From mboxrd@z Thu Jan  1 00:00:00 1970
From: arnd@arndb.de (Arnd Bergmann)
Date: Thu, 11 Aug 2016 17:54:08 +0200
Subject: [PATCH 2/2] arm: apply more __ro_after_init
In-Reply-To: <20160810230645.GP1041@n2100.armlinux.org.uk>
References: <1464979224-2085-1-git-send-email-keescook@chromium.org>
 <2760702.46Rp2Juk5b@wuerfel> <20160810230645.GP1041@n2100.armlinux.org.uk>
Message-ID: <2188666.QKZtt2vNXZ@wuerfel>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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