Subject: Re: Override default socket policy per cgroup

From: [email-masked]
Date: Wed, 9 Feb 2022 13:51:38 -0800

On 02/09, Martin KaFai Lau wrote:
> On Wed, Feb 09, 2022 at 09:03:45AM -0800, [email-masked] wrote:
> > Let's say I want to set some default sk_priority for all sockets in a
> > specific cgroup. I can do it right now using cgroup/sock_create, but it
> > applies only to AF_INET{,6} sockets. I'd like to do the same for raw
> > (AF_PACKET) sockets and cgroup/sock_create doesn't trigger for them :-(
> Other than AF_PACKET and INET[6], do you have use cases for other  
> families?

No, I only need AF_PACKET for now. But I feel like we should create
a more extensible hook point this time (if we go this route).

> > (1) My naive approach would be to add another cgroup/sock_post_create
> > which runs late from __sock_create and triggers on everything.
> >
> > (2) Another approach might be to move BPF_CGROUP_RUN_PROG_INET_SOCK and
> > make it work with AF_PACKET. This might be not 100% backwards compatible
> > but I'd assume that most users should look at the socket family before
> > doing anything. (in this case it feels like we can extend
> > sock_bind/release for af_packets as well, just for accounting purposes,
> > without any way to override the target ifindex).
> If adding a hook at __sock_create, I think having a new  
> CGROUP_POST_SOCK_CREATE
> may be better instead of messing with the current inet assumption
> in CGROUP_'INET'_SOCK_CREATE.  Running all CGROUP_*_SOCK_CREATE at
> __sock_create could be a nice cleanup such that a few lines can be
> removed from inet[6]_create but an extra family check will be needed.

SG. Hopefully I can at least reuse exiting progtype and just introduce
new hook point in __sock_create.

> The bpf prog has both bpf_sock->family and bpf_sock->protocol field to
> check with, so it should be able to decide the sk type if it is run
> at __sock_create.  All bpf_sock fields should make sense or at least 0
> to all families (?), please check.

Yeah, that's what I think as well, existing bpf_sock should work
as is (it might show empty ip/port for af_packet), but I'll do verify
that.

> For af_packet bind, the ip[46]/port probably won't be useful?  What
> the bpf prog will need?

For AF_PACKET bind we would need new ifindex and new protocol. I was  
thinking
maybe new bpf_packet_sock type+helper to convert from bpf_sock is the
way to go here.

For AF_PACKET bind we actually have another use-case where I think
generic bind hook might be helpful. I have a working prototype with  
fmod_ret,
but feels like per-cgroup hook is better (let's me access cgroup local
storage):
We'd like to have a cgroup-enforced TX-only form of raw socket (grant
CAP_NET_RAW+restrict RX path). For AF_INET{,6} it means allow only
socket(AF_INET{,6}, SOCK_RAW, IPPROTO_RAW); that's easily enforcible with
the current hooks. For AF_PACKET it means allow only
socket(AF_PACKET, SOCK_RAW, 0 == ETH_P_NONE) and prohibit bind to  
protocol !=
0.