Re: [PATCH v2 6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation

[Chao Gao]

On Tue, Oct 24, 2023 at 01:08:53AM -0700, Pawan Gupta wrote:
>During VMentry VERW is executed to mitigate MDS. After VERW, any memory
>access like register push onto stack may put host data in MDS affected
>CPU buffers. A guest can then use MDS to sample host data.
>
>Although likelihood of secrets surviving in registers at current VERW
>callsite is less, but it can't be ruled out. Harden the MDS mitigation
>by moving the VERW mitigation late in VMentry path.
>
>Note that VERW for MMIO Stale Data mitigation is unchanged because of
>the complexity of per-guest conditional VERW which is not easy to handle
>that late in asm with no GPRs available. If the CPU is also affected by
>MDS, VERW is unconditionally executed late in asm regardless of guest
>having MMIO access.
>
>Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
>---
> arch/x86/kvm/vmx/vmenter.S |  4 ++++
> arch/x86/kvm/vmx/vmx.c     | 10 +++++++---
> 2 files changed, 11 insertions(+), 3 deletions(-)
>
>diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
>index b3b13ec04bac..c566035938cc 100644
>--- a/arch/x86/kvm/vmx/vmenter.S
>+++ b/arch/x86/kvm/vmx/vmenter.S
>@@ -1,6 +1,7 @@
> /* SPDX-License-Identifier: GPL-2.0 */
> #include <linux/linkage.h>
> #include <asm/asm.h>
>+#include <asm/segment.h>

This header is already included a few lines below:

#include <asm/nospec-branch.h>
#include <asm/percpu.h>
#include <asm/segment.h>	<---

> #include <asm/bitsperlong.h>
> #include <asm/kvm_vcpu_regs.h>
> #include <asm/nospec-branch.h>