# CVE-2021-4440: x86/xen: Drop USERGS_SYSRET64 paravirt call

> A missing CPU buffer clearing in Xen PV guests can lead to denial of service or information leakage, requiring kernel patching.

- **Severity:** High 8.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/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:** 25 Jun 2024
- **Updated:** 11 May 2026
- **Also known as:** DEBIAN-CVE-2021-4440, GHSA-vc52-cgfp-jw27, UBUNTU-CVE-2021-4440

## What is CVE-2021-4440?

CVE-2021-4440 is a high-severity denial of service vulnerability in Linux kernel, published on 25 Jun 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:

x86/xen: Drop USERGS_SYSRET64 paravirt call

commit afd30525a659ac0ae0904f0cb4a2ca75522c3123 upstream.

USERGS_SYSRET64 is used to return from a syscall via SYSRET, but
a Xen PV guest will nevertheless use the IRET hypercall, as there
is no sysret PV hypercall defined.

So instead of testing all the prerequisites for doing a sysret and
then mangling the stack for Xen PV again for doing an iret just use
the iret exit from the beginning.

This can easily be done via an ALTERNATIVE like it is done for the
sysenter compat case already.

It should be noted that this drops the optimization in Xen for not
restoring a few registers when returning to user mode, but it seems
as if the saved instructions in the kernel more than compensate for
this drop (a kernel build in a Xen PV guest was slightly faster with
this patch applied).

While at it remove the stale sysret32 remnants.

  [ pawan: Brad Spengler and Salvatore Bonaccorso <carnil@debian.org>
	   reported a problem with the 5.10 backport commit edc702b4a820
	   ("x86/entry_64: Add VERW just before userspace transition").

	   When CONFIG_PARAVIRT_XXL=y, CLEAR_CPU_BUFFERS is not executed in
	   syscall_return_via_sysret path as USERGS_SYSRET64 is runtime
	   patched to:

	.cpu_usergs_sysret64    = { 0x0f, 0x01, 0xf8,
				    0x48, 0x0f, 0x07 }, // swapgs; sysretq

	   which is missing CLEAR_CPU_BUFFERS. It turns out dropping
	   USERGS_SYSRET64 simplifies the code, allowing CLEAR_CPU_BUFFERS
	   to be explicitly added to syscall_return_via_sysret path. Below
	   is with CONFIG_PARAVIRT_XXL=y and this patch applied:

	   syscall_return_via_sysret:
	   ...
	   <+342>:   swapgs
	   <+345>:   xchg   %ax,%ax
	   <+347>:   verw   -0x1a2(%rip)  <------
	   <+354>:   sysretq
  ]

Weakness: CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2021-4440?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-4440?

Apply the vendor's patch.

## Is CVE-2021-4440 being exploited?

There is no known exploitation of CVE-2021-4440 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-2021-4440 and fix it if it is affected.

What it is: CVE-2021-4440 is a high-severity denial of service vulnerability in Linux kernel, published on 25 Jun 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-2021-4440 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-2021-4440/ (Markdown: https://insights.sbomify.com/CVE-2021-4440/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-4440
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-4440
- OSV: https://osv.dev/vulnerability/CVE-2021-4440

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-4440/
Summaries are generated and may contain mistakes; the official description above is the record.