Re: [PATCH v5 4/5] KVM: x86: emulation: Apply LAM mask when emulating data access in 64-bit mode

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 27, 2023, Robert Hoo wrote:
> Emulate HW LAM masking when doing data access under 64-bit mode.
> 
> kvm_lam_untag_addr() implements this: per CR4/CR3 LAM bits configuration,
> firstly check the linear addr conforms LAM canonical, i.e. the highest
> address bit matches bit 63. Then mask out meta data per LAM configuration.
> If failed in above process, emulate #GP to guest.
> 
> Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
> ---
>  arch/x86/kvm/emulate.c | 13 ++++++++
>  arch/x86/kvm/x86.h     | 70 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 83 insertions(+)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 5cc3efa0e21c..77bd13f40711 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -700,6 +700,19 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
>  	*max_size = 0;
>  	switch (mode) {
>  	case X86EMUL_MODE_PROT64:
> +		/* LAM applies only on data access */
> +		if (!fetch && guest_cpuid_has(ctxt->vcpu, X86_FEATURE_LAM)) {

Derefencing ctxt->vcpu in the emulator is not allowed.

> +			enum lam_type type;
> +
> +			type = kvm_vcpu_lam_type(la, ctxt->vcpu);
> +			if (type == LAM_ILLEGAL) {
> +				*linear = la;
> +				goto bad;
> +			} else {
> +				la = kvm_lam_untag_addr(la, type);
> +			}
> +		}

This is wildly over-engineered.  Just do the untagging and let __is_canonical_address()
catch any non-canonical bits that weren't stripped.