Re: [PATCH v3 1/3] cpuset: Allow setscheduler regardless of manipulated task

[Waiman Long <longman@redhat.com>]

On 7/3/23 13:27, 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 when no effective change would happen.
> 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()).
>
> Notice that foreign task's affinity may still be modified if the user
> can modify destination cgroup's cpuset attributes
> (update_tasks_cpumask() does no permissions check). The permissions
> check could thus be skipped on v2 even when affinity changes. Stay
> conservative in this patch though.
>
> [1] https://github.com/systemd/systemd/issues/18293#issuecomment-831205649
>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> ---
>   kernel/cgroup/cpuset.c | 19 ++++++++++++++++---
>   1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 58e6f18f01c1..0a9b860844ca 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2487,6 +2487,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>   	struct cgroup_subsys_state *css;
>   	struct cpuset *cs, *oldcs;
>   	struct task_struct *task;
> +	bool cpus_updated, mems_updated;
>   	int ret;
>   
>   	/* used later by cpuset_attach() */
> @@ -2501,13 +2502,25 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>   	if (ret)
>   		goto out_unlock;
>   
> +	cpus_updated = !cpumask_equal(cs->effective_cpus, oldcs->effective_cpus);
> +	mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
> +
>   	cgroup_taskset_for_each(task, css, tset) {
>   		ret = task_can_attach(task);
>   		if (ret)
>   			goto out_unlock;
> -		ret = security_task_setscheduler(task);
> -		if (ret)
> -			goto out_unlock;
> +
> +		/*
> +		 * Skip rights over task check in v2 when nothing changes,
> +		 * migration permission derives from hierarchy ownership in
> +		 * cgroup_procs_write_permission()).
> +		 */
> +		if (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) ||
> +		    (cpus_updated || mems_updated)) {
> +			ret = security_task_setscheduler(task);
> +			if (ret)
> +				goto out_unlock;
> +		}
>   
>   		if (dl_task(task)) {
>   			cs->nr_migrate_dl_tasks++;
Reviewed-by: Waiman Long <longman@redhat.com>

Thanks,
Longman