linux-kernel - Re: [PATCH v2] x86/mm: warn on W+x mappings

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Date:	Wed, 14 Oct 2015 08:30:48 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Matt Fleming <matt@...eblueprint.co.uk>,
	Borislav Petkov <bp@...en8.de>,
	Stephen Smalley <sds@...ho.nsa.gov>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH v2] x86/mm: warn on W+x mappings

On Wed, Oct 14, 2015 at 8:18 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Matt Fleming <matt@...eblueprint.co.uk> wrote:
>
>> On Mon, 12 Oct, at 04:17:54PM, Ingo Molnar wrote:
>> >
>> > * Matt Fleming <matt@...eblueprint.co.uk> wrote:
>> >
>> > > On Mon, 12 Oct, at 02:49:36PM, Ingo Molnar wrote:
>> > > >
>> > > >
>> > > > So why not unmap them after bootup? Is there any reason to call into EFI code
>> > > > while the system is up and running?
>> > >
>> > > That's where the runtime services code lives. So if you want things like EFI
>> > > variables (used by the distro installer, among other things) you need to map the
>> > > runtime regions.
>> >
>> > So EFI variables could be queried during bootup and saved on the Linux side.
>>
>> Right, we could do that, but then we wouldn't be able to support
>> creation/updating variables at runtime, such as when you install a
>> distribution for the first time, or want to boot a new kernel filename
>> directly from the firmware without a boot loader (and need to modify
>> the BootXXXX variables).
>
> Do we know the precise position and address range of these variables?
>
> We could map them writable (but not executable), and the rest executable (but not
> writable).
>
> That raises the question whether the same physical page ever mixes variables and
> actual code - but the hope would be that it's suffiently page granular for this to
> work.

Can we just unmap these things until someone tries to do an EFI call,
and then unmap them again after the call returns?  We already switch
pgds for EFI IIRC.

--Andy
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