Bug report: Policy routed packets are dropped by wireguard

[Michael Wu <michael.apply@gmail.com>]

Hello,

I'd like to confirm that the following problem seemed to be a bug, and
not configuration error:
https://lists.zx2c4.com/pipermail/wireguard/2019-September/004545.html
(Content dupated at the end of this email for easier reference)

On system 1 (kernel 4.15), everything worked as expected. However, on
system 2 (kernel 5.4), the issue described above is observed.

Regards,
Michael


[System 1]:
Ubuntu 18.04.5 LTS
4.15.0-128-generic #131-Ubuntu SMP Wed Dec 9 06:57:35 UTC 2020 x86_64
x86_64 x86_64 GNU/Linux

[System 2]:
Ubuntu 20.04.2 LTS
5.4.0-73-generic #82-Ubuntu SMP Wed Apr 14 17:39:42 UTC 2021 x86_64
x86_64 x86_64 GNU/Linux

[Policy routed packets are dropped by wireguard]
Original mail: https://lists.zx2c4.com/pipermail/wireguard/2019-September/004545.html

Hello!

I'm looking for technical advice.
Currently I'm trying to pass marked sessions through wireguard VPN network.

Marking is done by cgroups classid matching:
> iptables -A OUTPUT -m cgroup --cgroup 3735928559 -j MARK --set-xmark 0x1c3/0xffffffff

The only route in the `vpn` table is default routing through wg0:
> ip route add default dev wg0 table vpn

Routing rule is pretty simple:
> ip rule add fwmark 451 table vpn

Now I pass some packets on the interface:
> cgexec -g net_cls:vpn ping 10.0.1.1

I see packets reaching interface but dropped in the driver:
> tcpdump -i wg0 host 10.0.1.1
> ...
> 6 packets dropped by interface

Value in 4th column (TX drop) is increasing in the `/proc/net/dev` for wg0.

If I add route to default routing table and do ping without assigning
cgroup to the process then all is perfectly fine.
> ip route add 10.0.1.0/24 dev wg0

> ping 10.0.1.1
> PING 10.0.1.1 (10.0.1.1) 56(84) bytes of data.
> 64 bytes from 10.0.1.1: icmp_seq=1 ttl=64 time=46.1 ms

Is it some kind of a bug of misconfiguration?

> uname -r
> 5.2.1-gentoo

Installed Gentoo package atom.
> =net-vpn/wireguard-0.0.20190913

Thanks for any help!