netdev - Re: [PATCH net 2/2] sch_cake: Make sure we can write the IP header before changing DSCP bits

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Date:   Thu, 04 Apr 2019 23:55:37 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        cake@...ts.bufferbloat.net
Subject: Re: [PATCH net 2/2] sch_cake: Make sure we can write the IP header before changing DSCP bits

Stephen Hemminger <stephen@...workplumber.org> writes:

> On Thu, 04 Apr 2019 22:44:33 +0200
> Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
>> Stephen Hemminger <stephen@...workplumber.org> writes:
>> 
>> > On Thu, 04 Apr 2019 15:01:33 +0200
>> > Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>> >  
>> >>  static u8 cake_handle_diffserv(struct sk_buff *skb, u16 wash)
>> >>  {
>> >> +	int wlen = skb_network_offset(skb);  
>> >
>> > In theory this could be negative, you should handle that?
>> > Rather than calling may_pull() with a huge unsigned value.  
>> 
>> Huh, that would imply that skb->network_header points to before
>> skb->head; when does that happen?
>> 
>> Also, pskb_may_pull() does check for len > skb->len, so I guess a
>> follow-up question would be, "does it happen often enough to warrant
>> handling at this level"?
>> 
>> Also, I copied that bit from sch_dsmark, so if you really thing it needs
>> to be fixed, I guess we should fix both...
>> 
>> -Toke
>
> It should never happen just paranoid

Ah, right. Well in that case I think any overflow is handled fine inside
pskb_may_pull():

	if (unlikely(len > skb->len))
		return 0;


-Toke