linux-kernel - Re: [kernel-hardening] Re: [PATCH 1/2] x86: introduce post-init read-only memory

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Date:	Wed, 25 Nov 2015 07:03:37 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Andy Lutomirski <luto@...capital.net>,
	linux-arch <linux-arch@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [kernel-hardening] Re: [PATCH 1/2] x86: introduce post-init
 read-only memory

On Tue, Nov 24, 2015 at 4:54 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
> On Tue, 2015-11-24 at 16:44 -0800, Kees Cook wrote:
>> On Tue, Nov 24, 2015 at 4:34 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> > On Nov 24, 2015 1:38 PM, "Kees Cook" <keescook@...omium.org> wrote:
>> > >
>> > > One of the easiest ways to protect the kernel from attack is to reduce
>> > > the internal attack surface exposed when a "write" flaw is available. By
>> > > making as much of the kernel read-only as possible, we reduce the
>> > > attack surface.
>> > >
>> > > Many things are written to only during __init, and never changed
>> > > again. These cannot be made "const" since the compiler will do the wrong
>> > > thing (we do actually need to write to them). Instead, move these items
>> > > into a memory region that will be made read-only during mark_rodata_ro()
>> > > which happens after all kernel __init code has finished.
>> > >
>> > > This introduces __read_only as a way to mark such memory, and adds some
>> > > documentation about the existing __read_mostly marking.
>> >
>> > Obligatory bikeshed:  __ro_after_init, please.  It's barely longer,
>> > and it directly explains what's going on.  __read_only makes me think
>> > that it's really read-only and could, for example, actually be in ROM.
>>
>> I'm fine with that. Anyone else want to chime in before I send a v2?
>
> I'm not clear on why this is x86 only?

I was initially looking at how __read_mostly got implemented, and it
seemed like section names were done on a per-arch basis. But it
doesn't seem like that needs to be true.

> It looks like it would work on any arch, or is there some toolchain
> requirement?

Given that the other sections are in the common linux.lds.h file, it
seems unlikely to me. I'll try it in an arch-agnostic way and see what
happens. :)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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