Re: [PATCH v5 5/9] KVM: VMX: Handle MMIO Stale Data in VM-Enter assembly via ALTERNATIVES_2

[Brendan Jackman <jackmanb@google.com>]

On Thu Nov 13, 2025 at 11:37 PM UTC, Sean Christopherson wrote:
> Rework the handling of the MMIO Stale Data mitigation to clear CPU buffers
> immediately prior to VM-Enter, i.e. in the same location that KVM emits a
> VERW for unconditional (at runtime) clearing.  Co-locating the code and
> using a single ALTERNATIVES_2 makes it more obvious how VMX mitigates the
> various vulnerabilities.
>
> Deliberately order the alternatives as:
>
>  0. Do nothing
>  1. Clear if vCPU can access MMIO
>  2. Clear always
>
> since the last alternative wins in ALTERNATIVES_2(), i.e. so that KVM will
> honor the strictest mitigation (always clear CPU buffers) if multiple
> mitigations are selected.  E.g. even if the kernel chooses to mitigate
> MMIO Stale Data via X86_FEATURE_CLEAR_CPU_BUF_VM_MMIO, another mitigation
> may enable X86_FEATURE_CLEAR_CPU_BUF_VM, and that other thing needs to win.
>
> Note, decoupling the MMIO mitigation from the L1TF mitigation also fixes
> a mostly-benign flaw where KVM wouldn't do any clearing/flushing if the
> L1TF mitigation is configured to conditionally flush the L1D, and the MMIO
> mitigation but not any other "clear CPU buffers" mitigation is enabled.
> For that specific scenario, KVM would skip clearing CPU buffers for the
> MMIO mitigation even though the kernel requested a clear on every VM-Enter.
>
> Note #2, the flaw goes back to the introduction of the MDS mitigation.  The
> MDS mitigation was inadvertently fixed by commit 43fb862de8f6 ("KVM/VMX:
> Move VERW closer to VMentry for MDS mitigation"), but previous kernels
> that flush CPU buffers in vmx_vcpu_enter_exit() are affected (though it's
> unlikely the flaw is meaningfully exploitable even older kernels).
>
> Fixes: 650b68a0622f ("x86/kvm/vmx: Add MDS protection when L1D Flush is not active")
> Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Brendan Jackman <jackmanb@google.com>