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Date: Thu, 11 Aug 2016 15:16:50 -0700
From: Kees Cook <>
To: Arnd Bergmann <>
Cc: Russell King - ARM Linux <>, 
	"" <>, 
	linux-arch <>, Ard Biesheuvel <>, 
	"" <>, LKML <>, 
	"" <>, 
	Andrew Morton <>, Mathias Krause <>
Subject: Re: [PATCH 2/2] arm: apply more __ro_after_init

On Thu, Aug 11, 2016 at 8:54 AM, Arnd Bergmann <> wrote:
> 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?

It wasn't no. I will add it. :)


Kees Cook
Nexus Security

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