Date:   Mon, 31 May 2021 11:46:13 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Dongliang Mu <mudongliangabcd@...il.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 Mon, May 31, 2021 at 04:08:04PM +0800, Dongliang Mu wrote:
> On Mon, May 31, 2021 at 3:34 PM Dongliang Mu <mudongliangabcd@...il.com> wrote:
> >
> > On Mon, May 31, 2021 at 3:03 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > >
> > > On Mon, May 31, 2021 at 02:20:37PM +0800, Dongliang Mu wrote:
> > > > On Mon, May 31, 2021 at 12:40 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > > > >
> > > > > On Mon, May 31, 2021 at 11:03:36AM +0800, Dongliang Mu wrote:
> > > > > > On Sat, May 29, 2021 at 5:35 AM 慕冬亮 <mudongliangabcd@...il.com> wrote:
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > > On May 28, 2021, at 10:05 PM, Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > > > > > > >
> > > > > > > > On Fri, May 28, 2021 at 09:50:49PM +0800, Dongliang Mu wrote:
> > > > > > > >>
> > > > > > > >> Can you please give some advise on how to fix this WARN issue?
> > > > > > > >
> > > > > > > > But it feels like it spoils the fun if I write the commit...  Anyway:
> > > > > > >
> > > > > > > It’s fine. I am still in the learning process. It’s also good to learn experience by comparing your patch and my patch.
> > > > > > >
> > > > > > > >
> > > > > > > > regards,
> > > > > > > > dan carpenter
> > > > > > > >
> > > > > > > > diff --git a/sound/core/control_led.c b/sound/core/control_led.c
> > > > > > > > index 25f57c14f294..dd357abc1b58 100644
> > > > > > > > --- a/sound/core/control_led.c
> > > > > > > > +++ b/sound/core/control_led.c
> > > > > > > > @@ -740,6 +740,7 @@ static int __init snd_ctl_led_init(void)
> > > > > > > >                       for (; group > 0; group--) {
> > > > > > > >                               led = &snd_ctl_leds[group - 1];
> > > > > > > >                               device_del(&led->dev);
> > > > > > > > +                             device_put(&led->dev);
> > > > > > > >                       }
> > > > > > > >                       device_del(&snd_ctl_led_dev);
> > > > > > > >                       return -ENOMEM;
> > > > > > > > @@ -768,6 +769,7 @@ static void __exit snd_ctl_led_exit(void)
> > > > > > > >       for (group = 0; group < MAX_LED; group++) {
> > > > > > > >               led = &snd_ctl_leds[group];
> > > > > > > >               device_del(&led->dev);
> > > > > > > > +             device_put(&led->dev);
> > > > > > > >       }
> > > > > > > >       device_del(&snd_ctl_led_dev);
> > > > > > > >       snd_ctl_led_clean(NULL);
> > > > > >
> > > > > > Hi Dan,
> > > > > >
> > > > > > I tried this patch, and it still triggers the memleak.
> > > > >
> > > > > Did your patch fix the leak?  Because my patch should have been
> > > > > equivalent except for it fixes an additional leak in the snd_ctl_led_init()
> > > > > error path.
> > > >
> > > > The syzbot link is [1]. I have tested my patch in the syzbot dashboard
> > > > and my local workspace.
> > > >
> > > > I think the reason why your patch did not work should be
> > > > led_card(struct snd_ctl_led_card) is already freed before returning in
> > > > snd_ctl_led_sysfs_remove, rather than led(struct snd_ctl_led). See the
> > > > implementation of snd_ctl_led_sysfs_remove for some details. Please
> > > > correct me if I make any mistakes.
> > > >
> > > > static void snd_ctl_led_sysfs_remove(struct snd_card *card)
> > > > {
> > > >         unsigned int group;
> > > >         struct snd_ctl_led_card *led_card;
> > > >         struct snd_ctl_led *led;
> > > >         char link_name[32];
> > > >
> > > >         for (group = 0; group < MAX_LED; group++) {
> > > >                 led = &snd_ctl_leds[group];
> > > >                 led_card = led->cards[card->number];
> > > >                 if (!led_card)
> > > >                         continue;
> > > >                 snprintf(link_name, sizeof(link_name), "led-%s", led->name);
> > > >                 sysfs_remove_link(&card->ctl_dev.kobj, link_name);
> > > >                 sysfs_remove_link(&led_card->dev.kobj, "card");
> > > >                 device_del(&led_card->dev);
> > > >                 put_device(&led_card->dev);
> > > >                 kfree(led_card);
> > > >                 led->cards[card->number] = NULL;
> > > >         }
> > > > }
> > >
> > > This is frustrating to look at because it's not a diff so it doesn't
> > > show what you changed.  I think you are saying that you added the
> > > put_device(&led_card->dev);.  That's true.  There are some other leaks
> > > as well.  We should just fix them all.  Use device_unregister() because
> > > it's cleaner.
> >
> > Oh, I see your point. Yeah, we should fix these memory leaks all. I
> > agree with device_unregister.
> >
> > >
> > > If both device_initialize() and device_add() succeed then call
> > > device_unregister() to unwind.
> >
> > BTW, have you tested this new patch on two memory leaks?
> >
> 
> Please keep in mind that if we don't have any release method for
> struct snd_ctl_led_card, it will trigger a WARN[1] in the
> device_release function. That's why I have to add one dummy release
> function.
> 
> if (dev->release)
>         dev->release(dev);
> else if (dev->type && dev->type->release)
>         dev->type->release(dev);
> else if (dev->class && dev->class->dev_release)
>         dev->class->dev_release(dev);
> else
>         WARN(1, KERN_ERR "Device '%s' does not have a release()
> function, it is broken and must be fixed. See
> Documentation/core-api/kobject.rst.\n",
> dev_name(dev));
> 

