Re: [PATCH 0/4] Rebase device_cgroup v2 patchset

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Hi,

so now that the device cgroup properly respects hierarchy, not allowing
a cgroup to be given greater permission than its parent, should we consider
relaxing the capability checks?

There are two capable(CAP_SYS_ADMIN) checks in deice_cgroup.c: one in
devcgroup_can_attach() to protect changing another task's cgroup, and
one in devcgroup_update_access() to protect writes to the devices.allow
and devices.deny files.

I think the first should be changed to a check for ns_capable() to
the victim's user_ns.  Something like 

--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -70,10 +70,16 @@ static int devcgroup_can_attach(struct cgroup *new_cgrp,
                                struct cgroup_taskset *set)
 {
        struct task_struct *task = cgroup_taskset_first(set);
+       struct user_namespace *ns;
+       int ret = -EPERM;

-       if (current != task && !capable(CAP_SYS_ADMIN))
-               return -EPERM;
-       return 0;
+       if (current == task)
+               return 0;
+
+       ns = userns_get(task);;
+       ret = ns_capable(ns, CAP_SYS_ADMIN) ? 0 : -EPERM;
+       put_user_ns(ns);
+       return ret;
 }

 /*

For the second, the hierarchy support should let us ignore concerns
about unprivileged users escalating privilege, but I'm trying to decide
whether we  need to worry about the sendmail capability class of bugs.
My sense is actually the answer is no, and we can drop the capable()
check altogether.  The reason is that while userspace frequently doesn't
properly handle a failing system call due to unexpected lack of partial
privilege, I wouldn't expect any setuid root program to ignore failure
to open or mknod a device file (and proceed into a bad failure mode).
Does this sound rasonable, or a recipe for disaster?

-serge