Re: [PATCH v2 3/4] cgroup: require write perm on common ancestor when moving processes on the default hierarchy

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jun 18, 2015 at 01:59:27PM -0400, Tejun Heo wrote:
> On traditional hierarchies, if a task has write access to "tasks" or
> "cgroup.procs" file of a cgroup and its euid agrees with the target,
> it can move the target to the cgroup; however, consider the following
> scenario.  The owner of each cgroup is in the parentheses.
> 
>  R (root) - 0 (root) - 00 (user1) - 000 (user1)
>           |                       \ 001 (user1)
>           \ 1 (root) - 10 (user1)
> 
> The subtrees of 00 and 10 are delegated to user1; however, while both
> subtrees may belong to the same user, it is clear that the two
> subtrees are to be isolated - they're under completely separate
> resource limits imposed by 0 and 1, respectively.  Note that 0 and 1
> aren't strictly necessary but added to ease illustrating the issue.
> 
> If user1 is allowed to move processes between the two subtrees, the
> intention of the hierarchy - keeping a given group of processes under
> a subtree with certain resource restrictions while delegating
> management of the subtree - can be circumvented by user1.
> 
> This happens because migration permission check doesn't consider the
> hierarchical nature of cgroups.  To fix the issue, this patch adds an
> extra permission requirement when userland tries to migrate a process
> in the default hierarchy - the issuing task must have write access to
> the common ancestor of "cgroup.procs" file of the ancestor in addition
> to the destination's.
> 
> Conceptually, the issuer must be able to move the target process from
> the source cgroup to the common ancestor of source and destination
> cgroups and then to the destination.  As long as delegation is done in
> a proper top-down way, this guarantees that a delegatee can't smuggle
> processes across disjoint delegation domains.
> 
> The next patch will add documentation on the delegation model on the
> default hierarchy.
> 
> v2: Fixed missing !ret test.  Spotted by Li Zefan.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Li Zefan <lizefan@huawei.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>