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SubjectRe: [PATCH 1/3] cpuset: Allow setscheduler regardless of manipulated task
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On 6/29/23 05:11, Michal Koutný wrote:
> When we migrate a task between two cgroups, one of the checks is a
> verification whether we can modify task's scheduler settings
> (cap_task_setscheduler()).
>
> An implicit migration occurs also when enabling a controller on the
> unified hierarchy (think of parent to child migration). The
> aforementioned check may be problematic if the caller of the migration
> (enabling a controller) has no permissions over migrated tasks.
> For instance, a user's cgroup that ends up running a process of a
> different user. Although cgroup permissions are configured favorably,
> the enablement fails due to the foreign process [1].
>
> Change the behavior by relaxing the permissions check on the unified
> hierarchy (or in v2 mode). This is in accordance with unified hierarchy
> attachment behavior when permissions of the source to target cgroups are
> decisive whereas the migrated task is opaque (as opposed to more
> restrictive check in __cgroup1_procs_write()).

The is_in_v2_mode() check is for supporting the v2 mode in cgroup v1.
However, there is no controller enabling in v1. So I think you should
just use cgroup_subsys_on_dfl(cpuset_cgrp_subsys) as the v2 check if
your focus is just to prevent problem when enabling cpuset controller.


>
> [1] https://github.com/systemd/systemd/issues/18293#issuecomment-831205649
>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> ---
> kernel/cgroup/cpuset.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index e4ca2dd2b764..3b5f87a9a150 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2495,6 +2495,13 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
> ret = task_can_attach(task, cs->effective_cpus);
> if (ret)
> goto out_unlock;
> +
> + /*
> + * Skip rights over task check in v2, migration permission derives
> + * from hierarchy ownership in cgroup_procs_write_permission()).
> + */
> + if (is_in_v2_mode())
> + continue;
> ret = security_task_setscheduler(task);
> if (ret)
> goto out_unlock;

This change will likely conflict with the latest cpuset change on
tracking # of dl tasks in a cpuset. You will have to, at least, move the
dl task check before the security_task_setscheduler() check.

Another fact about cpuset controller enabling is that both cpus_allowed
and mems_allowed are empty at that point. You may also add these checks
as a preconditions for disabling the security_task_setscheduler check.

Cheers,
Longman

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