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

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Date:   Mon, 17 Jan 2022 12:18:58 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu.linux@...il.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: [PATCH] KVM: x86: Fix the #GP(0) and #UD conditions for XSETBV
 emulation

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.

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;
>>
>>
>