Re: [Xen-devel] [PATCH net v3] xen-netback: fix fragment detection in checksum setup

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: David Miller [mailto:davem@xxxxxxxxxxxxx]
> Sent: 30 November 2013 21:14
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Wei Liu; Ian Campbell;
> David Vrabel
> Subject: Re: [PATCH net v3] xen-netback: fix fragment detection in checksum
> setup
> 
> From: Paul Durrant <paul.durrant@xxxxxxxxxx>
> Date: Fri, 29 Nov 2013 10:52:08 +0000
> 
> > @@ -1166,15 +1166,27 @@ static int checksum_setup_ip(struct xenvif *vif,
> struct sk_buff *skb,
> >     struct iphdr *iph = (void *)skb->data;
> >     unsigned int header_size;
> >     unsigned int off;
> > +   bool fragment;
> >     int err = -EPROTO;
> >
> > +   fragment = false;
> > +
> >     off = sizeof(struct iphdr);
> >
> >     header_size = skb->network_header + off + MAX_IPOPTLEN;
> >     maybe_pull_tail(skb, header_size);
> >
> > +   if (iph->frag_off & htons(IP_OFFSET | IP_MF))
> > +           fragment = true;
> 
> This function has a serious problem.
> 
> maybe_pull_tail() can change skb->data, therefore this "iph" pointer
> can become invalid, you're essentially dereferencing garbage if
> maybe_pull_tail() actually does any work.

Ok. Clearly I'm misunderstanding what __pskb_pull_tail() does then. I was under 
the impression that it moved skb->tail on but left skb->data alone (which is 
what I want to do). I will have another look.

> 
> Secondly, do you really (even rate limited) want to span the system
> log just because some ipv4 fragmented frames end up here?  That
> doesn't make any sense to me.  Maybe bump a statistic or something
> like that, but a log message triggerable by a remote entity?  No way.

Ok.

  Paul

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