[RFC PATCH] cpuset: Allow setscheduler regardless of manipulated task

[Michal Koutný]

When we migrate a task between two cgroups, one of the checks is a
verification that 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, 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 (for contrast, see more
restrictive check in __cgroup1_procs_write()).

[1] https://github.com/systemd/systemd/issues/18293#issuecomment-831205649

Reasons for RFC:

1) The unified hierarchy attachment behavior -- is that the
   right/consented model that migrated objects don't matter?

2) If 1) is true, have I missed any danger in allowing cpuset'ing a
   possibly privileged processes?

2.2) cpuset may be in v2 mode even on v1 hierarchy with different
   migration control rules (but checking migratee's creds in v1
   eliminates effect of this patch).

3) Alternative approach would be to allow cpuset migrations only when
   nothing effectively changes (which is the case for parent->child
   migration upon controller enablement).

4) This is just idea draft, not tested in the real case.

Thanks.

Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
---
 kernel/cgroup/cpuset.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 71a418858a5e..dbe78577de5b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2232,7 +2232,9 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 
 	percpu_down_write(&cpuset_rwsem);
 
-	/* allow moving tasks into an empty cpuset if on default hierarchy */
+	/* allow moving tasks into an empty cpuset if on default hierarchy,
+	 * also bypass permission check (access is controlled via cgroup(s)
+	 * owner in cgroup_procs_write_permission()) */
 	ret = -ENOSPC;
 	if (!is_in_v2_mode() &&
 	    (cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed)))
@@ -2242,6 +2244,9 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 		ret = task_can_attach(task, cs->cpus_allowed);
 		if (ret)
 			goto out_unlock;
+
+		if (is_in_v2_mode())
+			continue;
 		ret = security_task_setscheduler(task);
 		if (ret)
 			goto out_unlock;
-- 
2.35.3