[PATCH v10 27/27] KVM: x86: Don't emulate instructions guarded by CET

[Yang Weijiang]

Don't emulate the branch instructions, e.g., CALL/RET/JMP etc., when CET
is active in guest, return KVM_INTERNAL_ERROR_EMULATION to userspace to
handle it.

KVM doesn't emulate CPU behaviors to check CET protected stuffs while
emulating guest instructions, instead it stops emulation on detecting
the instructions in process are CET protected. By doing so, it can avoid
generating bogus #CP in guest and preventing CET protected execution flow
subversion from guest side.

Suggested-by: Chao Gao <chao.gao@xxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
 arch/x86/kvm/emulate.c | 46 ++++++++++++++++++++++++++++++++----------
 1 file changed, 35 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 8ccc17eb78ca..c18616d24ac9 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -178,6 +178,8 @@
 #define IncSP       ((u64)1 << 54)  /* SP is incremented before ModRM calc */
 #define TwoMemOp    ((u64)1 << 55)  /* Instruction has two memory operand */
 #define IsBranch    ((u64)1 << 56)  /* Instruction is considered a branch. */
+#define ShadowStack ((u64)1 << 57)  /* Instruction protected by Shadow Stack. */
+#define IndirBrnTrk ((u64)1 << 58)  /* Instruction protected by IBT. */
 
 #define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
 
@@ -4100,9 +4102,11 @@ static const struct opcode group4[] = {
 static const struct opcode group5[] = {
 	F(DstMem | SrcNone | Lock,		em_inc),
 	F(DstMem | SrcNone | Lock,		em_dec),
-	I(SrcMem | NearBranch | IsBranch,       em_call_near_abs),
-	I(SrcMemFAddr | ImplicitOps | IsBranch, em_call_far),
-	I(SrcMem | NearBranch | IsBranch,       em_jmp_abs),
+	I(SrcMem | NearBranch | IsBranch | ShadowStack | IndirBrnTrk,
+	em_call_near_abs),
+	I(SrcMemFAddr | ImplicitOps | IsBranch | ShadowStack | IndirBrnTrk,
+	em_call_far),
+	I(SrcMem | NearBranch | IsBranch | IndirBrnTrk, em_jmp_abs),
 	I(SrcMemFAddr | ImplicitOps | IsBranch, em_jmp_far),
 	I(SrcMem | Stack | TwoMemOp,		em_push), D(Undefined),
 };
@@ -4364,11 +4368,11 @@ static const struct opcode opcode_table[256] = {
 	/* 0xC8 - 0xCF */
 	I(Stack | SrcImmU16 | Src2ImmByte | IsBranch, em_enter),
 	I(Stack | IsBranch, em_leave),
-	I(ImplicitOps | SrcImmU16 | IsBranch, em_ret_far_imm),
-	I(ImplicitOps | IsBranch, em_ret_far),
-	D(ImplicitOps | IsBranch), DI(SrcImmByte | IsBranch, intn),
+	I(ImplicitOps | SrcImmU16 | IsBranch | ShadowStack, em_ret_far_imm),
+	I(ImplicitOps | IsBranch | ShadowStack, em_ret_far),
+	D(ImplicitOps | IsBranch), DI(SrcImmByte | IsBranch | ShadowStack, intn),
 	D(ImplicitOps | No64 | IsBranch),
-	II(ImplicitOps | IsBranch, em_iret, iret),
+	II(ImplicitOps | IsBranch | ShadowStack, em_iret, iret),
 	/* 0xD0 - 0xD7 */
 	G(Src2One | ByteOp, group2), G(Src2One, group2),
 	G(Src2CL | ByteOp, group2), G(Src2CL, group2),
@@ -4384,7 +4388,7 @@ static const struct opcode opcode_table[256] = {
 	I2bvIP(SrcImmUByte | DstAcc, em_in,  in,  check_perm_in),
 	I2bvIP(SrcAcc | DstImmUByte, em_out, out, check_perm_out),
 	/* 0xE8 - 0xEF */
-	I(SrcImm | NearBranch | IsBranch, em_call),
+	I(SrcImm | NearBranch | IsBranch | ShadowStack, em_call),
 	D(SrcImm | ImplicitOps | NearBranch | IsBranch),
 	I(SrcImmFAddr | No64 | IsBranch, em_jmp_far),
 	D(SrcImmByte | ImplicitOps | NearBranch | IsBranch),
@@ -4403,7 +4407,8 @@ static const struct opcode opcode_table[256] = {
 static const struct opcode twobyte_table[256] = {
 	/* 0x00 - 0x0F */
 	G(0, group6), GD(0, &group7), N, N,
-	N, I(ImplicitOps | EmulateOnUD | IsBranch, em_syscall),
+	N, I(ImplicitOps | EmulateOnUD | IsBranch | ShadowStack | IndirBrnTrk,
+	em_syscall),
 	II(ImplicitOps | Priv, em_clts, clts), N,
 	DI(ImplicitOps | Priv, invd), DI(ImplicitOps | Priv, wbinvd), N, N,
 	N, D(ImplicitOps | ModRM | SrcMem | NoAccess), N, N,
@@ -4434,8 +4439,9 @@ static const struct opcode twobyte_table[256] = {
 	IIP(ImplicitOps, em_rdtsc, rdtsc, check_rdtsc),
 	II(ImplicitOps | Priv, em_rdmsr, rdmsr),
 	IIP(ImplicitOps, em_rdpmc, rdpmc, check_rdpmc),
-	I(ImplicitOps | EmulateOnUD | IsBranch, em_sysenter),
-	I(ImplicitOps | Priv | EmulateOnUD | IsBranch, em_sysexit),
+	I(ImplicitOps | EmulateOnUD | IsBranch | ShadowStack | IndirBrnTrk,
+	em_sysenter),
+	I(ImplicitOps | Priv | EmulateOnUD | IsBranch | ShadowStack, em_sysexit),
 	N, N,
 	N, N, N, N, N, N, N, N,
 	/* 0x40 - 0x4F */
@@ -4973,6 +4979,24 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
 	if (ctxt->d == 0)
 		return EMULATION_FAILED;
 
+	if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_CET) {
+		u64 u_cet, s_cet;
+		bool stop_em;
+
+		if (ctxt->ops->get_msr(ctxt, MSR_IA32_U_CET, &u_cet) ||
+		    ctxt->ops->get_msr(ctxt, MSR_IA32_S_CET, &s_cet))
+			return EMULATION_FAILED;
+
+		stop_em = ((u_cet & CET_SHSTK_EN) || (s_cet & CET_SHSTK_EN)) &&
+			  (opcode.flags & ShadowStack);
+
+		stop_em |= ((u_cet & CET_ENDBR_EN) || (s_cet & CET_ENDBR_EN)) &&
+			   (opcode.flags & IndirBrnTrk);
+
+		if (stop_em)
+			return EMULATION_FAILED;
+	}
+
 	ctxt->execute = opcode.u.execute;
 
 	if (unlikely(emulation_type & EMULTYPE_TRAP_UD) &&
-- 
2.43.0