linux-kernel - Re: [PATCH v2 3/3] KVM: x86/emulator: Move the unhandled outer privilege level logic of far return into __load_segment

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Message-ID: <4896c092-a5cb-7ed0-a5d0-32b3abc352c4@redhat.com>
Date:   Fri, 25 Feb 2022 16:07:39 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Hou Wenlong <houwenlong.hwl@...group.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] KVM: x86/emulator: Move the unhandled outer
 privilege level logic of far return into __load_segment_descriptor()

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@...gle.com>
> Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.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