Re: [PATCH -next 2/2] cgroup: Disallow delegatee to write all interfaces outsize of cgroup ns

[chenridong <chenridong@huawei.com>]

On 2024/8/15 0:52, Tejun Heo wrote:
> Hello,
> 
> On Wed, Aug 14, 2024 at 04:09:59PM +0800, chenridong wrote:
> ...
>> Hi，TJ， We plan to use delegation in cgroup-v2, so I am conducting some
>> tests.
>> As doc mentions 'Because the resource control interface files in a given
>> directory control the distribution of the parent's resources, the delegatee
>> shouldn't be allowed to write to them.' However I found a root can write
>> parent's file(cgroup.subtree_control) to change the resource limits(a
>> fraudulent method). I believe this could pose a risk in some scenarios where
>> a root enters a new cgroup ns without unmounting original cgroup system, and
>> it can break limitations. For instance, running a docker with --privileged,
>> could this be a risk?
>>
>> So I sent this patch to discuss whether this case should be addressed?
> 
> That sounsd like a misconfiguration. cgroup NS doesn't make much sense if
> you don't limit the actual visibility. The interface is half broken in that
> situation anyway and if you're leaking filesystem visibility into a
> supposedly isolated container, relaxed resource limits aren't biggest of
> your problems.
> 
> While the proposed change isn't necessarily a bad idea, it's a behavior
> change and I don't either modifying existing behavior or introducing a new
> mount flag is justified here. Maybe just update the documentation indicating
> that the ancestral cgroups shouldn't be visible in a delegated ns?
> 
> Thanks.
> 

Thank you, TJ, I will send a patch to update comment and the documentation.

Thanks,
Ridong