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

[Kees Cook]

On Wed, Aug 10, 2016 at 12:41 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wednesday, August 10, 2016 11:32:07 AM CEST Kees Cook wrote:
>> On Wed, Aug 10, 2016 at 2:43 AM, Russell King - ARM Linux
>> <linux@xxxxxxxxxxxxxxx> 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().
>>
>> Ah, interesting. I guess none of that hardware is being tested on
>> linux-next.
>
> Right. The OMAP1 Amstrad Delta is a somewhat obscure machine, and that
> would be the most likely candidate to run into this.
>
> RiscPC also has FIQ support, but I have not heard of anyone other
> than Russell still using one with a modern kernel, and I doubt he
> tests linux-next on it.
>
> The s3c24xx and imx machines that could use FIQ probably don't
> use it in practice, last time I checked, I didn't see any DTS file
> or platform data definition in the kernel that activated that
> code path.
>
>> I'll drop that chunk and resubmit.
>
> Good enough for now, but it may be worth revisiting this, as the
> vector page might be a good target for an attack if you have a
> way to overwrite a few bytes in the kernel.
>
> Note that there are two mappings for the pages, and as Russell
> mentioned, the TLS emulation writes to the other one that is
> at a fixed virtual address.
>
> 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.

That sounds good (anyone want to work on this?). Does arm64 need a
similar vector page protection?

-Kees

-- 
Kees Cook
Nexus Security