KVM: x86: Generate #UD when memory operand is required

Certain x86 instructions that use modrm operands only allow memory operand
(i.e., mod012), and cause a #UD exception otherwise. KVM ignores this fact.
Currently, the instructions that are such and are emulated by KVM are MOVBE,
MOVNTPS, MOVNTPD and MOVNTI.  MOVBE is the most blunt example, since it may be
emulated by the host regardless of MMIO.

The fix introduces a new group for handling such instructions, marking mod3 as
illegal instruction.
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -123,6 +123,7 @@
 #define Prefix      (3<<15)     /* Instruction varies with 66/f2/f3 prefix */
 #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 Sse         (1<<18)     /* SSE Vector instruction */
 /* Generic ModRM decode. */
 #define ModRM       (1<<19)
@@ -211,6 +212,7 @@ struct opcode {
 		const struct group_dual *gdual;
 		const struct gprefix *gprefix;
 		const struct escape *esc;
+		const struct instr_dual *idual;
 		void (*fastop)(struct fastop *fake);
 	} u;
 	int (*check_perm)(struct x86_emulate_ctxt *ctxt);
@@ -233,6 +235,11 @@ struct escape {
 	struct opcode high[64];
 };
 
+struct instr_dual {
+	struct opcode mod012;
+	struct opcode mod3;
+};
+
 /* EFLAGS bit definitions. */
 #define EFLG_ID (1<<21)
 #define EFLG_VIP (1<<20)
@@ -3679,6 +3686,7 @@ static int check_perm_out(struct x86_emulate_ctxt *ctxt)
 #define EXT(_f, _e) { .flags = ((_f) | RMExt), .u.group = (_e) }
 #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 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) }
@@ -3840,8 +3848,12 @@ static const struct gprefix pfx_0f_6f_0f_7f = {
 	I(Mmx, em_mov), I(Sse | Aligned, em_mov), N, I(Sse | Unaligned, em_mov),
 };
 
+static const struct instr_dual instr_dual_0f_2b = {
+	I(0, em_mov), N
+};
+
 static const struct gprefix pfx_0f_2b = {
-	I(0, em_mov), I(0, em_mov), N, N,
+	ID(0, &instr_dual_0f_2b), ID(0, &instr_dual_0f_2b), N, N,
 };
 
 static const struct gprefix pfx_0f_28_0f_29 = {
@@ -3915,6 +3927,10 @@ static const struct escape escape_dd = { {
 	N, N, N, N, N, N, N, N,
 } };
 
+static const struct instr_dual instr_dual_0f_c3 = {
+	I(DstMem | SrcReg | ModRM | No16 | Mov, em_mov), N
+};
+
 static const struct opcode opcode_table[256] = {
 	/* 0x00 - 0x07 */
 	F6ALU(Lock, em_add),
@@ -4117,7 +4133,7 @@ static const struct opcode twobyte_table[256] = {
 	D(DstReg | SrcMem8 | ModRM | Mov), D(DstReg | SrcMem16 | ModRM | Mov),
 	/* 0xC0 - 0xC7 */
 	F2bv(DstMem | SrcReg | ModRM | SrcWrite | Lock, em_xadd),
-	N, I(DstMem | SrcReg | ModRM | No16 | Mov, em_mov),
+	N, ID(0, &instr_dual_0f_c3),
 	N, N, N, GD(0, &group9),
 	/* 0xC8 - 0xCF */
 	X8(I(DstReg, em_bswap)),
@@ -4130,12 +4146,20 @@ static const struct opcode twobyte_table[256] = {
 	N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N
 };
 
+static const struct instr_dual instr_dual_0f_38_f0 = {
+	I(DstReg | SrcMem | Mov, em_movbe), N
+};
+
+static const struct instr_dual instr_dual_0f_38_f1 = {
+	I(DstMem | SrcReg | Mov, em_movbe), N
+};
+
 static const struct gprefix three_byte_0f_38_f0 = {
-	I(DstReg | SrcMem | Mov, em_movbe), N, N, N
+	ID(0, &instr_dual_0f_38_f0), N, N, N
 };
 
 static const struct gprefix three_byte_0f_38_f1 = {
-	I(DstMem | SrcReg | Mov, em_movbe), N, N, N
+	ID(0, &instr_dual_0f_38_f1), N, N, N
 };
 
 /*
@@ -4536,6 +4560,12 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 			else
 				opcode = opcode.u.esc->op[(ctxt->modrm >> 3) & 7];
 			break;
+		case InstrDual:
+			if ((ctxt->modrm >> 6) == 3)
+				opcode = opcode.u.idual->mod3;
+			else
+				opcode = opcode.u.idual->mod012;
+			break;
 		default:
 			return EMULATION_FAILED;
 		}