Re: [PATCH v16 20/51] KVM: x86: Emulate SSP[63:32]!=0 #GP(0) for FAR JMP to 32-bit mode

Binbin Wu
2025-09-22T07:15:24Z

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> Emulate the Shadow Stack restriction that the current SSP must be a 32-bit
> value on a FAR JMP from 64-bit mode to compatibility mode. From the SDM's
> pseudocode for FAR JMP:
>
> IF ShadowStackEnabled(CPL)
> IF (IA32_EFER.LMA and DEST(segment selector).L) = 0
> (* If target is legacy or compatibility mode then the SSP must be in low 4GB *)
> IF (SSP & 0xFFFFFFFF00000000 != 0); THEN
> #GP(0);
> FI;
> FI;
> FI;
>
> Note, only the current CPL needs to be considered, as FAR JMP can't be
> used for inter-privilege level transfers, and KVM rejects emulation of all
> other far branch instructions when Shadow Stacks are enabled.
>
> To give the emulator access to GUEST_SSP, special case handling
> MSR_KVM_INTERNAL_GUEST_SSP in emulator_get_msr() to treat the access as a
> host access (KVM doesn't allow guest accesses to internal "MSRs"). The
> ->get_msr() API is only used for implicit accesses from the emulator, i.e.
> is only used with hardcoded MSR indices, and so any access to
> MSR_KVM_INTERNAL_GUEST_SSP is guaranteed to be from KVM, i.e. not from the
> guest via RDMSR.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
> arch/x86/kvm/emulate.c | 35 +++++++++++++++++++++++++++++++++++
> arch/x86/kvm/x86.c | 9 +++++++++
> 2 files changed, 44 insertions(+)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index dc0249929cbf..5c5fb6a6f7f9 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1605,6 +1605,37 @@ static int write_segment_descriptor(struct x86_emulate_ctxt *ctxt,
> return linear_write_system(ctxt, addr, desc, sizeof(*desc));
> }
>
> +static bool emulator_is_ssp_invalid(struct x86_emulate_ctxt *ctxt, u8 cpl)
> +{
> + const u32 MSR_IA32_X_CET = cpl == 3 ? MSR_IA32_U_CET : MSR_IA32_S_CET;
> + u64 efer = 0, cet = 0, ssp = 0;
> +
> + if (!(ctxt->ops->get_cr(ctxt, 4) & X86_CR4_CET))
> + return false;
> +
> + if (ctxt->ops->get_msr(ctxt, MSR_EFER, &efer))
> + return true;
> +
> + /* SSP is guaranteed to be valid if the vCPU was already in 32-bit mode. */
> + if (!(efer & EFER_LMA))
> + return false;
> +
> + if (ctxt->ops->get_msr(ctxt, MSR_IA32_X_CET, &cet))
> + return true;
> +
> + if (!(cet & CET_SHSTK_EN))
> + return false;
> +
> + if (ctxt->ops->get_msr(ctxt, MSR_KVM_INTERNAL_GUEST_SSP, &ssp))
> + return true;
> +
> + /*
> + * On transfer from 64-bit mode to compatibility mode, SSP[63:32] must
> + * be 0, i.e. SSP must be a 32-bit value outside of 64-bit mode.
> + */
> + return ssp >> 32;
> +}
> +
> static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
> u16 selector, int seg, u8 cpl,
> enum x86_transfer_type transfer,
> @@ -1745,6 +1776,10 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
> if (efer & EFER_LMA)
> goto exception;
> }
> + if (!seg_desc.l && emulator_is_ssp_invalid(ctxt, cpl)) {
> + err_code = 0;
> + goto exception;
> + }
>
> /* CS(RPL) <- CPL */
> selector = (selector & 0xfffc) | cpl;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0c060e506f9d..40596fc5142e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8741,6 +8741,15 @@ static int emulator_set_msr_with_filter(struct x86_emulate_ctxt *ctxt,
> static int emulator_get_msr(struct x86_emulate_ctxt *ctxt,
> u32 msr_index, u64 *pdata)
> {
> + /*
> + * Treat emulator accesses to the current shadow stack pointer as host-
> + * initiated, as they aren't true MSR accesses (SSP is a "just a reg"),
> + * and this API is used only for implicit accesses, i.e. not RDMSR, and
> + * so the index is fully KVM-controlled.
> + */
> + if (unlikely(msr_index == MSR_KVM_INTERNAL_GUEST_SSP))
> + return kvm_msr_read(emul_to_vcpu(ctxt), msr_index, pdata);
> +
> return __kvm_emulate_msr_read(emul_to_vcpu(ctxt), msr_index, pdata);
> }
>