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Subject: Re: [PATCH v2 2/3] KVM: x86: Move pkru save/restore to x86.c
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On 5/9/20 7:59 AM, Paolo Bonzini wrote:
> On 09/05/20 00:09, Jim Mattson wrote:
>>> +       if (static_cpu_has(X86_FEATURE_PKU) &&
>>> +           kvm_read_cr4_bits(vcpu, X86_CR4_PKE) &&
>>> +           vcpu->arch.pkru != vcpu->arch.host_pkru)
>>> +               __write_pkru(vcpu->arch.pkru);
>> This doesn't seem quite right to me. Though rdpkru and wrpkru are
>> contingent upon CR4.PKE, the PKRU resource isn't. It can be read with
>> XSAVE and written with XRSTOR. So, if we don't set the guest PKRU
>> value here, the guest can read the host value, which seems dodgy at
>> best.
>>
>> Perhaps the second conjunct should be: (kvm_read_cr4_bits(vcpu,
>> X86_CR4_PKE) || (vcpu->arch.xcr0 & XFEATURE_MASK_PKRU)).

Thanks Jim.
> 
> You're right.  The bug was preexistent, but we should fix it in 5.7 and
> stable as well.
Paolo, Do you want me to send this fix separately? Or I will send v3 just
adding this fix. Thanks

> 
>>>  }
>>>  EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_state);
>>>
>>>  void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
>>>  {
>>> +       /*
>>> +        * eager fpu is enabled if PKEY is supported and CR4 is switched
>>> +        * back on host, so it is safe to read guest PKRU from current
>>> +        * XSAVE.
>>> +        */
>> I don't understand the relevance of this comment to the code below.
>>
> 
> It's probably stale.

Will remove it.
> 
> Paolo
>