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

[Yang, Weijiang]

On 6/16/2022 7:19 PM, Peter Zijlstra wrote:
> 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?
With current solution, I think guest kernel will hit #GP while 
reading/writing PL0_SSP.
> 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).

You reminded me to think over these cases even I don't have a solution now,

thank you!

> > @@ -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.
Oops, last minute change messed it up :-(
> > +		if (!cet_kernel_ibt_supported())
> > +			kvm_cpu_cap_clear(X86_FEATURE_IBT);