Re: [PATCH net] sctp: get sctphdr by offset in sctp_compute_cksum

[Xin Long]

On Mon, Feb 25, 2019 at 10:08 PM Neil Horman <nhorman@xxxxxxxxxxxxx> wrote:
>
> On Mon, Feb 25, 2019 at 09:20:44PM +0800, Xin Long wrote:
> > On Mon, Feb 25, 2019 at 8:47 PM Neil Horman <nhorman@xxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Feb 25, 2019 at 07:25:37PM +0800, Xin Long wrote:
> > > > sctp_hdr(skb) only works when skb->transport_header is set properly.
> > > >
> > > > But in the path of nf_conntrack_in: sctp_packet() -> sctp_error()
> > > >
> > > > skb->transport_header is not guaranteed to be right value for sctp.
> > > > It will cause to fail to check the checksum for sctp packets.
> > > >
> > > > So fix it by using offset, which is always right in all places.
> > > >
> > > > Fixes: e6d8b64b34aa ("net: sctp: fix and consolidate SCTP checksumming code")
> > > > Reported-by: Li Shuang <shuali@xxxxxxxxxx>
> > > > Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
> > > > ---
> > > >  include/net/sctp/checksum.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/include/net/sctp/checksum.h b/include/net/sctp/checksum.h
> > > > index 32ee65a..1c6e6c0 100644
> > > > --- a/include/net/sctp/checksum.h
> > > > +++ b/include/net/sctp/checksum.h
> > > > @@ -61,7 +61,7 @@ static inline __wsum sctp_csum_combine(__wsum csum, __wsum csum2,
> > > >  static inline __le32 sctp_compute_cksum(const struct sk_buff *skb,
> > > >                                       unsigned int offset)
> > > >  {
> > > > -     struct sctphdr *sh = sctp_hdr(skb);
> > > > +     struct sctphdr *sh = (struct sctphdr *)(skb->data + offset);
> > > >       const struct skb_checksum_ops ops = {
> > > >               .update  = sctp_csum_update,
> > > >               .combine = sctp_csum_combine,
> > > > --
> > > > 2.1.0
> > > >
> > > >
> > > Shouldn't you use skb_set_transport_header and skb_transport_header here?
> > you mean:
> > skb_set_transport_header(skb, offset);
> > sh = sctp_hdr(skb);
> > ?
> >
> > There's no place counting on here to set transport_header.
> > It will be a kinda redundant job, yet skb is 'const'.
> >
> I'm not sure what you mean by "theres no place counting here".  We have the
> transport header offset, and you're doing the exact same computation that that
> function does.  It seems like we should use it in case the underlying
> implementation changes.
1. skb_set_transport_header() and sctp_hdr() are like:
skb->transport_header = skb->data - skb->head;
skb->transport_header += offset
sh = skb->head + skb->transport_header;

2. in this patch:
sh = (struct sctphdr *)(skb->data + offset);  only

I think the 2nd one is better.

I feel it's weird to set transport_header here if it's only for
sctp_hdr(skb) in here.

As for "underlying implementation changes", I don't know exactly the case
but there are quite a few places doing things like:
*hdr = (struct *hdr *)(skb->data + hdroff);

I'd think it's safe. no?

>
> I understand what you are saying regarding the use of a const variable there,
> but perhaps thats an argument for removing the const storage classifier.  Better
> still, it would be good to figure out why all paths to this function don't
> already set the transport header offset to begin with (addressing your redundant
> comment)
The issue was reported when going to nf_conntrack by br_netfilter's
bridge-nf-call-iptables.
As you can see on nf_conntrack_in() path, even iphdr is got by:
   iph = skb_header_pointer(skb, nhoff, sizeof(_iph), &_iph);
It's impossible to set skb->transport_header when we're not sure iphdr
in linearized memory.