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KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop

Type: advisorySources: OSV

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, K

Products
Kernel
Affected versions
Not specified
Fixed versions
5.15.182, 6.1.138, 6.12.16, 6.13.4, 6.6.90
CVEs
CVE-2025-21839
Confirmed fixed CVEs
Not specified
Updated in this feed
4 Nov 2026, 23:14 UTC · Revision 1

Source evidence

KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop

core-osv · Original source

Published 7 Mar 2025, 09:09 UTC

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, K

osv:CVE-2025-21839

Kernel

Fixed versions: 5.15.182, 6.1.138, 6.6.90, 6.12.16, 6.13.4

Complete source record
{
  "id": "32dbf2081cb01cfe98aa0c9e9184e11676223222806ed6664f523985b890d884",
  "url": "https://osv.dev/vulnerability/CVE-2025-21839",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop",
    "summary": "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, K",
    "evidence": "osv:CVE-2025-21839",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "5.15.182",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.1.138",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.6.90",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.12.16",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.13.4",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "CVE-2025-21839",
      "aliases": [
        "CVE-2025-21839"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "d67668e9dd76d98136048935723947156737932b"
                },
                {
                  "fixed": "9efb2b99b96c86664bbdbdd2cdb354ac9627eb20"
                },
                {
                  "fixed": "93eeb6df1605b3a24f38afdba7ab903ba6b64133"
                },
                {
                  "fixed": "a1723e9c53fe6431415be19302a56543daf503f5"
                },
                {
                  "fixed": "4eb063de686bfcdfd03a8c801d1bbe87d2d5eb55"
                },
                {
                  "fixed": "d456de38d9eb753a4e9fde053c18d4ef8e485339"
                },
                {
                  "fixed": "c2fee09fc167c74a64adb08656cb993ea475197e"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-21839.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "5.7.0"
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                  "fixed": "5.15.182"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "5.16.0"
                },
                {
                  "fixed": "6.1.138"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.2.0"
                },
                {
                  "fixed": "6.6.90"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.7.0"
                },
                {
                  "fixed": "6.12.16"
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              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.13.0"
                },
                {
                  "fixed": "6.13.4"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-21839.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:40.358296150Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/4eb063de686bfcdfd03a8c801d1bbe87d2d5eb55",
        "https://git.kernel.org/stable/c/93eeb6df1605b3a24f38afdba7ab903ba6b64133",
        "https://git.kernel.org/stable/c/9efb2b99b96c86664bbdbdd2cdb354ac9627eb20",
        "https://git.kernel.org/stable/c/a1723e9c53fe6431415be19302a56543daf503f5",
        "https://git.kernel.org/stable/c/c2fee09fc167c74a64adb08656cb993ea475197e",
        "https://git.kernel.org/stable/c/d456de38d9eb753a4e9fde053c18d4ef8e485339",
        "https://lists.debian.org/debian-lts-announce/2025/08/msg00010.html",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/21xxx/CVE-2025-21839.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2025-21839",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/21xxx/CVE-2025-21839.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2025-21839",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 507887,
  "published_at": "2025-03-07T09:09:58.22+00:00"
}