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Date:   Sat, 16 Jan 2021 11:44:26 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, Florian Westphal <fw@...len.de>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>
Subject: [PATCH net 2/2] netfilter: rpfilter: mask ecn bits before fib lookup

RT_TOS() only masks one of the two ECN bits. Therefore rpfilter_mt()
treats Not-ECT or ECT(1) packets in a different way than those with
ECT(0) or CE.

Reproducer:

  Create two netns, connected with a veth:
  $ ip netns add ns0
  $ ip netns add ns1
  $ ip link add name veth01 netns ns0 type veth peer name veth10 netns ns1
  $ ip -netns ns0 link set dev veth01 up
  $ ip -netns ns1 link set dev veth10 up
  $ ip -netns ns0 address add 192.0.2.10/32 dev veth01
  $ ip -netns ns1 address add 192.0.2.11/32 dev veth10

  Add a route to ns1 in ns0:
  $ ip -netns ns0 route add 192.0.2.11/32 dev veth01

  In ns1, only packets with TOS 4 can be routed to ns0:
  $ ip -netns ns1 route add 192.0.2.10/32 tos 4 dev veth10

  Ping from ns0 to ns1 works regardless of the ECN bits, as long as TOS
  is 4:
  $ ip netns exec ns0 ping -Q 4 192.0.2.11   # TOS 4, Not-ECT
    ... 0% packet loss ...
  $ ip netns exec ns0 ping -Q 5 192.0.2.11   # TOS 4, ECT(1)
    ... 0% packet loss ...
  $ ip netns exec ns0 ping -Q 6 192.0.2.11   # TOS 4, ECT(0)
    ... 0% packet loss ...
  $ ip netns exec ns0 ping -Q 7 192.0.2.11   # TOS 4, CE
    ... 0% packet loss ...

  Now use iptable's rpfilter module in ns1:
  $ ip netns exec ns1 iptables-legacy -t raw -A PREROUTING -m rpfilter --invert -j DROP

  Not-ECT and ECT(1) packets still pass:
  $ ip netns exec ns0 ping -Q 4 192.0.2.11   # TOS 4, Not-ECT
    ... 0% packet loss ...
  $ ip netns exec ns0 ping -Q 5 192.0.2.11   # TOS 4, ECT(1)
    ... 0% packet loss ...

  But ECT(0) and ECN packets are dropped:
  $ ip netns exec ns0 ping -Q 6 192.0.2.11   # TOS 4, ECT(0)
    ... 100% packet loss ...
  $ ip netns exec ns0 ping -Q 7 192.0.2.11   # TOS 4, CE
    ... 100% packet loss ...

After this patch, rpfilter doesn't drop ECT(0) and CE packets anymore.

Fixes: 8f97339d3feb ("netfilter: add ipv4 reverse path filter match")
Signed-off-by: Guillaume Nault <gnault@...hat.com>
---
 net/ipv4/netfilter/ipt_rpfilter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/netfilter/ipt_rpfilter.c b/net/ipv4/netfilter/ipt_rpfilter.c
index cc23f1ce239c..8cd3224d913e 100644
--- a/net/ipv4/netfilter/ipt_rpfilter.c
+++ b/net/ipv4/netfilter/ipt_rpfilter.c
@@ -76,7 +76,7 @@ static bool rpfilter_mt(const struct sk_buff *skb, struct xt_action_param *par)
 	flow.daddr = iph->saddr;
 	flow.saddr = rpfilter_get_saddr(iph->daddr);
 	flow.flowi4_mark = info->flags & XT_RPFILTER_VALID_MARK ? skb->mark : 0;
-	flow.flowi4_tos = RT_TOS(iph->tos);
+	flow.flowi4_tos = iph->tos & IPTOS_RT_MASK;
 	flow.flowi4_scope = RT_SCOPE_UNIVERSE;
 	flow.flowi4_oif = l3mdev_master_ifindex_rcu(xt_in(par));
 
-- 
2.21.3

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