From mboxrd@z Thu Jan 1 00:00:00 1970 From: arnd@arndb.de (Arnd Bergmann) Date: Wed, 10 Aug 2016 21:41:23 +0200 Subject: [PATCH 2/2] arm: apply more __ro_after_init In-Reply-To: References: <1464979224-2085-1-git-send-email-keescook@chromium.org> <20160810094339.GK1041@n2100.armlinux.org.uk> Message-ID: <2760702.46Rp2Juk5b@wuerfel> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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 > 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