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

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Message-ID: <303c4bb7-ad0f-7686-d4e6-b0640823cbc9@suse.cz>
Date:   Tue, 23 Nov 2021 09:55:07 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dave Hansen <dave.hansen@...el.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Peter Gonda <pgonda@...gle.com>
Cc:     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>, Joerg Roedel <jroedel@...e.de>,
        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 11/22/21 23:51, Dave Hansen wrote:
> On 11/22/21 12:33 PM, Brijesh Singh wrote:
>>> How do we, for example, prevent ptrace() from inducing a panic()?
>> 
>> In the current approach, this access will induce a panic(). 
> 
> That needs to get fixed before SEV-SNP is merged, IMNHO.  This behavior
> would effectively mean that any userspace given access to create SNP
> guests would panic the kernel.
> 
>> In general, supporting the ptrace() for the encrypted VM region is
>> going to be difficult.
> 
> By "supporting", do you mean doing something functional?  I don't really
> care if ptrace() to guest private memory returns -EINVAL or whatever.
> The most important thing is not crashing the host.
> 
> Also, as Sean mentioned, this isn't really about ptrace() itself.  It's
> really about ensuring that no kernel or devices accesses to guest
> private memory can induce bad behavior.

Then we need gup to block any changes from shared to guest private? I assume
there will be the usual issues of recognizing temporary elevated refcount vs
long-term gup, etc.

>> The upcoming TDX work to unmap the guest memory region from the
>> current process page table can easily extend for the SNP to cover the
>> current limitations.

By "current process page table" you mean userspace page tables?

> 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.
>