linux-kernel - Re: [PATCH V1 3/6] xen/virtio: Add option to restrict memory access under Xen

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Date:   Tue, 26 Apr 2022 07:16:16 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>, Oleksandr <olekstysh@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Julien Grall <julien@....org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH V1 3/6] xen/virtio: Add option to restrict memory access
 under Xen

On 25.04.22 23:25, Borislav Petkov wrote:
> On Mon, Apr 25, 2022 at 11:38:36PM +0300, Oleksandr wrote:
>> diff --git a/include/linux/cc_platform.h b/include/linux/cc_platform.h
>> index efd8205..d06bc7a 100644
>> --- a/include/linux/cc_platform.h
>> +++ b/include/linux/cc_platform.h
>> @@ -72,6 +72,19 @@ enum cc_attr {
>>           * Examples include TDX guest & SEV.
>>           */
>>          CC_ATTR_GUEST_UNROLL_STRING_IO,
>> +
>> +       /**
>> +        * @CC_ATTR_GUEST_MEM_ACCESS_RESTRICTED: Restricted memory access to
>> +        *                                       Guest memory is active
>> +        *
>> +        * The platform/OS is running as a guest/virtual machine and uses
>> +        * the restricted access to its memory. This attribute is set if
>> either
>> +        * Guest memory encryption or restricted memory access using Xen
>> grant
>> +        * mappings is active.
>> +        *
>> +        * Examples include Xen guest and SEV.
> 
> Wait, whaaat?
> 
> The cc_platform* stuff is for *confidential computing* guests to check
> different platform aspects.
> 
>  From quickly skimming over this, this looks like a misuse to me.

Christoph suggested (rather firmly) this would be the way to go.

> 
> Why can't you query this from the hypervisor just like you do your other
> querying about what is supported, etc? Hypercalls, CPUID, whatever...

This is needed on guest side at a rather hypervisor independent place.

So a capability of some sort seems appropriate.

Another suggestion of mine was to have a callback (or flag) in
struct x86_hyper_runtime for that purpose.


Juergen

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