linux-kernel - Re: [PATCH] ALSA: control led: fix memory leak in snd_ctl_led

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Message-ID: <CAD-N9QVWcEJjoziA6HVoQiUueVaKqAJS5Et60zvCvuUE7e6=gg@mail.gmail.com>
Date:   Fri, 28 May 2021 21:50:49 +0800
From:   Dongliang Mu <mudongliangabcd@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     perex@...ex.cz, tiwai@...e.com, alsa-devel@...a-project.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        syzbot+08a7d8b51ea048a74ffb@...kaller.appspotmail.com
Subject: Re: [PATCH] ALSA: control led: fix memory leak in snd_ctl_led_register

-

On Fri, May 28, 2021 at 9:33 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
>
> On Fri, May 28, 2021 at 09:17:57PM +0800, Dongliang Mu wrote:
> > The snd_ctl_led_sysfs_add and snd_ctl_led_sysfs_remove should contain
> > the refcount operations in pair. However, snd_ctl_led_sysfs_remove fails
> > to decrease the refcount to zero, which causes device_release never to
> > be invoked. This leads to memory leak to some resources, like struct
> > device_private.
> >
> > Fix this by calling put_device at the end of snd_ctl_led_sysfs_remove
> >
> > Reported-by: syzbot+08a7d8b51ea048a74ffb@...kaller.appspotmail.com
> > Fixes: a135dfb5de1 ("ALSA: led control - add sysfs kcontrol LED marking layer")
> > Signed-off-by: Dongliang Mu <mudongliangabcd@...il.com>
> > ---
> >  sound/core/control_led.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/sound/core/control_led.c b/sound/core/control_led.c
> > index 25f57c14f294..fff2688b5019 100644
> > --- a/sound/core/control_led.c
> > +++ b/sound/core/control_led.c
> > @@ -371,6 +371,10 @@ static void snd_ctl_led_disconnect(struct snd_card *card)
> >       snd_ctl_led_refresh();
> >  }
> >
> > +static void snd_ctl_led_release(struct device *dev)
> > +{
> > +}
>
> Whatever you're trying to do, adding a dummy function is never the
> answer.

I see your point. This function I added is not to fix the root cause,
but to fix an issue caused by the release function when the device is
released.

The put_device is to fix the root cause(i.e., decrease the refcount to
zero), however, the result is dev->p and kobject can be freed, but it
will trigger a WARN [1] as it has no release method.

I don't know how to craft a release method for such a device. So this
dummy function is generated following the default_release, also a
dummy function.

Can you please give some advise on how to fix this WARN issue?

[1] https://elixir.bootlin.com/linux/latest/source/drivers/base/core.c#L2110

>
> regards,
> dan carpenter
>