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@xxxxxxxx> --- 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++; -- 2.41.0
