Re: [Xen-devel] [PATCH 1/2] Revert "x86/hvm: disable pkeys for guests in non-paging mode"

["Han, Huaitong" <huaitong.han@xxxxxxxxx>]

On Fri, 2017-05-26 at 18:03 +0100, Andrew Cooper wrote:
> This reverts commit c41e0266dd59ab50b7a153157e9bd2a3ad114b53.
> 
> When determining Access Rights, Protection Keys only take effect when CR4.PKE
> it set, and 4-level paging is active.  All other circumstances (notibly, 32bit
> PAE paging) skip the Protection Key control mechanism.
> 
> Therefore, we do not need to clear CR4.PKE behind the back of a guest which is
> not using paging, as such a guest is necesserily running with EFER.LME
> disabled.

Yes, if EFER.LME = 0, Protection Keys would take no effect too, so it
isn't necessary to clear CR4.PKE in non-paging mode.

> 
> The {RD,WR}PKRU instructions are specified as being legal for use in any
> operating mode, but only if CR4.PKE is set.  By clearing CR4.PKE behind the
> back of an unpaged guest, these instructions yield #UD despite the guest
> seeing PKE set if it reads CR4, and OSPKE being visible in CPUID.
If CR4.PKE is cleared, OSPKE would be invisible at the same time. When
guest does set CR4_PKE in non-paging mode, then CR4_PKE would be cleared
in vmcs loading, so, OSPKE should be always invisible, and #UD should
not be yielded too.

 
Reviewed-by: Huaitong Han <huaitong.han@xxxxxxxxx>

> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> CC: Huaitong Han <huaitong.han@xxxxxxxxx>
> ---
>  xen/arch/x86/hvm/vmx/vmx.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index c8ef18a..58552c3 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -1673,13 +1673,12 @@ static void vmx_update_guest_cr(struct vcpu *v, 
> unsigned int cr)
>          if ( !hvm_paging_enabled(v) )
>          {
>              /*
> -             * SMEP/SMAP/PKU is disabled if CPU is in non-paging mode in
> -             * hardware. However Xen always uses paging mode to emulate guest
> -             * non-paging mode. To emulate this behavior, SMEP/SMAP/PKU needs
> -             * to be manually disabled when guest VCPU is in non-paging mode.
> +             * SMEP/SMAP is disabled if CPU is in non-paging mode in 
> hardware.
> +             * However Xen always uses paging mode to emulate guest 
> non-paging
> +             * mode. To emulate this behavior, SMEP/SMAP needs to be manually
> +             * disabled when guest VCPU is in non-paging mode.
>               */
> -            v->arch.hvm_vcpu.hw_cr[4] &=
> -                ~(X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE);
> +            v->arch.hvm_vcpu.hw_cr[4] &= ~(X86_CR4_SMEP | X86_CR4_SMAP);
>          }
>          __vmwrite(GUEST_CR4, v->arch.hvm_vcpu.hw_cr[4]);
>          break;

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