On 1/9/2024 11:10 PM, Sean Christopherson wrote:
On Mon, Jan 08, 2024, Weijiang Yang wrote:
On 1/6/2024 12:21 AM, Sean Christopherson wrote:
On Fri, Jan 05, 2024, Weijiang Yang wrote:
On 1/5/2024 8:54 AM, Sean Christopherson wrote:
On Fri, Jan 05, 2024, Rick P Edgecombe wrote:
For CALL/RET (and presumably any branch instructions with IBT?) other
instructions that are directly affected by CET, the simplest thing would
probably be to disable those in KVM's emulator if shadow stacks and/or IBT
are enabled, and let KVM's failure paths take it from there.
Right, that is what I was wondering might be the normal solution for
situations like this.
If KVM can't emulate something, it either retries the instruction (with some
decent logic to guard against infinite retries) or punts to userspace.
What kind of error is proper if KVM has to punt to userspace?
KVM_INTERNAL_ERROR_EMULATION. See prepare_emulation_failure_exit().
Or just inject #UD into guest on detecting this case?
No, do not inject #UD or do anything else that deviates from architecturally
defined behavior.
Thanks!
But based on current KVM implementation and patch 24, seems that if CET is exposed
to guest, the emulation code or shadow paging mode couldn't be activated at the same time:
No, requiring unrestricted guest only disables the paths where KVM *delibeately*
emulates the entire guest code stream. In no way, shape, or form does it prevent
KVM from attempting to emulate arbitrary instructions.
Yes, also need to prevent sporadic emulation, how about adding below patch in emulator?
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index e223043ef5b2..e817d8560ceb 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 +4972,12 @@ 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)) {
+ WARN_ONCE(1, "CET is active, emulation aborted.\n");
+ return EMULATION_FAILED;
+ }
+
ctxt->execute = opcode.u.execute;
if (unlikely(emulation_type & EMULTYPE_TRAP_UD) &&
