KVM: x86: Refactor prefix decoding to prevent Spectre-v1/L1TF attacks

This fixes Spectre-v1/L1TF vulnerabilities in
vmx_read_guest_seg_selector(), vmx_read_guest_seg_base(),
vmx_read_guest_seg_limit() and vmx_read_guest_seg_ar().  When
invoked from emulation, these functions contain index computations
based on the (attacker-influenced) segment value.  Using constants
prevents the attack.

Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -5195,16 +5195,28 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 				ctxt->ad_bytes = def_ad_bytes ^ 6;
 			break;
 		case 0x26:	/* ES override */
+			has_seg_override = true;
+			ctxt->seg_override = VCPU_SREG_ES;
+			break;
 		case 0x2e:	/* CS override */
+			has_seg_override = true;
+			ctxt->seg_override = VCPU_SREG_CS;
+			break;
 		case 0x36:	/* SS override */
+			has_seg_override = true;
+			ctxt->seg_override = VCPU_SREG_SS;
+			break;
 		case 0x3e:	/* DS override */
 			has_seg_override = true;
-			ctxt->seg_override = (ctxt->b >> 3) & 3;
+			ctxt->seg_override = VCPU_SREG_DS;
 			break;
 		case 0x64:	/* FS override */
+			has_seg_override = true;
+			ctxt->seg_override = VCPU_SREG_FS;
+			break;
 		case 0x65:	/* GS override */
 			has_seg_override = true;
-			ctxt->seg_override = ctxt->b & 7;
+			ctxt->seg_override = VCPU_SREG_GS;
 			break;
 		case 0x40 ... 0x4f: /* REX */
 			if (mode != X86EMUL_MODE_PROT64)