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

[Chao Gao <chao.gao@intel.com>]

On Mon, Feb 27, 2023 at 04:45:46PM +0800, 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)) {
>+			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);
>+			}
>+		}
>+
> 		*linear = la;
> 		va_bits = ctxt_virt_addr_bits(ctxt);
> 		if (!__is_canonical_address(la, va_bits))

...

>+static inline u64 kvm_lam_untag_addr(u64 addr, enum lam_type type)
>+{
>+	switch (type) {
>+	case LAM_U57:
>+	case LAM_S57:
>+		addr = __canonical_address(addr, 57);
>+		break;
>+	case LAM_U48:
>+	case LAM_S48:
>+		addr = __canonical_address(addr, 48);
>+		break;
>+	case LAM_NONE:
>+	default:
>+		break;
>+	}
>+
>+	return addr;
>+}

LAM's change to canonicality check is:
before performing the check, software metadata in pointers is masked by
sign-extending the value of bit 56/47.

so, to emulate this behavior, in kvm_lam_untag_addr(), we can simply:
1. determine which LAM configuration is enabled, LAM57 or LAM48.
2. mask software metadata by sign-extending the bit56/47, i.e.,

	addr = (sign_extern64(addr, X) & ~BIT_ULL(63)) |
	       (addr & BIT_ULL(63));

	where X=56 for LAM57 and X=47 for LAM48.

Note that this doesn't ensure the resulting @addr is canonical. It
isn't a problem because the original canonicality check
(__is_canonical_address() above) can identify non-canonical addresses
and raise #GP/#SS to the guest.