[PATCH 6.18 020/175] cgroup/dmem: avoid pool UAF

Greg Kroah-Hartman
2026-02-09T14:28:03Z

From: Chen Ridong <chenridong@huawei.com>

commit 99a2ef500906138ba58093b9893972a5c303c734 upstream.

An UAF issue was observed:

BUG: KASAN: slab-use-after-free in page_counter_uncharge+0x65/0x150
Write of size 8 at addr ffff888106715440 by task insmod/527

CPU: 4 UID: 0 PID: 527 Comm: insmod 6.19.0-rc7-next-20260129+ #11
Tainted: [O]=OOT_MODULE
Call Trace:
<TASK>
dump_stack_lvl+0x82/0xd0
kasan_report+0xca/0x100
kasan_check_range+0x39/0x1c0
page_counter_uncharge+0x65/0x150
dmem_cgroup_uncharge+0x1f/0x260

Allocated by task 527:

Freed by task 0:

The buggy address belongs to the object at ffff888106715400
which belongs to the cache kmalloc-512 of size 512
The buggy address is located 64 bytes inside of
freed 512-byte region [ffff888106715400, ffff888106715600)

The buggy address belongs to the physical page:

Memory state around the buggy address:
ffff888106715300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff888106715380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888106715400: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888106715480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888106715500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

The issue occurs because a pool can still be held by a caller after its
associated memory region is unregistered. The current implementation frees
the pool even if users still hold references to it (e.g., before uncharge
operations complete).

This patch adds a reference counter to each pool, ensuring that a pool is
only freed when its reference count drops to zero.

Fixes: b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting cgroup")
Cc: stable@vger.kernel.org # v6.14+
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/cgroup/dmem.c | 60 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 58 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
index 787b334e0f5d..1ea6afffa985 100644
--- a/kernel/cgroup/dmem.c
+++ b/kernel/cgroup/dmem.c
@@ -14,6 +14,7 @@
#include <linux/mutex.h>
#include <linux/page_counter.h>
#include <linux/parser.h>
+#include <linux/refcount.h>
#include <linux/rculist.h>
#include <linux/slab.h>

@@ -71,7 +72,9 @@ struct dmem_cgroup_pool_state {
struct rcu_head rcu;

struct page_counter cnt;
+ struct dmem_cgroup_pool_state *parent;

+ refcount_t ref;
bool inited;
};

@@ -88,6 +91,9 @@ struct dmem_cgroup_pool_state {
static DEFINE_SPINLOCK(dmemcg_lock);
static LIST_HEAD(dmem_cgroup_regions);

+static void dmemcg_free_region(struct kref *ref);
+static void dmemcg_pool_free_rcu(struct rcu_head *rcu);
+
static inline struct dmemcg_state *
css_to_dmemcs(struct cgroup_subsys_state *css)
{
@@ -104,10 +110,38 @@ static struct dmemcg_state *parent_dmemcs(struct dmemcg_state *cg)
return cg->css.parent ? css_to_dmemcs(cg->css.parent) : NULL;
}

+static void dmemcg_pool_get(struct dmem_cgroup_pool_state *pool)
+{
+ refcount_inc(&pool->ref);
+}
+
+static bool dmemcg_pool_tryget(struct dmem_cgroup_pool_state *pool)
+{
+ return refcount_inc_not_zero(&pool->ref);
+}
+
+static void dmemcg_pool_put(struct dmem_cgroup_pool_state *pool)
+{
+ if (!refcount_dec_and_test(&pool->ref))
+ return;
+
+ call_rcu(&pool->rcu, dmemcg_pool_free_rcu);
+}
+
+static void dmemcg_pool_free_rcu(struct rcu_head *rcu)
+{
+ struct dmem_cgroup_pool_state *pool = container_of(rcu, typeof(*pool), rcu);
+
+ if (pool->parent)
+ dmemcg_pool_put(pool->parent);
+ kref_put(&pool->region->ref, dmemcg_free_region);
+ kfree(pool);
+}
+
static void free_cg_pool(struct dmem_cgroup_pool_state *pool)
{
list_del(&pool->region_node);
- kfree(pool);
+ dmemcg_pool_put(pool);
}

