Re: [PATCH net 1/2] ipv6: honor rule suppression in raw FIB lookups

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- [Medium] For raw lookups, fib6_rule_suppress() (net/ipv6/fib6_rules.c)
  evaluates suppress_ifgroup against a different nexthop than the dst-…

--

ipv6: honor rule suppression in raw FIB lookups

This patch makes fib6_rule_suppress() apply suppress_prefixlength and
suppress_ifgroup to raw fib6_table_lookup() results. For those results it
takes the prefix length from res->f6i and the device from res->nh. Raw
results marked RTF_REJECT are never suppressed, so a later rule cannot
bypass the rejection.

Link: https://github.com/firewalld/firewalld/issues/1600

> diff --git a/net/ipv6/fib6_rules.c b/net/ipv6/fib6_rules.c
> index 0ffd7b47a0460..e8a44b540291d 100644
> --- a/net/ipv6/fib6_rules.c
> +++ b/net/ipv6/fib6_rules.c
> @@ -283,19 +283,29 @@ INDIRECT_CALLABLE_SCOPE bool fib6_rule_suppress(struct fib_rule *rule,
>  						struct fib_lookup_arg *arg)
>  {
>  	struct fib6_result *res = arg->result;
> -	struct rt6_info *rt = res->rt6;
> +	struct rt6_info *rt = NULL;
>  	struct net_device *dev = NULL;
> -
> -	if (!rt)
> -		return false;
> -
> -	if (rt->rt6i_idev)
> -		dev = rt->rt6i_idev->dev;
> +	int plen;
> +
> +	if (arg->lookup_ptr == fib6_table_lookup) {
> +		/* Raw lookups report reject routes without an error. */
> +		if (res->fib6_flags & RTF_REJECT)
> +			return false;
> +		plen = res->f6i->fib6_dst.plen;
> +		dev = res->nh->fib_nh_dev;

[Severity: Medium]
For multipath routes, is this the same nexthop that the dst-based path
checks?

On the raw path, res->nh is whatever fib6_table_lookup() picked. For
multipath that is usually the first sibling or the first usable member of
the nexthop group. fib6_rule_action_alt() never calls fib6_select_path().

The dst-based path selects the path before it builds the rt6_info whose
rt6i_idev the old code looked at:

net/ipv6/route.c:ip6_pol_route() {
    ...
	fib6_select_path(net, &res, fl6, oif, have_oif_match, skb, strict);
    ...
}

fib6_select_path() then replaces res->nh (and res->f6i for siblings) with
the hash-selected path, which may be on a different device:

net/ipv6/route.c:fib6_select_path() {
    ...
	if (unlikely(match->nh)) {
		nexthop_path_fib6_result(res, fl6->mp_hash);
		return;
	}
    ...
out:
	res->f6i = match;
	res->nh = match->fib6_nh;
}

bpf_ipv6_fib_lookup() runs path selection only after fib6_lookup() has
already made the suppression decision:

net/core/filter.c:bpf_ipv6_fib_lookup() {
    ...
	err = fib6_lookup(net, oif, &fl6, &res, strict);
    ...
	fib6_select_path(net, &res, &fl6, fl6.flowi6_oif,
			 fl6.flowi6_oif != 0, NULL, strict);
    ...
	dev = res.nh->fib_nh_dev;
	params->ifindex = dev->ifindex;
    ...
}

Consider a multipath route whose nexthops are on devices in different
interface groups, plus a rule like "ip -6 rule ... lookup <tbl>
suppress_ifgroup N". Could a flow be suppressed by the ordinary route
lookup but not by the nft fib or BPF raw lookup, or the other way round?

Can bpf_fib_lookup also return a device in the suppressed group that
suppression never evaluated?

That would leave the same raw versus ordinary disagreement the commit
message says this patch removes. suppress_prefixlength does not appear to
be affected, since siblings share one prefix.

> +	} else {
> +		rt = res->rt6;
> +		if (!rt)
> +			return false;
> +		plen = rt->rt6i_dst.plen;
> +		if (rt->rt6i_idev)
> +			dev = rt->rt6i_idev->dev;
> +	}

[ ... ]

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