Re: Support for loading firewall rules with cgroup(v2) expressions early

[Topi Miettinen <toiwoton@gmail.com>]

On 2.4.2022 11.12, Topi Miettinen wrote:
> On 30.3.2022 5.53, Pablo Neira Ayuso wrote:
>> On Wed, Mar 30, 2022 at 12:25:25AM +0200, Pablo Neira Ayuso wrote:
>>> On Tue, Mar 29, 2022 at 09:20:25PM +0300, Topi Miettinen wrote:
>> [...]
>>> You could define a ruleset that describes the policy following the
>>> cgroupsv2 hierarchy. Something like this:
>>>
>>>   table inet filter {
>>>          map dict_cgroup_level_1 {
>>>                  type cgroupsv2 : verdict;
>>>                  elements = { "system.slice" : jump system_slice }
>>>          }
>>>
>>>          map dict_cgroup_level_2 {
>>>                  type cgroupsv2 : verdict;
>>>                  elements = { 
>>> "system.slice/systemd-timesyncd.service" : jump systemd_timesyncd }
>>>          }
>>>
>>>          chain systemd_timesyncd {
>>>                  # systemd-timesyncd policy
>>>          }
>>>
>>>          chain system_slice {
>>>                  socket cgroupv2 level 2 vmap @dict_cgroup_level_2
>>>                  # policy for system.slice process
>>>          }
>>>
>>>          chain input {
>>>                  type filter hook input priority filter; policy drop;
>>
>> This example should use the output chain instead:
>>
>>            chain output {
>>                    type filter hook output priority filter; policy drop;
>>
>>  From the input chain, the packet relies on early demux to have access
>> to the socket.
>>
>> The idea would be to filter out outgoing traffic and rely on conntrack
>> for (established) input traffic.
> 
> Is it really so that 'socket cgroupv2' can't be used on input side at 
> all? At least 'ss' can display the cgroup for listening sockets 
> correctly, so the cgroup information should be available somewhere:
> 
> $ ss -lt --cgroup
> State    Recv-Q   Send-Q       Local Address:Port       Peer 
> Address:Port   Process
> LISTEN   0        4096                  *%lo:ssh                   *:* 
>       cgroup:/system.slice/ssh.socket

Also 'meta skuid' doesn't seem to work in input filters. It would have 
been simple to use 'meta skuid < 1000' to simulate 'system.slice' vs. 
'user.slice' cgroups.

If this is intentional, the manual page should make this much clearer. 
There's no warning and the kernel doesn't reject the useless input rules.

I think it should be possible to do filtering on input side based on the 
socket properties (UID, GID, cgroup). Especially with UDP, it should be 
possible to drop all packets if the listening process is not OK.

My use case is that I need to open ports for Steam games (TCP and UDP 
ports 27015-27030) but I don't want to make them available for system 
services or any other apps besides Steam games. SELinux SECMARKs and TE 
rules for sockets help me here but there are other problems.

-Topi