CVE-2025-21839

/// Technical 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

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References