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: 8f97339d3f ("netfilter: add ipv4 reverse path filter match")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2e5a6266fb