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@xxxxxxxx> --- 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
