apparmor: allow SYS_CAP_RESOURCE to be sufficient to prlimit another task

While using AppArmor, SYS_CAP_RESOURCE is insufficient to call prlimit
on another task. The only other example of a AppArmor mediating access to
another, already running, task (ignoring fork+exec) is ptrace.

The AppArmor model for ptrace is that one of the following must be true:
1) The tracer is unconfined
2) The tracer is in complain mode
3) The tracer and tracee are confined by the same profile
4) The tracer is confined but has SYS_CAP_PTRACE

1), 2, and 3) are already true for setrlimit.

We can match the ptrace model just by allowing CAP_SYS_RESOURCE.

We still test the values of the rlimit since it can always be overridden
using a value that means unlimited for a particular resource.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/resource.c

@@ -101,9 +101,11 @@ int aa_task_setrlimit(struct aa_profile *profile, struct task_struct *task,
 	/* TODO: extend resource control to handle other (non current)
 	 * profiles.  AppArmor rules currently have the implicit assumption
 	 * that the task is setting the resource of a task confined with
-	 * the same profile.
+	 * the same profile or that the task setting the resource of another
+	 * task has CAP_SYS_RESOURCE.
 	 */
-	if (profile != task_profile ||
+	if ((profile != task_profile &&
+	     aa_capable(profile, CAP_SYS_RESOURCE, 1)) ||
 	    (profile->rlimits.mask & (1 << resource) &&
 	     new_rlim->rlim_max > profile->rlimits.limits[resource].rlim_max))
 		error = -EACCES;