Re: [PATCH v2 3/3] KVM: x86/emulator: Move the unhandled outer privilege level logic of far return into __load_segment_descriptor()

[Paolo Bonzini <pbonzini@redhat.com>]

On 2/8/22 10:34, Hou Wenlong wrote:
> Outer-privilege level return is not implemented in emulator,
> move the unhandled logic into __load_segment_descriptor to
> make it easier to understand why the checks for RET are
> incomplete.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> ---
>   arch/x86/kvm/emulate.c | 14 ++++++++------
>   1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 37c4213bdcc1..bd91b952eb0a 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1631,9 +1631,14 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
>   		if (!(seg_desc.type & 8))
>   			goto exception;
>   
> -		/* RET can never return to an inner privilege level. */
> -		if (transfer == X86_TRANSFER_RET && rpl < cpl)
> -			goto exception;
> +		if (transfer == X86_TRANSFER_RET) {
> +			/* RET can never return to an inner privilege level. */
> +			if (rpl < cpl)
> +				goto exception;
> +			/* Outer-privilege level return is not implemented */
> +			if (rpl > cpl)
> +				return X86EMUL_UNHANDLEABLE;
> +		}
>   		if (transfer == X86_TRANSFER_RET || transfer == X86_TRANSFER_TASK_SWITCH) {
>   			if (seg_desc.type & 4) {
>   				/* conforming */
> @@ -2228,9 +2233,6 @@ static int em_ret_far(struct x86_emulate_ctxt *ctxt)
>   	rc = emulate_pop(ctxt, &cs, ctxt->op_bytes);
>   	if (rc != X86EMUL_CONTINUE)
>   		return rc;
> -	/* Outer-privilege level return is not implemented */
> -	if (ctxt->mode >= X86EMUL_MODE_PROT16 && (cs & 3) > cpl)
> -		return X86EMUL_UNHANDLEABLE;
>   	rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, cpl,
>   				       X86_TRANSFER_RET,
>   				       &new_desc);

Queued all three, thanks!

Paolo