[kernel-hardening] Re: [PATCH 2/2] arm: apply more __ro_after_init

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday, August 10, 2016 11:12:53 AM CEST Russell King - ARM Linux wrote:
> On Wed, Aug 10, 2016 at 12:00:53PM +0200, Arnd Bergmann wrote:
> > On Wednesday, August 10, 2016 10:43:39 AM CEST Russell King - ARM Linux wrote:
> > > On Fri, Jun 03, 2016 at 11:40:24AM -0700, Kees Cook wrote:
> > > > @@ -1309,16 +1309,11 @@ void __init arm_mm_memblock_reserve(void)
> > > >   * Any other function or debugging method which may touch any device _will_
> > > >   * crash the kernel.
> > > >   */
> > > > +static char vectors[PAGE_SIZE * 2] __ro_after_init __aligned(PAGE_SIZE);
> > > >  static void __init devicemaps_init(const struct machine_desc *mdesc)
> > > >  {
> > > >       struct map_desc map;
> > > >       unsigned long addr;
> > > > -     void *vectors;
> > > > -
> > > > -     /*
> > > > -      * Allocate the vector page early.
> > > > -      */
> > > > -     vectors = early_alloc(PAGE_SIZE * 2);
> > > 
> > > This one is not appropriate.  We _do_ write to these pages after init
> > > for FIQ handler updates.  See set_fiq_handler().
> > 
> > Is that the only thing that modifies the page? If we think this is a
> > valuable change, we could make it depend on the absence of FIQ
> > support, as very few platforms (rpc, omap1, s3c24xx and possibly
> > imx) seem to even use it.
> 
> There's the TLS emulation too, but that writes via the vectors mapping
> at 0xffff0ff0.

Ok, so that should be safe. Can we change the fiq code to also use the
high mapping and then take the __ro_after_init patch on top?

	Arnd