linux-kernel - [PATCH] mm/memcg: v1: account event registrations and drop world-writable cgroup.event

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Message-Id: <20250904181248.5527-1-disclosure@aisle.com>
Date: Thu,  4 Sep 2025 21:12:48 +0300
From: Stanislav Fort <stanislav.fort@...le.com>
To: linux-mm@...ck.org,
	cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: hannes@...xchg.org,
	mhocko@...nel.org,
	roman.gushchin@...ux.dev,
	shakeel.butt@...ux.dev,
	muchun.song@...ux.dev,
	akpm@...ux-foundation.org,
	stable@...r.kernel.org,
	Stanislav Fort <disclosure@...le.com>
Subject: [PATCH] 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.

Reported-by: Stanislav Fort <disclosure@...le.com>
Acked-by: Johannes Weiner <hannes@...xchg.org>
Cc: stable@...r.kernel.org
Signed-off-by: Stanislav Fort <disclosure@...le.com>
---
 mm/memcontrol-v1.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
index 4b94731305b9..9374785319ab 100644
--- a/mm/memcontrol-v1.c
+++ b/mm/memcontrol-v1.c
@@ -761,7 +761,7 @@ static int __mem_cgroup_usage_register_event(struct mem_cgroup *memcg,
 	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 | __GFP_ACCOUNT);
 	if (!new) {
 		ret = -ENOMEM;
 		goto unlock;
@@ -924,7 +924,7 @@ static int mem_cgroup_oom_register_event(struct mem_cgroup *memcg,
 {
 	struct mem_cgroup_eventfd_list *event;
 
-	event = kmalloc(sizeof(*event),	GFP_KERNEL);
+	event = kmalloc(sizeof(*event),	GFP_KERNEL | __GFP_ACCOUNT);
 	if (!event)
 		return -ENOMEM;
 
@@ -1087,7 +1087,7 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
 
 	CLASS(fd, cfile)(cfd);
 
-	event = kzalloc(sizeof(*event), GFP_KERNEL);
+	event = kzalloc(sizeof(*event), GFP_KERNEL | __GFP_ACCOUNT);
 	if (!event)
 		return -ENOMEM;
 
@@ -2053,7 +2053,7 @@ struct cftype mem_cgroup_legacy_files[] = {
 	{
 		.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",
-- 
2.39.3 (Apple Git-146)