Oh yeah.  You're right.  The "kfree(led_card);" needs to be moved to a
release function or it can lead to a use after free.  For the others,
I think a dummy release function is ok (because it is static data).

It feels like there should be a standard way to say that there is no
need to release any data.  That way it could be verified by static
analysis tools.

regards,
dan carpenter

> [1] https://elixir.bootlin.com/linux/latest/source/drivers/base/core.c#L2110 
> 
> > >
> > > diff --git a/sound/core/control_led.c b/sound/core/control_led.c
> > > index 25f57c14f294..561fe45e4449 100644
> > > --- a/sound/core/control_led.c
> > > +++ b/sound/core/control_led.c
> > > @@ -700,7 +700,7 @@ static void snd_ctl_led_sysfs_remove(struct snd_card *card)
> > >                 snprintf(link_name, sizeof(link_name), "led-%s", led->name);
> > >                 sysfs_remove_link(&card->ctl_dev.kobj, link_name);
> > >                 sysfs_remove_link(&led_card->dev.kobj, "card");
> > > -               device_del(&led_card->dev);
> > > +               device_unregister(&led_card->dev);
> > >                 kfree(led_card);
> > >                 led->cards[card->number] = NULL;
> > >         }
> > > @@ -739,9 +739,9 @@ static int __init snd_ctl_led_init(void)
> > >                         put_device(&led->dev);
> > >                         for (; group > 0; group--) {
> > >                                 led = &snd_ctl_leds[group - 1];
> > > -                               device_del(&led->dev);
> > > +                               device_unregister(&led->dev);
> > >                         }
> > > -                       device_del(&snd_ctl_led_dev);
> > > +                       device_unregister(&snd_ctl_led_dev);
> > >                         return -ENOMEM;
> > >                 }
> > >         }
> > > @@ -767,9 +767,9 @@ static void __exit snd_ctl_led_exit(void)
> > >         }
> > >         for (group = 0; group < MAX_LED; group++) {
> > >                 led = &snd_ctl_leds[group];
> > > -               device_del(&led->dev);
> > > +               device_unregister(&led->dev);
> > >         }
> > > -       device_del(&snd_ctl_led_dev);
> > > +       device_unregister(&snd_ctl_led_dev);
> > >         snd_ctl_led_clean(NULL);
> > >  }
> > >

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