Re: [PATCH v16 18/51] KVM: x86: Don't emulate instructions affected by CET features
From: Xiaoyao Li
Date: Tue Sep 23 2025 - 10:13:01 EST
On 9/23/2025 4:04 AM, Sean Christopherson wrote:
From: Sean Christopherson<seanjc@xxxxxxxxxx>
Date: Fri, 19 Sep 2025 15:32:25 -0700
Subject: [PATCH] KVM: x86: Don't emulate instructions affected by CET features
Don't emulate branch instructions, e.g. CALL/RET/JMP etc., that are
affected by Shadow Stacks and/or Indirect Branch Tracking when said
features are enabled in the guest, as fully emulating CET would require
significant complexity for no practical benefit (KVM shouldn't need to
emulate branch instructions on modern hosts). Simply doing nothing isn't
an option as that would allow a malicious entity to subvert CET
protections via the emulator.
To detect instructions that are subject to IBT or affect IBT state, use
the existing IsBranch flag along with the source operand type to detect
indirect branches, and the existing NearBranch flag to detect far JMPs
and CALLs, all of which are effectively indirect. Explicitly check for
emulation of IRET, FAR RET (IMM), and SYSEXIT (the ret-like far branches)
instead of adding another flag, e.g. IsRet, as it's unlikely the emulator
will ever need to check for return-like instructions outside of this one
specific flow. Use an allow-list instead of a deny-list because (a) it's
a shorter list and (b) so that a missed entry gets a false positive, not a
false negative (i.e. reject emulation instead of clobbering CET state).
For Shadow Stacks, explicitly track instructions that directly affect the
current SSP, as KVM's emulator doesn't have existing flags that can be
used to precisely detect such instructions. Alternatively, the em_xxx()
helpers could directly check for ShadowStack interactions, but using a
dedicated flag is arguably easier to audit, and allows for handling both
IBT and SHSTK in one fell swoop.
Note! On far transfers, do NOT consult the current privilege level and
instead treat SHSTK/IBT as being enabled if they're enabled for User*or*
Supervisor mode. On inter-privilege level far transfers, SHSTK and IBT
can be in play for the target privilege level, i.e. checking the current
privilege could get a false negative, and KVM doesn't know the target
privilege level until emulation gets under way.
Note #2, FAR JMP from 64-bit mode to compatibility mode interacts with
the current SSP, but only to ensure SSP[63:32] == 0. Don't tag FAR JMP
as SHSTK, which would be rather confusing and would result in FAR JMP
being rejected unnecessarily the vast majority of the time (ignoring that
it's unlikely to ever be emulated). A future commit will add the #GP(0)
check for the specific FAR JMP scenario.
Note #3, task switches also modify SSP and so need to be rejected. That
too will be addressed in a future commit.
Suggested-by: Chao Gao<chao.gao@xxxxxxxxx>
Originally-by: Yang Weijiang<weijiang.yang@xxxxxxxxx>
Cc: Mathias Krause<minipli@xxxxxxxxxxxxxx>
Cc: John Allen<john.allen@xxxxxxx>
Cc: Rick Edgecombe<rick.p.edgecombe@xxxxxxxxx>
Reviewed-by: Chao Gao<chao.gao@xxxxxxxxx>
Reviewed-by: Binbin Wu<binbin.wu@xxxxxxxxxxxxxxx>
Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
Two nits besides,
Link:https://lore.kernel.org/r/20250919223258.1604852-19-seanjc@xxxxxxxxxx
Signed-off-by: Sean Christopherson<seanjc@xxxxxxxxxx>
---
arch/x86/kvm/emulate.c | 117 ++++++++++++++++++++++++++++++++++++-----
1 file changed, 103 insertions(+), 14 deletions(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 23929151a5b8..a7683dc18405 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 ShadowStack ((u64)1 << 57) /* Instruction affects Shadow Stacks. */
#define DstXacc (DstAccLo | SrcAccHi | SrcWrite)
@@ -4068,9 +4069,9 @@ 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 | ShadowStack, em_call_near_abs),
+ I(SrcMemFAddr | ImplicitOps | IsBranch | ShadowStack, em_call_far),
+ I(SrcMem | NearBranch | IsBranch, em_jmp_abs),
The change of this line is unexpected, since it only changes the indentation of 'em_jmp_abs'
static unsigned imm_size(struct x86_emulate_ctxt *ctxt)
{
unsigned size;
@@ -4943,6 +4998,40 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
ctxt->execute = opcode.u.execute;
+ /*
+ * Reject emulation if KVM might need to emulate shadow stack updates
+ * and/or indirect branch tracking enforcement, which the emulator
+ * doesn't support.
+ */
+ if ((is_ibt_instruction(ctxt) || is_shstk_instruction(ctxt)) &&
+ ctxt->ops->get_cr(ctxt, 4) & X86_CR4_CET) {
+ u64 u_cet = 0, s_cet = 0;
+
+ /*
+ * Check both User and Supervisor on far transfers as inter-
+ * privilege level transfers are impacted by CET at the target
+ * privilege level, and that is not known at this time. The
+ * the expectation is that the guest will not require emulation
Dobule 'the'
+ * of any CET-affected instructions at any privilege level.
+ */
+ if (!(ctxt->d & NearBranch))
+ u_cet = s_cet = CET_SHSTK_EN | CET_ENDBR_EN;
+ else if (ctxt->ops->cpl(ctxt) == 3)
+ u_cet = CET_SHSTK_EN | CET_ENDBR_EN;
+ else
+ s_cet = CET_SHSTK_EN | CET_ENDBR_EN;
+
+ if ((u_cet && ctxt->ops->get_msr(ctxt, MSR_IA32_U_CET, &u_cet)) ||
+ (s_cet && ctxt->ops->get_msr(ctxt, MSR_IA32_S_CET, &s_cet)))
+ return EMULATION_FAILED;
+
+ if ((u_cet | s_cet) & CET_SHSTK_EN && is_shstk_instruction(ctxt))
+ return EMULATION_FAILED;
+
+ if ((u_cet | s_cet) & CET_ENDBR_EN && is_ibt_instruction(ctxt))
+ return EMULATION_FAILED;
+ }
+
if (unlikely(emulation_type & EMULTYPE_TRAP_UD) &&
likely(!(ctxt->d & EmulateOnUD)))
return EMULATION_FAILED;
base-commit: 88539a6a25bc7a7ed96952775152e0c3331fdcaf