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

[chenridong <chenridong@huawei.com>]

On 2024/8/14 3:02, Tejun Heo wrote:
> Hello,
> 
> On Mon, Aug 12, 2024 at 06:57:06PM +0200, Michal Koutný wrote:
> ...
>> You could also have increased the ancestral limit (if there was any)
>> echo max > dlgt_grp_ns/pids.max // similarly allowed
>>
>> If you're a root (or otherwise have sufficient permissions) and you can
>> _see_ an ancestral cgroup, you can write to its attributes according to
>> permissions. Thus the delegation works via cgroup ns (in)visibility but
>> cgroup ns root is visible on both sides of the boundary hence the extra
>> check.
> 
> Yeah, the intended usage scenario w/ NS delegation is that the delegatee
> won't be able to see the ancetral cgroups beyond the delegation point. Chen,
> is this from an actual usecase? If so, can you describe what's going on?
> 
> Thanks.
> 
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?

Thanks,
Ridong