Re: [PATCH v7 02/20] KVM: VMX: Save guest EGPRs in VCPU cache

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Host kernel data in Extended General Purpose Registers (EGPRs) is leaked to a malicious guest via speculative execution when the guest operates with APX disabled.
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commit 70442dd137e1b61dbdd5b674afc21cfb7948b2b6
Author: Chang S. Bae <chang.seok.bae@intel.com>
KVM: VMX: Save guest EGPRs in VCPU cache

This commit saves and restores the guest EGPRs on VM exit/entry if the
system supports Advanced Performance Extensions (APX). It skips loading the
EGPR state for guests that do not set XCR0[APX], and places the register
saving path after SPEC_CTRL is restored.

Link: https://lore.kernel.org/adPRA4ZhnvbaXSn0@google.com

> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index 00d807505fc88..02308fb36f0f5 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S

[ ... ]

> @@ -46,6 +47,16 @@ SYM_FUNC_START(__vmx_vcpu_run)
>  	/* Reload @vmx, _ASM_ARG1 may be modified by vmx_update_host_rsp().  */
>  	mov WORD_SIZE(%_ASM_SP), %_ASM_DI
>  
> +#ifdef CONFIG_KVM_APX
> +	ALTERNATIVE "jmp .Lload_egprs_done", "", X86_FEATURE_APX
> +	testl	$KVM_ENTER_EGPR_SWITCH, (%_ASM_SP)
> +	jz	.Lload_egprs_done
> +	LOAD_REGS %_ASM_DI, VMX_vcpu_arch_regs, \
> +		  %r16, %r17, %r18, %r19, %r20, %r21, %r22, %r23, \
> +		  %r24, %r25, %r26, %r27, %r28, %r29, %r30, %r31
> +.Lload_egprs_done:
> +#endif

[Severity: High]
In __vmx_vcpu_run(), when a guest runs with APX disabled,
KVM_ENTER_EGPR_SWITCH is clear and this branch skips loading the guest's
EGPRs. Are the host's physical %r16 through %r31 register values left
intact upon entering the guest when this happens?

If so, could a malicious guest with APX disabled speculatively execute APX
instructions before the architectural #UD exception retires, allowing it to
read the host's %r16 through %r31 values and leak them via a cache
side-channel?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260902021804.718676-1-chang.seok.bae@intel.com?part=2