[PATCH v9 27/27] KVM: x86: Stop emulating for CET protected branch instructions

Yang Weijiang
2024-01-24T02:42:51Z

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@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
---
arch/x86/kvm/emulate.c | 27 ++++++++++++++++-----------
1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index e223043ef5b2..ad15ce055a1d 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -178,6 +178,7 @@
#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 IsProtected ((u64)1 << 57) /* Instruction is protected by CET. */

#define DstXacc (DstAccLo | SrcAccHi | SrcWrite)

@@ -4098,9 +4099,9 @@ 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 | IsProtected, em_call_near_abs),
+ I(SrcMemFAddr | ImplicitOps | IsBranch | IsProtected, em_call_far),
+ I(SrcMem | NearBranch | IsBranch | IsProtected, em_jmp_abs),
I(SrcMemFAddr | ImplicitOps | IsBranch, em_jmp_far),
I(SrcMem | Stack | TwoMemOp, em_push), D(Undefined),
};
@@ -4362,11 +4363,11 @@ 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 | IsProtected, em_ret_far_imm),
+ I(ImplicitOps | IsBranch | IsProtected, em_ret_far),
+ D(ImplicitOps | IsBranch), DI(SrcImmByte | IsBranch | IsProtected, intn),
D(ImplicitOps | No64 | IsBranch),
- II(ImplicitOps | IsBranch, em_iret, iret),
+ II(ImplicitOps | IsBranch | IsProtected, em_iret, iret),
/* 0xD0 - 0xD7 */
G(Src2One | ByteOp, group2), G(Src2One, group2),
G(Src2CL | ByteOp, group2), G(Src2CL, group2),
@@ -4382,7 +4383,7 @@ 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 | IsProtected, em_call),
D(SrcImm | ImplicitOps | NearBranch | IsBranch),
I(SrcImmFAddr | No64 | IsBranch, em_jmp_far),
D(SrcImmByte | ImplicitOps | NearBranch | IsBranch),
@@ -4401,7 +4402,7 @@ 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 | IsProtected, 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,
@@ -4432,8 +4433,8 @@ 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 | IsProtected, em_sysenter),
+ I(ImplicitOps | Priv | EmulateOnUD | IsBranch | IsProtected, em_sysexit),
N, N,
N, N, N, N, N, N, N, N,
/* 0x40 - 0x4F */
@@ -4971,6 +4972,10 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
if (ctxt->d == 0)
return EMULATION_FAILED;

+ if ((opcode.flags & IsProtected) &&
+ (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_CET))
+ return EMULATION_FAILED;
+
ctxt->execute = opcode.u.execute;

if (unlikely(emulation_type & EMULTYPE_TRAP_UD) &&
--
2.39.3