Re: [PATCH v16 19/51] KVM: x86: Don't emulate task switches when IBT or SHSTK is enabled

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

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> Exit to userspace with KVM_INTERNAL_ERROR_EMULATION if the guest triggers
> task switch emulation with Indirect Branch Tracking or Shadow Stacks
> enabled,

The code just does it when shadow stack is enabled.

> as attempting to do the right thing would require non-trivial
> effort and complexity, KVM doesn't support emulating CET generally, and
> it's extremely unlikely that any guest will do task switches while also
> utilizing CET.  Defer taking on the complexity until someone cares enough
> to put in the time and effort to add support.
>
> Per the SDM:
>
>    If shadow stack is enabled, then the SSP of the task is located at the
>    4 bytes at offset 104 in the 32-bit TSS and is used by the processor to
>    establish the SSP when a task switch occurs from a task associated with
>    this TSS. Note that the processor does not write the SSP of the task
>    initiating the task switch to the TSS of that task, and instead the SSP
>    of the previous task is pushed onto the shadow stack of the new task.
>
> Note, per the SDM's pseudocode on TASK SWITCHING, IBT state for the new
> privilege level is updated.  To keep things simple, check both S_CET and
> U_CET (again, anyone that wants more precise checking can have the honor
> of implementing support).
>
> Reported-by: Binbin Wu <binbin.wu@linux.intel.com>
> Closes: https://lore.kernel.org/all/819bd98b-2a60-4107-8e13-41f1e4c706b1@linux.intel.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/x86.c | 35 ++++++++++++++++++++++++++++-------
>   1 file changed, 28 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d2cccc7594d4..0c060e506f9d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12178,6 +12178,25 @@ int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int idt_index,
>   	struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
>   	int ret;
>   
> +	if (kvm_is_cr4_bit_set(vcpu, X86_CR4_CET)) {
> +		u64 u_cet, s_cet;
> +
> +		/*
> +		 * Check both User and Supervisor on task switches as inter-
> +		 * privilege level task switches are impacted by CET at both
> +		 * the current privilege level and the new privilege level, and
> +		 * that information is not known at this time.  The expectation
> +		 * is that the guest won't require emulation of task switches
> +		 * while using IBT or Shadow Stacks.
> +		 */
> +		if (__kvm_emulate_msr_read(vcpu, MSR_IA32_U_CET, &u_cet) ||
> +		    __kvm_emulate_msr_read(vcpu, MSR_IA32_S_CET, &s_cet))
> +			return EMULATION_FAILED;
> +
> +		if ((u_cet | s_cet) & CET_SHSTK_EN)
> +			goto unhandled_task_switch;
> +	}
> +
>   	init_emulate_ctxt(vcpu);
>   
>   	ret = emulator_task_switch(ctxt, tss_selector, idt_index, reason,
> @@ -12187,17 +12206,19 @@ int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int idt_index,
>   	 * Report an error userspace if MMIO is needed, as KVM doesn't support
>   	 * MMIO during a task switch (or any other complex operation).
>   	 */
> -	if (ret || vcpu->mmio_needed) {
> -		vcpu->mmio_needed = false;
> -		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> -		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
> -		vcpu->run->internal.ndata = 0;
> -		return 0;
> -	}
> +	if (ret || vcpu->mmio_needed)
> +		goto unhandled_task_switch;
>   
>   	kvm_rip_write(vcpu, ctxt->eip);
>   	kvm_set_rflags(vcpu, ctxt->eflags);
>   	return 1;
> +
> +unhandled_task_switch:
> +	vcpu->mmio_needed = false;
> +	vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> +	vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
> +	vcpu->run->internal.ndata = 0;
> +	return 0;
>   }
>   EXPORT_SYMBOL_GPL(kvm_task_switch);
>