linux-kernel - Re: [PATCH v2 2/3] KVM: x86: Move pkru save/restore to x86.c

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Message-ID: <90657d4b-cb2b-0678-fd9c-a281bb85fadf@redhat.com>
Date:   Sat, 9 May 2020 14:59:17 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>, Babu Moger <babu.moger@....com>
Cc:     Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        mchehab+samsung@...nel.org, changbin.du@...el.com,
        Nadav Amit <namit@...are.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        yang.shi@...ux.alibaba.com, asteinhauser@...gle.com,
        anshuman.khandual@....com, Jan Kiszka <jan.kiszka@...mens.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        steven.price@....com, rppt@...ux.vnet.ibm.com, peterx@...hat.com,
        Dan Williams <dan.j.williams@...el.com>, arjunroy@...gle.com,
        logang@...tatee.com, Thomas Hellstrom <thellstrom@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>, justin.he@....com,
        robin.murphy@....com, ira.weiny@...el.com,
        Kees Cook <keescook@...omium.org>,
        Juergen Gross <jgross@...e.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        pawan.kumar.gupta@...ux.intel.com,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        vineela.tummalapalli@...el.com, yamada.masahiro@...ionext.com,
        sam@...nborg.org, acme@...hat.com, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] KVM: x86: Move pkru save/restore to x86.c

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)).

You're right.  The bug was preexistent, but we should fix it in 5.7 and
stable as well.

>>  }
>>  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.

Paolo