# CVE-2023-53756: KVM: VMX: Fix crash due to uninitialized current_vmcs

> A local attacker with low privileges can cause a kernel crash in KVM VMX by triggering a null pointer dereference during MSR bitmap updates on nested Hyper-V hosts.

- **Severity:** Not scored
- **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:** 8 Dec 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2023-53756, DEBIAN-CVE-2023-53756, GHSA-5h86-42q5-w9hr, UBUNTU-CVE-2023-53756

## What is CVE-2023-53756?

CVE-2023-53756 is a vulnerability in Linux, published on 8 Dec 2025. 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:

KVM: VMX: Fix crash due to uninitialized current_vmcs

KVM enables 'Enlightened VMCS' and 'Enlightened MSR Bitmap' when running as
a nested hypervisor on top of Hyper-V. When MSR bitmap is updated,
evmcs_touch_msr_bitmap function uses current_vmcs per-cpu variable to mark
that the msr bitmap was changed.

vmx_vcpu_create() modifies the msr bitmap via vmx_disable_intercept_for_msr
-> vmx_msr_bitmap_l01_changed which in the end calls this function. The
function checks for current_vmcs if it is null but the check is
insufficient because current_vmcs is not initialized. Because of this, the
code might incorrectly write to the structure pointed by current_vmcs value
left by another task. Preemption is not disabled, the current task can be
preempted and moved to another CPU while current_vmcs is accessed multiple
times from evmcs_touch_msr_bitmap() which leads to crash.

The manipulation of MSR bitmaps by callers happens only for vmcs01 so the
solution is to use vmx->vmcs01.vmcs instead of current_vmcs.

  BUG: kernel NULL pointer dereference, address: 0000000000000338
  PGD 4e1775067 P4D 0
  Oops: 0002 [#1] PREEMPT SMP NOPTI
  ...
  RIP: 0010:vmx_msr_bitmap_l01_changed+0x39/0x50 [kvm_intel]
  ...
  Call Trace:
   vmx_disable_intercept_for_msr+0x36/0x260 [kvm_intel]
   vmx_vcpu_create+0xe6/0x540 [kvm_intel]
   kvm_arch_vcpu_create+0x1d1/0x2e0 [kvm]
   kvm_vm_ioctl_create_vcpu+0x178/0x430 [kvm]
   kvm_vm_ioctl+0x53f/0x790 [kvm]
   __x64_sys_ioctl+0x8a/0xc0
   do_syscall_64+0x5c/0x90
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

## Which versions are affected by CVE-2023-53756?


## How do I fix CVE-2023-53756?

Apply the vendor's patch.

## Is CVE-2023-53756 being exploited?

There is no known exploitation of CVE-2023-53756 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-53756 and fix it if it is affected.

What it is: CVE-2023-53756 is a vulnerability in Linux, published on 8 Dec 2025. 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-2023-53756 in the wild and no public exploit code.

Affected product: Linux. 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-2023-53756/ (Markdown: https://insights.sbomify.com/CVE-2023-53756/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-53756
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-53756
- OSV: https://osv.dev/vulnerability/CVE-2023-53756

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-53756/
Summaries are generated and may contain mistakes; the official description above is the record.