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Date: Tue, 23 Apr 2019 12:14:36 -0700
From: Kees Cook <keescook@...omium.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Alexander Potapenko <glider@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Lameter <cl@...ux.com>, Dmitry Vyukov <dvyukov@...gle.com>, Laura Abbott <labbott@...hat.com>, 
	Linux-MM <linux-mm@...ck.org>, 
	linux-security-module <linux-security-module@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 2/3] gfp: mm: introduce __GFP_NOINIT

On Thu, Apr 18, 2019 at 9:52 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 4/18/19 8:42 AM, Alexander Potapenko wrote:
> > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > index be84f5f95c97..f9d1f1236cd0 100644
> > --- a/kernel/kexec_core.c
> > +++ b/kernel/kexec_core.c
> > @@ -302,7 +302,7 @@ static struct page *kimage_alloc_pages(gfp_t gfp_mask, unsigned int order)
> >  {
> >       struct page *pages;
> >
> > -     pages = alloc_pages(gfp_mask & ~__GFP_ZERO, order);
> > +     pages = alloc_pages((gfp_mask & ~__GFP_ZERO) | __GFP_NOINIT, order);
> >       if (pages) {
> >               unsigned int count, i;
>
> While this is probably not super security-sensitive, it's also not
> performance sensitive.

It is, however, a pretty clear case of "and then we immediately zero it".

> These sl*b ones seem like a bad idea.  We already have rules that sl*b
> allocations must be initialized by callers, and we have reasonably
> frequent bugs where the rules are broken.

Hm? No, this is saying that the page allocator can skip the auto-init
because the slab internals will be doing it later.

> Setting __GFP_NOINIT might be reasonable to do, though, for slabs that
> have a constructor.  We have much higher confidence that *those* are
> going to get initialized properly.

That's already handled in patch 1.

-- 
Kees Cook

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