On Thu, Aug 11, 2016 at 8:54 AM, Arnd Bergmann [off-list ref] wrote:

On Thursday, August 11, 2016 12:06:45 AM CEST Russell King - ARM Linux wrote:

quoted

On Wed, Aug 10, 2016 at 09:41:23PM +0200, Arnd Bergmann wrote:

quoted

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

-- 
Kees Cook
Nexus Security