linux-kernel - Re: [PATCH 2/2] arm: apply more __ro_after

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <2760702.46Rp2Juk5b@wuerfel>
Date:	Wed, 10 Aug 2016 21:41:23 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Kees Cook <keescook@...omium.org>,
	Russell King - ARM Linux <linux@...linux.org.uk>,
	linux-arch <linux-arch@...r.kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathias Krause <minipli@...glemail.com>
Subject: Re: [PATCH 2/2] arm: apply more __ro_after_init

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@...linux.org.uk> 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.

	Arnd