# CVE-2025-21839: KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop

> A flaw in Linux KVM allows a guest to leak stale debug register values to the host, potentially exposing sensitive information.

- **Severity:** High 7.3 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:L/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:** 7 Mar 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2025-21839, DEBIAN-CVE-2025-21839, GHSA-m8v3-m369-c2xp, ROOT-OS-DEBIAN-11-CVE-2025-21839, ROOT-OS-DEBIAN-12-CVE-2025-21839, ROOT-OS-UBUNTU-2204-CVE-2025-21839, ROOT-OS-UBUNTU-2404-CVE-2025-21839, UBUNTU-CVE-2025-21839

## What is CVE-2025-21839?

CVE-2025-21839 is a high-severity vulnerability in Linux kernel, published on 7 Mar 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: x86: Load DR6 with guest value only before entering .vcpu_run() loop

Move the conditional loading of hardware DR6 with the guest's DR6 value
out of the core .vcpu_run() loop to fix a bug where KVM can load hardware
with a stale vcpu->arch.dr6.

When the guest accesses a DR and host userspace isn't debugging the guest,
KVM disables DR interception and loads the guest's values into hardware on
VM-Enter and saves them on VM-Exit.  This allows the guest to access DRs
at will, e.g. so that a sequence of DR accesses to configure a breakpoint
only generates one VM-Exit.

For DR0-DR3, the logic/behavior is identical between VMX and SVM, and also
identical between KVM_DEBUGREG_BP_ENABLED (userspace debugging the guest)
and KVM_DEBUGREG_WONT_EXIT (guest using DRs), and so KVM handles loading
DR0-DR3 in common code, _outside_ of the core kvm_x86_ops.vcpu_run() loop.

But for DR6, the guest's value doesn't need to be loaded into hardware for
KVM_DEBUGREG_BP_ENABLED, and SVM provides a dedicated VMCB field whereas
VMX requires software to manually load the guest value, and so loading the
guest's value into DR6 is handled by {svm,vmx}_vcpu_run(), i.e. is done
_inside_ the core run loop.

Unfortunately, saving the guest values on VM-Exit is initiated by common
x86, again outside of the core run loop.  If the guest modifies DR6 (in
hardware, when DR interception is disabled), and then the next VM-Exit is
a fastpath VM-Exit, KVM will reload hardware DR6 with vcpu->arch.dr6 and
clobber the guest's actual value.

The bug shows up primarily with nested VMX because KVM handles the VMX
preemption timer in the fastpath, and the window between hardware DR6
being modified (in guest context) and DR6 being read by guest software is
orders of magnitude larger in a nested setup.  E.g. in non-nested, the
VMX preemption timer would need to fire precisely between #DB injection
and the #DB handler's read of DR6, whereas with a KVM-on-KVM setup, the
window where hardware DR6 is "dirty" extends all the way from L1 writing
DR6 to VMRESUME (in L1).

    L1's view:
    ==========
    <L1 disables DR interception>
           CPU 0/KVM-7289    [023] d....  2925.640961: kvm_entry: vcpu 0
 A:  L1 Writes DR6
           CPU 0/KVM-7289    [023] d....  2925.640963: <hack>: Set DRs, DR6 = 0xffff0ff1

 B:        CPU 0/KVM-7289    [023] d....  2925.640967: kvm_exit: vcpu 0 reason EXTERNAL_INTERRUPT intr_info 0x800000ec

 D: L1 reads DR6, arch.dr6 = 0
           CPU 0/KVM-7289    [023] d....  2925.640969: <hack>: Sync DRs, DR6 = 0xffff0ff0

           CPU 0/KVM-7289    [023] d....  2925.640976: kvm_entry: vcpu 0
    L2 reads DR6, L1 disables DR interception
           CPU 0/KVM-7289    [023] d....  2925.640980: kvm_exit: vcpu 0 reason DR_ACCESS info1 0x0000000000000216
           CPU 0/KVM-7289    [023] d....  2925.640983: kvm_entry: vcpu 0

           CPU 0/KVM-7289    [023] d....  2925.640983: <hack>: Set DRs, DR6 = 0xffff0ff0

    L2 detects failure
           CPU 0/KVM-7289    [023] d....  2925.640987: kvm_exit: vcpu 0 reason HLT
    L1 reads DR6 (confirms failure)
           CPU 0/KVM-7289    [023] d....  2925.640990: <hack>: Sync DRs, DR6 = 0xffff0ff0

    L0's view:
    ==========
    L2 reads DR6, arch.dr6 = 0
          CPU 23/KVM-5046    [001] d....  3410.005610: kvm_exit: vcpu 23 reason DR_ACCESS info1 0x0000000000000216
          CPU 23/KVM-5046    [001] .....  3410.005610: kvm_nested_vmexit: vcpu 23 reason DR_ACCESS info1 0x0000000000000216

    L2 => L1 nested VM-Exit
          CPU 23/KVM-5046    [001] .....  3410.005610: kvm_nested_vmexit_inject: reason: DR_ACCESS ext_inf1: 0x0000000000000216

          CPU 23/KVM-5046    [001] d....  3410.005610: kvm_entry: vcpu 23
          CPU 23/KVM-5046    [001] d....  3410.005611: kvm_exit: vcpu 23 reason VMREAD
          CPU 23/KVM-5046    [001] d....  3410.005611: kvm_entry: vcpu 23
          CPU 23/KVM-5046    [001] d....  3410.
---truncated---

## Which versions are affected by CVE-2025-21839?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21839?

Apply the vendor's patch.

## Is CVE-2025-21839 being exploited?

There is no known exploitation of CVE-2025-21839 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-2025-21839 and fix it if it is affected.

What it is: CVE-2025-21839 is a high-severity vulnerability in Linux kernel, published on 7 Mar 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-2025-21839 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-2025-21839/ (Markdown: https://insights.sbomify.com/CVE-2025-21839/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21839
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21839
- OSV: https://osv.dev/vulnerability/CVE-2025-21839

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21839/
Summaries are generated and may contain mistakes; the official description above is the record.