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

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Date:   Mon, 22 Nov 2021 18:03:40 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     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 16:23, Brijesh Singh wrote:
> Hi Peter,
> 
> On 11/12/21 9:43 AM, Peter Gonda wrote:
>> Hi Brijesh,,
>>
>> One high level discussion I'd like to have on these SNP KVM patches.
>>
>> In these patches (V5) if a host userspace process writes a guest
>> private page a SIGBUS is issued to that process. If the kernel writes
>> a guest private page then the kernel panics due to the unhandled RMP
>> fault page fault. This is an issue because not all writes into guest
>> memory may come from a bug in the host. For instance a malicious or
>> even buggy guest could easily point the host to writing a private page
>> during the emulation of many virtual devices (virtio, NVMe, etc). For
>> example if a well behaved guests behavior is to: start up a driver,
>> select some pages to share with the guest, ask the host to convert
>> them to shared, then use those pages for virtual device DMA, if a
>> buggy guest forget the step to request the pages be converted to
>> shared its easy to see how the host could rightfully write to private
>> memory. I think we can better guarantee host reliability when running
>> SNP guests without changing SNP’s security properties.
>>
>> Here is an alternative to the current approach: On RMP violation (host
>> or userspace) the page fault handler converts the page from private to
>> shared to allow the write to continue. This pulls from s390’s error
>> handling which does exactly this. See ‘arch_make_page_accessible()’.
>> Additionally it adds less complexity to the SNP kernel patches, and
>> requires no new ABI.
>>
>> In the current (V5) KVM implementation if a userspace process
>> generates an RMP violation (writes to guest private memory) the
>> process receives a SIGBUS. At first glance, it would appear that
>> user-space shouldn’t write to private memory. However, guaranteeing
>> this in a generic fashion requires locking the RMP entries (via locks
>> external to the RMP). Otherwise, a user-space process emulating a
>> guest device IO may be vulnerable to having the guest memory
>> (maliciously or by guest bug) converted to private while user-space
>> emulation is happening. This results in a well behaved userspace
>> process receiving a SIGBUS.
>>
>> This proposal allows buggy and malicious guests to run under SNP
>> without jeopardizing the reliability / safety of host processes. This
>> is very important to a cloud service provider (CSP) since it’s common
>> to have host wide daemons that write/read all guests, i.e. a single
>> process could manage the networking for all VMs on the host. Crashing
>> that singleton process kills networking for all VMs on the system.
>>
> Thank you for starting the thread; based on the discussion, I am keeping the
> current implementation as-is and *not* going with the auto conversion from
> private to shared. To summarize what we are doing in the current SNP series:
> 
> - If userspace accesses guest private memory, it gets SIGBUS.

So, is there anything protecting host userspace processes from malicious guests?

> - If kernel accesses[*] guest private memory, it does panic.
> 
> [*] Kernel consults the RMP table for the page ownership before the access.
> If the page is shared, then it uses the locking mechanism to ensure that a
> guest will not be able to change the page ownership while kernel has it mapped.
> 
> thanks
>