linux-kernel - Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

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Date:   Tue, 30 Nov 2021 20:40:05 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Brijesh Singh <brijesh.singh@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Peter Gonda <pgonda@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support

On 11/29/21 17:13, Brijesh Singh wrote:
>>
>> That could work for the kmap() context.
>> What to do for the userspace context (host userspace)?
>> - shared->private transition - page has to be unmapped from all userspace,
>> elevated refcount (gup() in progress) can block this unmap until it goes
>> away - could be doable
> 
> An unmap of the page from all the userspace process during the page state
> transition will be great. If we can somehow store the state information in
> the 'struct page' then it can be later used to make better decision. I am
> not sure that relying on the elevated refcount is the correct approach. e.g
> in the case of encrypted guests, the HV may pin the page to prevent it from
> migration.
> 
> Thoughts on how you want to approach unmaping the page from userspace page
> table?

After giving it more thought and rereading the threads here it seems I
thought it would be easier than it really is, and it would have to be
something at least like Kirill's hwpoison based approach.

>> - still, what to do if host userspace then tries to access the unmapped
>> page? SIGSEGV instead of SIGBUS and it can recover?
>>
> 
> Yes, SIGSEGV makes sense to me.

OTOH the newer fd-based proposal also IIUC takes care of this part better -
the host userspace controls the guest's shared->private conversion requests
so it can't be tricked to access a page that's changed under it.

>>
>>
>>> Thoughts ?
>>>
>>>>
>>>> This should turn an RMP fault in the kernel which is not covered in the
>>>> uaccess exception table into a fatal error.
>>>>
>>>> Regards,
>>>>
>>