Re: [PATCH 19/19] KVM: x86: Enable supervisor IBT support for guest

[Peter Zijlstra]

On Thu, Jun 16, 2022 at 04:46:43AM -0400, Yang Weijiang wrote:
> Mainline kernel now supports supervisor IBT for kernel code,
> to make s-IBT work in guest(nested guest), pass through
> MSR_IA32_S_CET to guest(nested guest) if host kernel and KVM
> enabled IBT. Note, s-IBT can work independent to host xsaves
> support because guest MSR_IA32_S_CET can be stored/loaded from
> specific VMCS field.


> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fe049d0e5ecc..c0118b33806a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1463,6 +1463,7 @@ static const u32 msrs_to_save_all[] = {
>  	MSR_IA32_XFD, MSR_IA32_XFD_ERR,
>  	MSR_IA32_XSS,
>  	MSR_IA32_U_CET, MSR_IA32_PL3_SSP, MSR_KVM_GUEST_SSP,
> +	MSR_IA32_S_CET,
>  };


So much like my local kvm/qemu hacks; this patch suffers the problem of
not exposing S_SHSTK. What happens if a guest tries to use that?

Should we intercept and reject setting those bits or complete this patch
and support full S_SHSTK? (with all the warts and horrors that entails)

I don't think throwing this out in this half-finished state makes much
sense (which is why I never much shared my hacks).


> @@ -11830,7 +11835,13 @@ int kvm_arch_hardware_setup(void *opaque)
>  	/* Update CET features now as kvm_caps.supported_xss is finalized. */
>  	if (!kvm_cet_user_supported()) {
>  		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
> -		kvm_cpu_cap_clear(X86_FEATURE_IBT);
> +		/* If CET user bit is disabled due to cmdline option such as
> +		 * noxsaves, but kernel IBT is on, this means we can expose
> +		 * kernel IBT alone to guest since CET user mode msrs are not
> +		 * passed through to guest.
> +		 */

Invalid multi-line comment style.

> +		if (!cet_kernel_ibt_supported())
> +			kvm_cpu_cap_clear(X86_FEATURE_IBT);