linux-kernel - Re: [PATCH v5 32/32] x86/mm: Add support to make use of Secure Memory Encryption

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Date:   Tue, 30 May 2017 10:37:03 -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>, <kexec@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <kasan-dev@...glegroups.com>,
        <linux-mm@...ck.org>, <iommu@...ts.linux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dave Young <dyoung@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 32/32] x86/mm: Add support to make use of Secure Memory
 Encryption

On 5/30/2017 9:55 AM, Borislav Petkov wrote:
> On Tue, May 30, 2017 at 09:38:36AM -0500, Tom Lendacky wrote:
>> In this case we're running identity mapped and the "on" constant ends up
>> as kernel address (0xffffffff81...) which results in a segfault.
> 
> Would
> 
> 	static const char *__on_str = "on";
> 
> 	...
> 
> 	if (!strncmp(buffer, __pa_nodebug(__on_str), 2))
> 		...
> 
> work?
> 
> __phys_addr_nodebug() seems to pay attention to phys_base and
> PAGE_OFFSET and so on...

Except that phys_base hasn't been adjusted yet so that doesn't work
either.

> 
> I'd like to avoid that rip-relative address finding in inline asm which
> looks fragile to me.

I can define the command line option and the "on" and "off" values as
character buffers in the function and initialize them on a per character
basis (using a static string causes the same issues as referencing a
string constant), i.e.:

char cmdline_arg[] = {'m', 'e', 'm', '_', 'e', 'n', 'c', 'r', 'y', 'p', 't', '\0'};
char cmdline_off[] = {'o', 'f', 'f', '\0'};
char cmdline_on[] = {'o', 'n', '\0'};

It doesn't look the greatest, but it works and removes the need for the
rip-relative addressing.

Thanks,
Tom

> 
> Thanks.
>