# CVE-2021-47011: mm: memcontrol: slab: fix obtain a reference to a freeing memcg

> A use-after-free vulnerability in the Linux kernel memory control subsystem allows attackers to cause a denial of service.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 28 Feb 2024
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2021-47011, GHSA-jvv7-jhh3-m96v, UBUNTU-CVE-2021-47011

## What is CVE-2021-47011?

CVE-2021-47011 is a high-severity use after free vulnerability in Linux kernel, published on 28 Feb 2024. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

mm: memcontrol: slab: fix obtain a reference to a freeing memcg

Patch series "Use obj_cgroup APIs to charge kmem pages", v5.

Since Roman's series "The new cgroup slab memory controller" applied.
All slab objects are charged with the new APIs of obj_cgroup.  The new
APIs introduce a struct obj_cgroup to charge slab objects.  It prevents
long-living objects from pinning the original memory cgroup in the
memory.  But there are still some corner objects (e.g.  allocations
larger than order-1 page on SLUB) which are not charged with the new
APIs.  Those objects (include the pages which are allocated from buddy
allocator directly) are charged as kmem pages which still hold a
reference to the memory cgroup.

E.g.  We know that the kernel stack is charged as kmem pages because the
size of the kernel stack can be greater than 2 pages (e.g.  16KB on
x86_64 or arm64).  If we create a thread (suppose the thread stack is
charged to memory cgroup A) and then move it from memory cgroup A to
memory cgroup B.  Because the kernel stack of the thread hold a
reference to the memory cgroup A.  The thread can pin the memory cgroup
A in the memory even if we remove the cgroup A.  If we want to see this
scenario by using the following script.  We can see that the system has
added 500 dying cgroups (This is not a real world issue, just a script
to show that the large kmallocs are charged as kmem pages which can pin
the memory cgroup in the memory).

	#!/bin/bash

	cat /proc/cgroups | grep memory

	cd /sys/fs/cgroup/memory
	echo 1 > memory.move_charge_at_immigrate

	for i in range{1..500}
	do
		mkdir kmem_test
		echo $$ > kmem_test/cgroup.procs
		sleep 3600 &
		echo $$ > cgroup.procs
		echo `cat kmem_test/cgroup.procs` > cgroup.procs
		rmdir kmem_test
	done

	cat /proc/cgroups | grep memory

This patchset aims to make those kmem pages to drop the reference to
memory cgroup by using the APIs of obj_cgroup.  Finally, we can see that
the number of the dying cgroups will not increase if we run the above test
script.

This patch (of 7):

The rcu_read_lock/unlock only can guarantee that the memcg will not be
freed, but it cannot guarantee the success of css_get (which is in the
refill_stock when cached memcg changed) to memcg.

  rcu_read_lock()
  memcg = obj_cgroup_memcg(old)
  __memcg_kmem_uncharge(memcg)
      refill_stock(memcg)
          if (stock->cached != memcg)
              // css_get can change the ref counter from 0 back to 1.
              css_get(&memcg->css)
  rcu_read_unlock()

This fix is very like the commit:

  eefbfa7fd678 ("mm: memcg/slab: fix use after free in obj_cgroup_charge")

Fix this by holding a reference to the memcg which is passed to the
__memcg_kmem_uncharge() before calling __memcg_kmem_uncharge().

## Which versions are affected by CVE-2021-47011?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47011?

Apply the vendor's patch.

## Is CVE-2021-47011 being exploited?

There is no known exploitation of CVE-2021-47011 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-47011 and fix it if it is affected.

What it is: CVE-2021-47011 is a high-severity use after free vulnerability in Linux kernel, published on 28 Feb 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-47011 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-47011/ (Markdown: https://insights.sbomify.com/CVE-2021-47011/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47011
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47011
- OSV: https://osv.dev/vulnerability/CVE-2021-47011

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47011/
Summaries are generated and may contain mistakes; the official description above is the record.