linux-kernel - Re: [DROP][PATCH] KVM: x86: Fix the #GP(0) and #UD conditions for XSETBV emulation

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Message-ID: <92b16faf-c9a7-4be3-43f7-3450259346e9@gmail.com>
Date:   Thu, 20 Jan 2022 15:48:19 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [DROP][PATCH] KVM: x86: Fix the #GP(0) and #UD conditions for
 XSETBV emulation

On 17/1/2022 7:18 pm, Paolo Bonzini wrote:
> On 1/17/22 10:44, Like Xu wrote:
>>> Also, the "Fixes" tag is not really correct as the behavior was the same 
>>> before.  Rather, it fixes commit 02d4160fbd76 ("x86: KVM: add xsetbv to the 
>>
>> It seems the original code comes from 81dd35d42c9a ("KVM: SVM: Add xsetbv 
>> intercept").
>> 2acf923e38 ("KVM: VMX: Enable XSAVE/XRSTOR for guest") and 92f9895c146d.
>>
>>> emulator", 2019-08-22).  Checking OSXSAVE is a bug in the emulator path, even 
>>> though it's not needed in the XSETBV vmexit case.
>>
>> The kvm_emulate_xsetbv() has been removed from the emulator path.
>> I'm not really sure why it's not needed in the XSETBV vmexit case. More details ?
> 
> Nevermind, I confused AMD (where #UD is generated before checking for 
> exceptions) with Intel where it's unconditional.
> 
> So the bug should be there since 2acf923e38 by executing XSETBV with 
> CR4.XSAVE=0.  If so, please include a testcase.

In the testcase "executing XSETBV with CR4.XSAVE=0",

- on the VMX, #UD delivery does not require vm-exit;
- on the SVM, #UD is trapped but goes to the ud_interception() path;

There is no room to run this pointless patch.
Thank you and sorry for the noise.

> 
> Paolo
> 
>>>
>>> Thanks,
>>>
>>> Paolo
>>>
>>>> +    if ((is_protmode(vcpu) && static_call(kvm_x86_get_cpl)(vcpu) != 0) ||
>>>>           __kvm_set_xcr(vcpu, kvm_rcx_read(vcpu), kvm_read_edx_eax(vcpu))) {
>>>>           kvm_inject_gp(vcpu, 0);
>>>>           return 1;
>>>
>>>
>>
> 
>