From: Guillaume Nault <[email protected]>
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 <[email protected]>
Applied and queued up for -stable, thanks.
