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

[Xin Long]

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'.