Re: [nf-next PATCH] netfilter: nft_exthdr: Search chunks in SCTP packets only

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Fri, Jun 11, 2021 at 12:26:24PM +0200, Phil Sutter wrote:
> On Thu, Jun 10, 2021 at 07:43:34PM +0200, Pablo Neira Ayuso wrote:
> > On Thu, Jun 10, 2021 at 04:23:16PM +0200, Phil Sutter wrote:
> > > Since user space does not generate a payload dependency, plain sctp
> > > chunk matches cause searching in non-SCTP packets, too. Avoid this
> > > potential mis-interpretation of packet data by checking pkt->tprot.
> > > 
> > > Fixes: 133dc203d77df ("netfilter: nft_exthdr: Support SCTP chunks")
> > > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > > ---
> > >  net/netfilter/nft_exthdr.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
> > > index 7f705b5c09de8..1093bb83f8aeb 100644
> > > --- a/net/netfilter/nft_exthdr.c
> > > +++ b/net/netfilter/nft_exthdr.c
> > > @@ -312,6 +312,9 @@ static void nft_exthdr_sctp_eval(const struct nft_expr *expr,
> > >  	const struct sctp_chunkhdr *sch;
> > >  	struct sctp_chunkhdr _sch;
> > >  
> > > +	if (!pkt->tprot_set || pkt->tprot != IPPROTO_SCTP)
> > > +		goto err;
> > 
> > nft_set_pktinfo_unspec() already initializes pkt->tprot to zero.
> > 
> > I think it's safe to simplify this to:
> > 
> > 	if (pkt->tprot != IPPROTO_SCTP)
> 
> Are you sure? Checking the spots that (should) initialize
> tprot/tprot_set, in nft_do_chain_inet() it seems that if state->pf is
> neither NFPROTO_IPV4 nor NFPROTO_IPV6, nft_do_chain() is called without
> prior init. Maybe default case should call nft_set_pktinfo_unspec()?

state->pf in nft_do_chain_inet() can only be either NFPROTO_IPV4 or
NFPROTO_IPV6.

pkt->tprot_set is there to deal with a corner case: IPPROTO_IP (0).
If pkt->tprot_set == true and pkt->tprot == 0, it means: "match on
IPPROTO_IP". For other IPPROTO_*, checking pkt->tprot looks safe to me.

> BTW: The final return call in nft_do_chain_inet_ingress() is dead code,
> right?

You mean the default case of nft_do_chain_inet_ingress()? inet/ingress
is special, it allows you to filter IPv4 and IPv6 traffic only.
Anything else from ingress is accepted (you should filter it via
netdev family).