Re: [PATCH v2 1/2] of: overlay: Fix memory leak in of_overlay_apply() error path

[Frank Rowand <frowand.list@gmail.com>]

On 12/05/17 05:49, Geert Uytterhoeven wrote:
> Hi Frank,
> 
> On Tue, Dec 5, 2017 at 9:01 AM, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>> On Tue, Dec 5, 2017 at 3:07 AM, Frank Rowand <frowand.list@gmail.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@canonical.com>
>>>> Fixes: f948d6d8b792bb90 ("of: overlay: avoid race condition between applying multiple overlays")
>>>> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.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.
> 
> Actually we should use a temporary variable id here, just like for cnt
> and fragments, and store into ovcs->id if everything succeeds.

OK.  That would make the flow in init_overlay_changeset() more consistent.
And of course the idr_remove() after err_free_idr: would use that temporary
variable id.


> Else both init_overlay_changeset() and free_overlay_changeset() will
> free the ID if something goes wrong. It seems IDR can handle that, but
> better safe than sorry.> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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
>