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Date: Thu, 11 Aug 2016 17:54:08 +0200
From: Arnd Bergmann <arnd@...db.de>
To: Russell King - ARM Linux <linux@...linux.org.uk>
Cc: linux-arm-kernel@...ts.infradead.org, Kees Cook <keescook@...omium.org>, linux-arch <linux-arch@...r.kernel.org>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, "x86@...nel.org" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Andrew Morton <akpm@...ux-foundation.org>, Mathias Krause <minipli@...glemail.com>
Subject: Re: [PATCH 2/2] arm: apply more __ro_after_init
On Thursday, August 11, 2016 12:06:45 AM CEST Russell King - ARM Linux wrote:
> On Wed, Aug 10, 2016 at 09:41:23PM +0200, Arnd Bergmann wrote:
> > It might be better to start by making the fixed mapping readonly,
> > as KASLR doesn't protect that one at all, and change the TLS
> > code accordingly.
>
> I think that's impossible, because we gave userspace permission to
> read 0xffff0ff0 directly without using __kuser_get_tls. You're
> talking about potentially breaking userspace.
>
> If you disable kuser helpers, then the page becomes read-only and
> invisible to userspace anyway. So, everything is being done there
> which can be done - if you have kuser helpers enabled, then you
> lose some opportunities for these security improvements.
What I meant was writing to the page through the linear mapping
rather than the virtual mapping at 0xffff0000 so we can leave that
one read-only (I did not consider whether that might cause cache
aliasing problems when reading from the other address).
Your other point is more important though: if one really cares
about optimizing security here, they probably should disable
kuser helpers completely anyway.
Kees, is that something you have on your radar already?
Arnd
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