mm/memcg: v1: account event registrations and drop world-writable cgroup.event_control

In cgroup v1, the legacy cgroup.event_control file is world-writable and
allows unprivileged users to register unbounded events and thresholds.
Each registration allocates kernel memory without capping or memcg
charging, which can be abused to exhaust kernel memory in affected
configurations.

Make the following minimal changes:
- Account allocations with __GFP_ACCOUNT in event and threshold registration.
- Remove CFTYPE_WORLD_WRITABLE from cgroup.event_control to make it
owner-writable.

This does not affect cgroup v2. Allocations are still subject to kmem
accounting being enabled, but this reduces unbounded global growth.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Stanislav Fort <[e-mail]>
Acked-by: Roman Gushchin <[e-mail]>
Acked-by: Shakeel Butt <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Muchun Song <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/memcontrol-v1.c

···
 	size = thresholds->primary ? thresholds->primary->size + 1 : 1;
 
 	/* Allocate memory for new array of thresholds */
-	new = kmalloc(struct_size(new, entries, size), GFP_KERNEL);
+	new = kmalloc(struct_size(new, entries, size), GFP_KERNEL_ACCOUNT);
 	if (!new) {
 		ret = -ENOMEM;
 		goto unlock;
···
 {
 	struct mem_cgroup_eventfd_list *event;
 
-	event = kmalloc(sizeof(*event),	GFP_KERNEL);
+	event = kmalloc(sizeof(*event),	GFP_KERNEL_ACCOUNT);
 	if (!event)
 		return -ENOMEM;
 
···
 
 	CLASS(fd, cfile)(cfd);
 
-	event = kzalloc(sizeof(*event), GFP_KERNEL);
+	event = kzalloc(sizeof(*event), GFP_KERNEL_ACCOUNT);
 	if (!event)
 		return -ENOMEM;
 
···
 	{
 		.name = "cgroup.event_control",		/* XXX: for compat */
 		.write = memcg_write_event_control,
-		.flags = CFTYPE_NO_PREFIX | CFTYPE_WORLD_WRITABLE,
+		.flags = CFTYPE_NO_PREFIX,
 	},
 	{
 		.name = "swappiness",