netdev - Re: [Patch net] gre: refetch erspan header from skb->data after pskb_may

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Date:   Sat, 7 Dec 2019 18:04:15 +0100
From:   Simon Horman <simon.horman@...ronome.com>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [Patch net] gre: refetch erspan header from skb->data after
 pskb_may_pull()

On Fri, Dec 06, 2019 at 01:55:25PM +0200, Lorenzo Bianconi wrote:
> >
> > Hi Cong,
> >
> > On Thu, Dec 05, 2019 at 07:39:02PM -0800, Cong Wang wrote:
> > > After pskb_may_pull() we should always refetch the header
> > > pointers from the skb->data in case it got reallocated.
> > >
> > > In gre_parse_header(), the erspan header is still fetched
> > > from the 'options' pointer which is fetched before
> > > pskb_may_pull().
> > >
> > > Found this during code review of a KMSAN bug report.
> > >
> > > Fixes: cb73ee40b1b3 ("net: ip_gre: use erspan key field for tunnel lookup")
> > > Cc: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
> > > Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> > > ---
> > >  net/ipv4/gre_demux.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/ipv4/gre_demux.c b/net/ipv4/gre_demux.c
> > > index 44bfeecac33e..5fd6e8ed02b5 100644
> > > --- a/net/ipv4/gre_demux.c
> > > +++ b/net/ipv4/gre_demux.c
> > > @@ -127,7 +127,7 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
> > >               if (!pskb_may_pull(skb, nhs + hdr_len + sizeof(*ershdr)))
> > >                       return -EINVAL;
> > >
> > > -             ershdr = (struct erspan_base_hdr *)options;
> > > +             ershdr = (struct erspan_base_hdr *)(skb->data + nhs + hdr_len);
> >
> > It seems to me that in the case of WCCPv2 hdr_len will be 4 bytes longer
> > than where options would be advanced to. Is that a problem here?
> >
> 
> Hi Simon,
> 
> I guess the two conditions are mutually exclusive since tpi->proto is
> initialized with greh->protocol. Am I missing something?

Thanks Lorenzo,

I see that now and agree that this patch is correct.

Reviewed-by: Simon Horman <simon.horman@...ronome.com>

> > >               tpi->key = cpu_to_be32(get_session_id(ershdr));
> > >       }
> > >
> > > --
> > > 2.21.0
> > >
> >
>