netdev - Re: [PATCH net] ipv4: Ignore ECN bits for fib lookups in fib_compute_spec

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Message-Id: <20201228.144522.2164224163258675861.davem@davemloft.net>
Date:   Mon, 28 Dec 2020 14:45:22 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     gnault@...hat.com
Cc:     kuba@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net] ipv4: Ignore ECN bits for fib lookups in
 fib_compute_spec_dst()

From: Guillaume Nault <gnault@...hat.com>
Date: Thu, 24 Dec 2020 20:01:09 +0100

> 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>

Applied and queued up for -stable, thanks.