KVM: x86: ARPL emulation can cause spurious exceptions

ARPL and MOVSXD are encoded the same and their execution depends on the
execution mode.  The operand sizes of each instruction are different.
Currently, ARPL is detected too late, after the decoding was already done, and
therefore may result in spurious exception (instead of failed emulation).

Introduce a group to the emulator to handle instructions according to execution
mode (32/64 bits). Note: in order not to make changes that may affect
performance, the new ModeDual can only be applied to instructions with ModRM.
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -125,6 +125,7 @@
 #define RMExt       (4<<15)     /* Opcode extension in ModRM r/m if mod == 3 */
 #define Escape      (5<<15)     /* Escape to coprocessor instruction */
 #define InstrDual   (6<<15)     /* Alternate instruction decoding of mod == 3 */
+#define ModeDual    (7<<15)     /* Different instruction for 32/64 bit */
 #define Sse         (1<<18)     /* SSE Vector instruction */
 /* Generic ModRM decode. */
 #define ModRM       (1<<19)
@@ -215,6 +216,7 @@ struct opcode {
 		const struct gprefix *gprefix;
 		const struct escape *esc;
 		const struct instr_dual *idual;
+		const struct mode_dual *mdual;
 		void (*fastop)(struct fastop *fake);
 	} u;
 	int (*check_perm)(struct x86_emulate_ctxt *ctxt);
@@ -242,6 +244,11 @@ struct instr_dual {
 	struct opcode mod3;
 };
 
+struct mode_dual {
+	struct opcode mode32;
+	struct opcode mode64;
+};
+
 /* EFLAGS bit definitions. */
 #define EFLG_ID (1<<21)
 #define EFLG_VIP (1<<20)
@@ -3530,6 +3537,12 @@ static int em_clflush(struct x86_emulate_ctxt *ctxt)
 	return X86EMUL_CONTINUE;
 }
 
+static int em_movsxd(struct x86_emulate_ctxt *ctxt)
+{
+	ctxt->dst.val = (s32) ctxt->src.val;
+	return X86EMUL_CONTINUE;
+}
+
 static bool valid_cr(int nr)
 {
 	switch (nr) {
@@ -3729,6 +3742,7 @@ static int check_perm_out(struct x86_emulate_ctxt *ctxt)
 #define G(_f, _g) { .flags = ((_f) | Group | ModRM), .u.group = (_g) }
 #define GD(_f, _g) { .flags = ((_f) | GroupDual | ModRM), .u.gdual = (_g) }
 #define ID(_f, _i) { .flags = ((_f) | InstrDual | ModRM), .u.idual = (_i) }
+#define MD(_f, _m) { .flags = ((_f) | ModeDual), .u.mdual = (_m) }
 #define E(_f, _e) { .flags = ((_f) | Escape | ModRM), .u.esc = (_e) }
 #define I(_f, _e) { .flags = (_f), .u.execute = (_e) }
 #define F(_f, _e) { .flags = (_f) | Fastop, .u.fastop = (_e) }
@@ -3973,6 +3987,10 @@ static const struct instr_dual instr_dual_0f_c3 = {
 	I(DstMem | SrcReg | ModRM | No16 | Mov, em_mov), N
 };
 
+static const struct mode_dual mode_dual_63 = {
+	N, I(DstReg | SrcMem32 | ModRM | Mov, em_movsxd)
+};
+
 static const struct opcode opcode_table[256] = {
 	/* 0x00 - 0x07 */
 	F6ALU(Lock, em_add),
@@ -4007,7 +4025,7 @@ static const struct opcode opcode_table[256] = {
 	/* 0x60 - 0x67 */
 	I(ImplicitOps | Stack | No64, em_pusha),
 	I(ImplicitOps | Stack | No64, em_popa),
-	N, D(DstReg | SrcMem32 | ModRM | Mov) /* movsxd (x86/64) */ ,
+	N, MD(ModRM, &mode_dual_63),
 	N, N, N, N,
 	/* 0x68 - 0x6F */
 	I(SrcImm | Mov | Stack, em_push),
@@ -4227,6 +4245,7 @@ static const struct opcode opcode_map_0f_38[256] = {
 #undef I
 #undef GP
 #undef EXT
+#undef MD
 
 #undef D2bv
 #undef D2bvIP
@@ -4616,6 +4635,12 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 			else
 				opcode = opcode.u.idual->mod012;
 			break;
+		case ModeDual:
+			if (ctxt->mode == X86EMUL_MODE_PROT64)
+				opcode = opcode.u.mdual->mode64;
+			else
+				opcode = opcode.u.mdual->mode32;
+			break;
 		default:
 			return EMULATION_FAILED;
 		}
@@ -4956,11 +4981,6 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 		goto threebyte_insn;
 
 	switch (ctxt->b) {
-	case 0x63:		/* movsxd */
-		if (ctxt->mode != X86EMUL_MODE_PROT64)
-			goto cannot_emulate;
-		ctxt->dst.val = (s32) ctxt->src.val;
-		break;
 	case 0x70 ... 0x7f: /* jcc (short) */
 		if (test_cc(ctxt->b, ctxt->eflags))
 			rc = jmp_rel(ctxt, ctxt->src.val);