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, 4 Apr 2019 14:40:07 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
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

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