Re: [PATCH 2/2] arm: apply more __ro_after_init

[Russell King - ARM Linux]

On Thu, Aug 11, 2016 at 05:54:08PM +0200, 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).

Kees original patch was about moving the vector pages into the
read-only area after init, so the linear mapping of them becomes
read-only as well.  So that won't work.  We need at least one
read-write mapping for FIQ, and for context switching for kuser
helpers.

> Your other point is more important though: if one really cares
> about optimizing security here, they probably should disable
> kuser helpers completely anyway.

We could probably predicate moving the vectors page into the RO
section when kuser helpers are enabled.

> Kees, is that something you have on your radar already?

I believe Android already disable kuser helpers as of a few years ago.

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