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

[Binbin Wu <binbin.wu@linux.intel.com>]

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);
>   }
>