Message-ID: <303c4bb7-ad0f-7686-d4e6-b0640823cbc9@suse.cz>
Date:   Tue, 23 Nov 2021 09:55:07 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.3.2
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support
Content-Language: en-US
To:     Dave Hansen <dave.hansen@intel.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Peter Gonda <pgonda@google.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org,
        linux-crypto@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Borislav Petkov <bp@alien8.de>,
        Michael Roth <michael.roth@amd.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com
References: <20210820155918.7518-1-brijesh.singh@amd.com>
 <CAMkAt6o0ySn1=iLYsH0LCnNARrUbfaS0cvtxB__y_d+Q6DUzfA@mail.gmail.com>
 <daf5066b-e89b-d377-ed8a-9338f1a04c0d@amd.com>
 <d673f082-9023-dafb-e42e-eab32a3ddd0c@intel.com>
 <f15597a0-e7e0-0a57-39fd-20715abddc7f@amd.com>
 <5f3b3aab-9ec2-c489-eefd-9136874762ee@intel.com>
 <d83e6668-bec4-8d1f-7f8a-085829146846@amd.com>
 <38282b0c-7eb5-6a91-df19-2f4cfa8549ce@intel.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <38282b0c-7eb5-6a91-df19-2f4cfa8549ce@intel.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.
>