static void
@@ -342,6 +376,12 @@ alloc_pool_single(struct dmemcg_state *dmemcs, struct dmem_cgroup_region *region
page_counter_init(&pool->cnt,
ppool ? &ppool->cnt : NULL, true);
reset_all_resource_limits(pool);
+ refcount_set(&pool->ref, 1);
+ kref_get(&region->ref);
+ if (ppool && !pool->parent) {
+ pool->parent = ppool;
+ dmemcg_pool_get(ppool);
+ }

list_add_tail_rcu(&pool->css_node, &dmemcs->pools);
list_add_tail(&pool->region_node, &region->pools);
@@ -389,6 +429,10 @@ get_cg_pool_locked(struct dmemcg_state *dmemcs, struct dmem_cgroup_region *regio

/* Fix up parent links, mark as inited. */
pool->cnt.parent = &ppool->cnt;
+ if (ppool && !pool->parent) {
+ pool->parent = ppool;
+ dmemcg_pool_get(ppool);
+ }
pool->inited = true;

pool = ppool;
@@ -435,6 +479,8 @@ void dmem_cgroup_unregister_region(struct dmem_cgroup_region *region)

list_for_each_entry_safe(pool, next, &region->pools, region_node) {
list_del_rcu(&pool->css_node);
+ list_del(&pool->region_node);
+ dmemcg_pool_put(pool);
}

/*
@@ -515,8 +561,10 @@ static struct dmem_cgroup_region *dmemcg_get_region_by_name(const char *name)
*/
void dmem_cgroup_pool_state_put(struct dmem_cgroup_pool_state *pool)
{
- if (pool)
+ if (pool) {
css_put(&pool->cs->css);
+ dmemcg_pool_put(pool);
+ }
}
EXPORT_SYMBOL_GPL(dmem_cgroup_pool_state_put);

@@ -530,6 +578,8 @@ get_cg_pool_unlocked(struct dmemcg_state *cg, struct dmem_cgroup_region *region)
pool = find_cg_pool_locked(cg, region);
if (pool && !READ_ONCE(pool->inited))
pool = NULL;
+ if (pool && !dmemcg_pool_tryget(pool))
+ pool = NULL;
rcu_read_unlock();

while (!pool) {
@@ -538,6 +588,8 @@ get_cg_pool_unlocked(struct dmemcg_state *cg, struct dmem_cgroup_region *region)
pool = get_cg_pool_locked(cg, region, &allocpool);
else
pool = ERR_PTR(-ENODEV);
+ if (!IS_ERR(pool))
+ dmemcg_pool_get(pool);
spin_unlock(&dmemcg_lock);

if (pool == ERR_PTR(-ENOMEM)) {
@@ -573,6 +625,7 @@ void dmem_cgroup_uncharge(struct dmem_cgroup_pool_state *pool, u64 size)

page_counter_uncharge(&pool->cnt, size);
css_put(&pool->cs->css);
+ dmemcg_pool_put(pool);
}
EXPORT_SYMBOL_GPL(dmem_cgroup_uncharge);

@@ -624,7 +677,9 @@ int dmem_cgroup_try_charge(struct dmem_cgroup_region *region, u64 size,
if (ret_limit_pool) {
*ret_limit_pool = container_of(fail, struct dmem_cgroup_pool_state, cnt);
css_get(&(*ret_limit_pool)->cs->css);
+ dmemcg_pool_get(*ret_limit_pool);
}
+ dmemcg_pool_put(pool);
ret = -EAGAIN;
goto err;
}
@@ -719,6 +774,7 @@ static ssize_t dmemcg_limit_write(struct kernfs_open_file *of,

/* And commit */
apply(pool, new_limit);
+ dmemcg_pool_put(pool);

out_put:
kref_put(&region->ref, dmemcg_free_region);
--
2.53.0