CVE-2025-21839

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.

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