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Date:   Thu, 24 Dec 2020 20:01:09 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org
Subject: [PATCH net] ipv4: Ignore ECN bits for fib lookups in
 fib_compute_spec_dst()

RT_TOS() only clears one of the ECN bits. Therefore, when
fib_compute_spec_dst() resorts to a fib lookup, it can return
different results depending on the value of the second ECN bit.

For example, ECT(0) and ECT(1) packets could be treated differently.

  $ 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 lo up
  $ ip -netns ns1 link set dev lo up
  $ ip -netns ns0 link set dev veth01 up
  $ ip -netns ns1 link set dev veth10 up

  $ ip -netns ns0 address add 192.0.2.10/24 dev veth01
  $ ip -netns ns1 address add 192.0.2.11/24 dev veth10

  $ ip -netns ns1 address add 192.0.2.21/32 dev lo
  $ ip -netns ns1 route add 192.0.2.10/32 tos 4 dev veth10 src 192.0.2.21
  $ ip netns exec ns1 sysctl -wq net.ipv4.icmp_echo_ignore_broadcasts=0

With TOS 4 and ECT(1), ns1 replies using source address 192.0.2.21
(ping uses -Q to set all TOS and ECN bits):

  $ ip netns exec ns0 ping -c 1 -b -Q 5 192.0.2.255
  [...]
  64 bytes from 192.0.2.21: icmp_seq=1 ttl=64 time=0.544 ms

But with TOS 4 and ECT(0), ns1 replies using source address 192.0.2.11
because the "tos 4" route isn't matched:

  $ ip netns exec ns0 ping -c 1 -b -Q 6 192.0.2.255
  [...]
  64 bytes from 192.0.2.11: icmp_seq=1 ttl=64 time=0.597 ms

After this patch the ECN bits don't affect the result anymore:

  $ ip netns exec ns0 ping -c 1 -b -Q 6 192.0.2.255
  [...]
  64 bytes from 192.0.2.21: icmp_seq=1 ttl=64 time=0.591 ms

Fixes: 35ebf65e851c ("ipv4: Create and use fib_compute_spec_dst() helper.")
Signed-off-by: Guillaume Nault <gnault@...hat.com>
---
 net/ipv4/fib_frontend.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index cdf6ec5aa45d..84bb707bd88d 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -292,7 +292,7 @@ __be32 fib_compute_spec_dst(struct sk_buff *skb)
 			.flowi4_iif = LOOPBACK_IFINDEX,
 			.flowi4_oif = l3mdev_master_ifindex_rcu(dev),
 			.daddr = ip_hdr(skb)->saddr,
-			.flowi4_tos = RT_TOS(ip_hdr(skb)->tos),
+			.flowi4_tos = ip_hdr(skb)->tos & IPTOS_RT_MASK,
 			.flowi4_scope = scope,
 			.flowi4_mark = vmark ? skb->mark : 0,
 		};
-- 
2.21.3

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