lkml.org 
[lkml]   [2016]   [Aug]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH 2/2] arm: apply more __ro_after_init
Date
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

\
 
 \ /
  Last update: 2016-08-11 18:42    [W:0.136 / U:1.684 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site