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

Type: advisorySource: Official CVE List1 source record

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

Announced Source revision 1
Original source ↗

CVE-2025-21839

Primary source · Revision 1

Imported from structured advisory data. Source references identify upstream records or fields; verify affected versions and your deployed configuration.

Security context

Source reference: cve:CVE-2025-21839

Linux

Source reference: /containers/cna/affected/0/product

Affected versions
Not stated in this record
Fixed versions
Not stated in this record
Prerequisites
Not stated in this record

Linux

Source reference: /containers/cna/affected/1/product

Affected versions
Not stated in this record
Fixed versions
Not stated in this record
Prerequisites
Not stated in this record

Structured affected-version data

Original package identities and version rules from this source.

[
  {
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "vendor": "Linux",
    "product": "Linux",
    "versions": [
      {
        "status": "affected",
        "version": "d67668e9dd76d98136048935723947156737932b",
        "lessThan": "9efb2b99b96c86664bbdbdd2cdb354ac9627eb20",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "d67668e9dd76d98136048935723947156737932b",
        "lessThan": "93eeb6df1605b3a24f38afdba7ab903ba6b64133",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "d67668e9dd76d98136048935723947156737932b",
        "lessThan": "a1723e9c53fe6431415be19302a56543daf503f5",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "d67668e9dd76d98136048935723947156737932b",
        "lessThan": "4eb063de686bfcdfd03a8c801d1bbe87d2d5eb55",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "d67668e9dd76d98136048935723947156737932b",
        "lessThan": "d456de38d9eb753a4e9fde053c18d4ef8e485339",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "d67668e9dd76d98136048935723947156737932b",
        "lessThan": "c2fee09fc167c74a64adb08656cb993ea475197e",
        "versionType": "git"
      }
    ],
    "programFiles": [
      "arch/x86/include/asm/kvm-x86-ops.h",
      "arch/x86/include/asm/kvm_host.h",
      "arch/x86/kvm/svm/svm.c",
      "arch/x86/kvm/vmx/main.c",
      "arch/x86/kvm/vmx/vmx.c",
      "arch/x86/kvm/vmx/x86_ops.h",
      "arch/x86/kvm/x86.c"
    ],
    "defaultStatus": "unaffected"
  },
  {
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "vendor": "Linux",
    "product": "Linux",
    "versions": [
      {
        "status": "affected",
        "version": "5.7"
      },
      {
        "status": "unaffected",
        "version": "0",
        "lessThan": "5.7",
        "versionType": "semver"
      },
      {
        "status": "unaffected",
        "version": "5.15.182",
        "versionType": "semver",
        "lessThanOrEqual": "5.15.*"
      },
      {
        "status": "unaffected",
        "version": "6.1.138",
        "versionType": "semver",
        "lessThanOrEqual": "6.1.*"
      },
      {
        "status": "unaffected",
        "version": "6.6.90",
        "versionType": "semver",
        "lessThanOrEqual": "6.6.*"
      },
      {
        "status": "unaffected",
        "version": "6.12.16",
        "versionType": "semver",
        "lessThanOrEqual": "6.12.*"
      },
      {
        "status": "unaffected",
        "version": "6.13.4",
        "versionType": "semver",
        "lessThanOrEqual": "6.13.*"
      },
      {
        "status": "unaffected",
        "version": "6.14",
        "versionType": "original_commit_for_fix",
        "lessThanOrEqual": "*"
      }
    ],
    "programFiles": [
      "arch/x86/include/asm/kvm-x86-ops.h",
      "arch/x86/include/asm/kvm_host.h",
      "arch/x86/kvm/svm/svm.c",
      "arch/x86/kvm/vmx/main.c",
      "arch/x86/kvm/vmx/vmx.c",
      "arch/x86/kvm/vmx/x86_ops.h",
      "arch/x86/kvm/x86.c"
    ],
    "defaultStatus": "affected"
  }
]
Related CVEs
CVE-2025-21839

Source reference: /cveMetadata/cveId

Publisher severity

high

Source reference: /containers/cna/metrics

Event
#30438 · Revision 1
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Publisher date
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