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

[Phil Sutter <phil@nwl.cc>]

On Fri, Jun 11, 2021 at 01:56:54PM +0200, Pablo Neira Ayuso wrote:
> 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.

Shouldn't there be a WARN_ON_ONCE or something in the default case then?
Looking at nf_hook(), it seems entirely possible to me that state->pf
might be NFPROTO_ARP, for instance. That's probably just me not getting
it, but things we rely upon shouldn't be hidden that well, right?

> 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.

Ah, thanks for clarifying! So whenever I check a specific value that's
non-zero, tprot_set doesn't matter. Should I send a patch for the same
change in nft_tcp_header_pointer(), too? (That's where I copied the code
from. ;)

> > 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).

Oh, sorry. Looks like I had tomatoes on the eyes[1]: I missed that the
non-default cases just 'break' and therefore hit the function's last
line.

Thanks, Phil

[1] Famous German idiom.