Re: [PATCH] cls_u32: use skb_copy_bits() to dereference data safely

[David Miller <davem@davemloft.net>]

From: Changli Gao <xiaosuo@gmail.com>
Date: Wed, 2 Jun 2010 21:14:55 +0800

> On Wed, Jun 2, 2010 at 8:45 PM, David Miller <davem@davemloft.net> wrote:
>> From: jamal <hadi@cyberus.ca>
>> Date: Wed, 02 Jun 2010 08:25:38 -0400
>>
>>> --- a/net/sched/cls_u32.c
>>> +++ b/net/sched/cls_u32.c
>>> @@ -135,6 +135,9 @@ next_knode:
>>>
>>>  for (i = n->sel.nkeys; i>0; i--, key++) {
>>>
>>> +        int toff = key->off+(off2&key->offmask)- 4;
>>> +        if (unlikely(toff > skb->len))
>>> +              /* bailout here - needs some thought */
>>>          if ((*(__be32*)(ptr+key->off+(off2&key->offmask))^key->v
>>
>> I don't think it's that simple.
>>
>> You can't dereference from the skb->data linear area if your offset is
>> beyond "skb->len - skb->data_len" (aka. skb_headlen()) since that's
>> where the paged or fragmented portion starts.
>>
>> We really need to use skb_copy_bits() if we want to allow
>> any offset into the SKB, and because of all the ways
>> packets can be transformed and constructed we absolutely
>> have to.
>>
> 
> Maybe skb_header_pointer() is lighter.

Yes, it should be.  In fact, it's designed for this kind of situation
and that's why it's used extensively in netfilter.