Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <4896c092-a5cb-7ed0-a5d0-32b3abc352c4@redhat.com>
Date:   Fri, 25 Feb 2022 16:07:39 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.5.0
Subject: Re: [PATCH v2 3/3] KVM: x86/emulator: Move the unhandled outer
 privilege level logic of far return into __load_segment_descriptor()
Content-Language: en-US
To:     Hou Wenlong <houwenlong.hwl@antgroup.com>, kvm@vger.kernel.org
Cc:     Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org
References: <cover.1644292363.git.houwenlong.hwl@antgroup.com>
 <5b7188e6388ac9f4567d14eab32db9adf3e00119.1644292363.git.houwenlong.hwl@antgroup.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <5b7188e6388ac9f4567d14eab32db9adf3e00119.1644292363.git.houwenlong.hwl@antgroup.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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