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

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Message-ID: <CAJ0CqmWjh1bAOwx25tVE_yDbzCbf9dCXsFE7ZV_1N7Tt-DF64A@mail.gmail.com>
Date:   Fri, 6 Dec 2019 13:55:25 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Simon Horman <simon.horman@...ronome.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()

>
> 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?

Regards,
Lorenzo

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