0 in such case, thus
such setups could fail then?