linux-kernel - Re: [PATCH] kvm/x86/mmu: use the correct inherited permissions to get shadow page

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Message-ID: <7a9d3517-7bcc-723a-5ec5-80018d0850d7@redhat.com>
Date:   Mon, 30 Nov 2020 18:53:41 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Avi Kivity <avi@...ranet.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH] kvm/x86/mmu: use the correct inherited permissions to get
 shadow page

On 30/11/20 18:41, Sean Christopherson wrote:
>>
>> pmd1 and pmd2 point to the same pte table, so:
>> ptr1 and ptr3 points to the same page.
>> ptr2 and ptr4 points to the same page.
>>
>>    The guess read-accesses to ptr1 first. So the hypervisor gets the
>> shadow pte page table with role.access=u-- among other things.
>>     (Note the shadowed pmd1's access is uwx)
>>
>>    And then the guest write-accesses to ptr2, and the hypervisor
>> set up shadow page for ptr2.
>>     (Note the hypervisor silencely accepts the role.access=u--
>>      shadow pte page table in FNAME(fetch))
>>
>>    After that, the guess read-accesses to ptr3, the hypervisor
>> reused the same shadow pte page table as above.
>>
>>    At last, the guest writes to ptr4 without vmexit nor pagefault,
>> Which should cause vmexit as the guest expects.
>
> Hmm, yes, KVM would incorrectly handle this scenario.  But, the proposed patch
> would not address the issue as KVM always maps non-leaf shadow pages with full
> access permissions.

Can we have a testcase in kvm-unit-tests?  It's okay of course if it 
only fails with ept=0.

Paolo