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SubjectRe: [PATCH v2 1/3] cpuset: Allow setscheduler regardless of manipulated task
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On 6/30/23 14:39, 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()).
>
> [1] https://github.com/systemd/systemd/issues/18293#issuecomment-831205649
>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> ---
> kernel/cgroup/cpuset.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 58e6f18f01c1..41d3ed14b0f4 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2505,9 +2505,16 @@ static int cpuset_can_attach(struct cgroup_taskset *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, migration permission derives
> + * from hierarchy ownership in cgroup_procs_write_permission()).
> + */
> + if (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys)) {
> + ret = security_task_setscheduler(task);
> + if (ret)
> + goto out_unlock;
> + }

I am somewhat hesitant to skip the security_task_setscheduler() check
for all cgroup v2 task migrations. The check is controlled by SElinux
which is a different subsystem. I believe the scheduler property here
refer's to the task cpu affinity and node mask. If you look at
cpuset_attach(), we have actually skipped the task iteration process to
change them if cpu affinity and node mask aren't changed at all.

I don't want to introduce a possible security vulnerability because of
this relaxation. I would suggest you skip it under the same condition of
no change to cpu affinity and node mask for cgroup v2.

Thanks,
Longman

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