linux-kernel - Re: [PATCH v2 1/2] of: overlay: Fix memory leak in of_overlay

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Message-ID: <87ecaacc-1969-c4ef-9c1d-f4279144e93b@gmail.com>
Date:   Tue, 5 Dec 2017 08:45:52 -0500
From:   Frank Rowand <frowand.list@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Rob Herring <robh+dt@...nel.org>,
        Colin King <colin.king@...onical.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] of: overlay: Fix memory leak in of_overlay_apply()
 error path

On 12/05/17 03:01, Geert Uytterhoeven wrote:
> Hi Frank,
> 
> On Tue, Dec 5, 2017 at 3:07 AM, Frank Rowand <frowand.list@...il.com> wrote:
>> On 12/04/17 10:47, Geert Uytterhoeven wrote:
>>> If of_resolve_phandles() fails, free_overlay_changeset() is called in
>>> the error path.  However, that function returns early if the list hasn't
>>> been initialized yet, before freeing the object.
>>>
>>> Explicitly calling kfree() instead would solve that issue. However, that
>>> complicates matter, by having to consider which of two different methods
>>> to use to dispose of the same object.
>>>
>>> Hence make free_overlay_changeset() consider initialization state of the
>>> different parts of the object, making it always safe to call (once!) to
>>> dispose of a (partially) initialized overlay_changeset:
>>>   - Only destroy the changeset if the list was initialized,
>>>   - Ignore uninitialized IDs (zero).
>>>
>>> Reported-by: Colin King <colin.king@...onical.com>
>>> Fixes: f948d6d8b792bb90 ("of: overlay: avoid race condition between applying multiple overlays")
>>> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>>> ---
>>>  drivers/of/overlay.c | 7 +++----
>>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
>>> index 3b7a3980ff50d6bf..312cd658bec0083b 100644
>>> --- a/drivers/of/overlay.c
>>> +++ b/drivers/of/overlay.c
>>> @@ -630,11 +630,10 @@ static void free_overlay_changeset(struct overlay_changeset *ovcs)
>>>  {
>>>       int i;
>>>
>>> -     if (!ovcs->cset.entries.next)
>>> -             return;
>>> -     of_changeset_destroy(&ovcs->cset);
>>> +     if (ovcs->cset.entries.next)
>>> +             of_changeset_destroy(&ovcs->cset);
>>>
>>
>> OK
>>
>>> -     if (ovcs->id)
>>> +     if (ovcs->id > 0)
>>
>> Instead of this change, could you please make a change in init_overlay_changeset()?
>>
>> Current init_overlay_changeset():
>>
>>         ovcs->id = idr_alloc(&ovcs_idr, ovcs, 1, 0, GFP_KERNEL);
>>         if (ovcs->id <= 0)
>>                 return ovcs->id;
>>
>> My proposed version:
>>
>>         ret = idr_alloc(&ovcs_idr, ovcs, 1, 0, GFP_KERNEL);
>>         if (ret <= 0)
>>                 return ret;
>>         ovcs->id = ret;
> 
> Sure.
> 
>>>               idr_remove(&ovcs_idr, ovcs->id);
>>>
>>>       for (i = 0; i < ovcs->count; i++) {
>>>
>>
>> Also, the previous version of the patch, and the discussion around the resulting
>> bug make me think that I should not have moved 'kfree(ovcs)' into
>> free_overlay_changeset(), because that kfree is then not very visible in the
>> error path of of_overlay_apply().  Could you remove 'kfree(ovcs)' from
>> free_overlay_changeset(), and instead call it immediately after each call
>> to free_overlay_changeset()?
> 
> Actually I like that free_overlay_changeset() takes care of the deallocation,
> especially in light of the kojectification op top from bbb-overlays, which
> means you cannot just call kfree(ovcs) anymore (I know this won't go upstream
> anytime soon, but I need overlay configfs for my development and testing).

OK, knowing that kobjectification is being considered I am willing to leave the
kfree(ovcs) where it is for now.


> Perhaps the allocation of ovcs should be moved into free_overlay_changeset(),

                                          I think this ^^^^^^^^^^^^^^^^^^^^^^^
is a typo, and you meant init_overlay_changeset().

> and the latter being renamed to alloc_overlay_changeset()?
> That way allocation and freeing become symmetrical.
> It would move the allocation under the mutexes, though.

I considered moving the kzalloc() into init_overlay_changeset() when I
created it, but decided not to because the type of the first argument of
init_overlay_changeset() would change from
   struct overlay_changeset *
to
   struct overlay_changeset **,
and usage of ovcs would become _slightly_ more ugly and complex in
init_overlay_changeset().


> 
> What do you think?
> 
> Thanks!
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
>