linux-kernel - Re: [RFC PATCH v2 19/20] x86: Access the setup data through debugfs un-encrypted

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Date:   Thu, 15 Sep 2016 12:08:04 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>
CC:     <linux-arch@...r.kernel.org>, <linux-efi@...r.kernel.org>,
        <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <kasan-dev@...glegroups.com>, <linux-mm@...ck.org>,
        <iommu@...ts.linux-foundation.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Joerg Roedel <joro@...tes.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [RFC PATCH v2 19/20] x86: Access the setup data through debugfs
 un-encrypted

On 09/14/2016 09:51 AM, Borislav Petkov wrote:
> On Wed, Sep 14, 2016 at 09:29:41AM -0500, Tom Lendacky wrote:
>> This is still required because just using the __va() would still cause
>> the mapping created to have the encryption bit set. The ioremap call
>> will result in the mapping not having the encryption bit set.
> 
> I meant this: https://lkml.kernel.org/r/20160902181447.GA25328@nazgul.tnic
> 
> Wouldn't simply clearing the SME mask work?
> 
> #define __va(x)			((void *)(((unsigned long)(x)+PAGE_OFFSET) & ~sme_me_mask))
> 
> Or are you saying, one needs the whole noodling through ioremap_cache()
> because the data is already encrypted and accessing it with sme_me_mask
> cleared would simply give you the encrypted garbage?

The problem is that this physical address does not contain the
encryption bit, and even if it did, it wouldn't matter.  The __va()
define creates a virtual address that will be mapped as encrypted given
the current approach (which is how I found this).  It's only ioremap()
that would create a mapping without the encryption attribute and since
this is unencrypted data it needs to be access accordingly.

Thanks,
Tom

>