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

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Date:   Wed, 24 Nov 2021 20:34:40 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dave Hansen <dave.hansen@...el.com>, Joerg Roedel <jroedel@...e.de>
Cc:     Brijesh Singh <brijesh.singh@....com>,
        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 11/24/21 18:48, Dave Hansen wrote:
> On 11/24/21 8:03 AM, Joerg Roedel wrote:
>> On Mon, Nov 22, 2021 at 02:51:35PM -0800, Dave Hansen wrote:
>>> My preference would be that we never have SEV-SNP code in the kernel
>>> that can panic() the host from guest userspace.  If that means waiting
>>> until there's common guest unmapping infrastructure around, then I think
>>> we should wait.
>> Can you elaborate how to crash host kernel from guest user-space? If I
>> understood correctly it was about crashing host kernel from _host_
>> user-space.
> 
> Sorry, I misspoke there.
> 
> My concern is about crashing the host kernel.  It appears that *host*
> userspace can do that quite easily by inducing the host kernel to access
> some guest private memory via a kernel mapping.

I thought some of the scenarios discussed here also went along "guest
(doesn't matter if userspace or kernel) shares a page with host, invokes
some host kernel operation and in parallel makes the page private again".

>> I think the RMP-fault path in the page-fault handler needs to take the
>> uaccess exception tables into account before actually causing a panic.
>> This should solve most of the problems discussed here.
> 
> That covers things like copy_from_user().  It does not account for
> things where kernel mappings are used, like where a
> get_user_pages()/kmap() is in play.
>