[kernel-hardening] Re: [PATCH 04/13] mm: Track NR_KERNEL_STACK in pages instead of number of stacks

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Nadav Amit <nadav.amit@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Brian Gerst <brgerst@gmail.com>,
	"kernel-hardening@lists.openwall.com"
	<kernel-hardening@lists.openwall.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Vladimir Davydov <vdavydov@virtuozzo.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: [kernel-hardening] Re: [PATCH 04/13] mm: Track NR_KERNEL_STACK in pages instead of number of stacks
Date: Thu, 16 Jun 2016 14:39:23 -0500	[thread overview]
Message-ID: <20160616193923.hyma4vcmr7lvklcx@treble> (raw)
In-Reply-To: <CALCETrXRONH1K1zAWAFN--Lsza+5bkgtmcMrgAY_nvT-e21C3w@mail.gmail.com>

On Thu, Jun 16, 2016 at 10:39:43AM -0700, Andy Lutomirski wrote:
> On Thu, Jun 16, 2016 at 8:33 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > On Wed, Jun 15, 2016 at 05:28:26PM -0700, Andy Lutomirski wrote:
> >> Currently, NR_KERNEL_STACK tracks the number of kernel stacks in a
> >> zone.  This only makes sense if each kernel stack exists entirely in
> >> one zone, and allowing vmapped stacks could break this assumption.
> >>
> >> It turns out that the code for tracking kernel stack allocations in
> >> units of pages is slightly simpler, so just switch to counting
> >> pages.
> >>
> >> Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
> >> Cc: Johannes Weiner <hannes@cmpxchg.org>
> >> Cc: Michal Hocko <mhocko@kernel.org>
> >> Cc: linux-mm@kvack.org
> >> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> >> ---
> >>  fs/proc/meminfo.c | 2 +-
> >>  kernel/fork.c     | 3 ++-
> >>  mm/page_alloc.c   | 3 +--
> >>  3 files changed, 4 insertions(+), 4 deletions(-)
> >
> > You missed another usage of NR_KERNEL_STACK in drivers/base/node.c.
> 
> Thanks.
> 
> The real reason I cc'd you was so you could look at
> rewind_stack_do_exit and the sneaky trick I did in no_context in the
> last patch, though.  :)  Both survive objtool, but I figured I'd check
> with objtool's author as well.  If there was a taint bit I could set
> saying "kernel is hosed -- don't try to apply live patches any more",
> I'd have extra confidence.

I think it all looks fine from an objtool and a live patching
standpoint.  Other than my previous comment about setting the stack
pointer correctly before calling do_exit(), I didn't see anything else
which would mess up the stack of a sleeping task, which is all I really
care about.

-- 
Josh