linux-kernel - Re: [PATCH v5 31/32] x86: Add sysfs support for Secure Memory Encryption

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Date:   Thu, 4 May 2017 09:17:48 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...el.com>, Dave Young <dyoung@...hat.com>
CC:     <linux-arch@...r.kernel.org>, <linux-efi@...r.kernel.org>,
        <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <x86@...nel.org>, <kexec@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <kasan-dev@...glegroups.com>,
        <linux-mm@...ck.org>, <iommu@...ts.linux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Rik van Riel <riel@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Joerg Roedel <joro@...tes.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 31/32] x86: Add sysfs support for Secure Memory
 Encryption

On 4/27/2017 10:52 AM, Dave Hansen wrote:
> On 04/27/2017 12:25 AM, Dave Young wrote:
>> On 04/21/17 at 02:55pm, Dave Hansen wrote:
>>> On 04/18/2017 02:22 PM, Tom Lendacky wrote:
>>>> Add sysfs support for SME so that user-space utilities (kdump, etc.) can
>>>> determine if SME is active.
>>>>
>>>> A new directory will be created:
>>>>   /sys/kernel/mm/sme/
>>>>
>>>> And two entries within the new directory:
>>>>   /sys/kernel/mm/sme/active
>>>>   /sys/kernel/mm/sme/encryption_mask
>>>
>>> Why do they care, and what will they be doing with this information?
>>
>> Since kdump will copy old memory but need this to know if the old memory
>> was encrypted or not. With this sysfs file we can know the previous SME
>> status and pass to kdump kernel as like a kernel param.
>>
>> Tom, have you got chance to try if it works or not?
>
> What will the kdump kernel do with it though?  We kexec() into that
> kernel so the SME keys will all be the same, right?  So, will the kdump

Yes, the SME key will be same after a kexec.

> kernel be just setting the encryption bit in the PTE so it can copy the
> old plaintext out?

Yes, the idea would be to set the encryption bit in the PTE when mapping
and copying encrypted pages and not set it for unencrypted pages.

>
> Why do we need both 'active' and 'encryption_mask'?  How could it be
> that the hardware-enumerated 'encryption_mask' changes across a kexec()?
>

We don't need both, I just added the 'encryption mask' entry for
information. It won't change across a kexec, I can remove it.

Thanks,
Tom