linux-kernel - Re: [PATCH] x86/boot: Refuse to build with data relocations

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Date:	Wed, 18 May 2016 10:11:02 -0400
From:	Kees Cook <keescook@...omium.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
	Baoquan He <bhe@...hat.com>, Yinghai Lu <yinghai@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/boot: Refuse to build with data relocations

On Wed, May 18, 2016 at 4:29 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Kees Cook <keescook@...omium.org> wrote:
>
>> > I think there is something way more subtle going on here, and it bothers me
>> > exactly because it is subtle.  It may be that it is OK right now, but there
>> > are alarm bells going on all over my brain on this.  I'm going to stare at
>> > this for a bit and see if I can make sense of it; but if it turns out that
>> > what we have is something really problematic it might be better to apply a big
>> > hammer and avoid future breakage once and for all.
>>
>> Sounds good. I would just like to decouple this from the KASLR improvements.
>> This fragility hasn't changed as a result of that work, but I'd really like to
>> have that series put to bed -- I've spent a lot of time already cleaning up it
>> and other areas of the compressed kernel code. :)
>
> So I disagree on that: while technically kASLR is independent of relocations, your
> series already introduced such a relocation bug and I don't want to further
> increase complexity via kASLR without first increasing robustness.

Well, in my defense, the bug was never actually reachable.

> So could we try something to either detect or avoid such subtle and hard to debug
> relocation bugs in very early boot code?

I've sent this (the readelf patch which detects the bug from the KASLR
series), but hpa wants to do a more comprehensive version. Could we
temporarily use my version of this, since it appears to accomplish at
least a subset of the new goal?

And on a related topic, how would you like me to send Thomas Garnier's
memory base randomization series? Pull request, or as a series like
I've done with the other KASLR improvements?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security