linux-kernel - Re: [PATCH v6 11/12] libnvdimm/pfn: Fix fsdax-mode namespace info-block zero-fields

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Date:   Wed, 17 Apr 2019 15:09:42 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     stable <stable@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v6 11/12] libnvdimm/pfn: Fix fsdax-mode namespace
 info-block zero-fields

On Wed, Apr 17, 2019 at 3:02 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed, 17 Apr 2019 11:39:52 -0700 Dan Williams <dan.j.williams@...el.com> wrote:
>
> > At namespace creation time there is the potential for the "expected to
> > be zero" fields of a 'pfn' info-block to be filled with indeterminate
> > data. While the kernel buffer is zeroed on allocation it is immediately
> > overwritten by nd_pfn_validate() filling it with the current contents of
> > the on-media info-block location. For fields like, 'flags' and the
> > 'padding' it potentially means that future implementations can not rely
> > on those fields being zero.
> >
> > In preparation to stop using the 'start_pad' and 'end_trunc' fields for
> > section alignment, arrange for fields that are not explicitly
> > initialized to be guaranteed zero. Bump the minor version to indicate it
> > is safe to assume the 'padding' and 'flags' are zero. Otherwise, this
> > corruption is expected to benign since all other critical fields are
> > explicitly initialized.
> >
> > Fixes: 32ab0a3f5170 ("libnvdimm, pmem: 'struct page' for pmem")
> > Cc: <stable@...r.kernel.org>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>
> Buried at the end of a 12 patch series.  Should this be a standalone
> patch, suitable for a prompt merge?

It's not a problem unless a kernel implementation is explicitly
expecting those fields to be zero-initialized. I only marked it for
-stable in case some future kernel backports patch12. Otherwise it's
benign on older kernels that don't have patch12 since all fields are
indeed initialized.