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

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Date:   Tue, 1 Jun 2021 16:46:07 +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 Tue, Jun 01, 2021 at 09:17:04PM +0800, Dongliang Mu wrote:
> On Mon, May 31, 2021 at 7:02 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > > @@ -701,6 +706,7 @@ static void snd_ctl_led_sysfs_remove(struct snd_card *card)
> > >               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;
> > >       }
> >
> > Btw, I have created a Smatch warning for this type of code where we
> > have:
> >
> >         put_device(&foo->dev);
> >         kfree(foo);
> 
> I don't think this should be a bug pattern. put_device will drop the
> final reference of one object with struct device and invoke
> device_release to release some resources.
> 
> The release function should only clean up the internal resources in
> the device object. It should not touch the led_card which contains the
> device object.
> 

It's only a use after free if you turn CONFIG_DEBUG_KOBJECT_RELEASE
debugging on, which you would never do in a production environment.  The
put_device() function calls kobject_release():

lib/kobject.c
   725  static void kobject_release(struct kref *kref)
   726  {
   727          struct kobject *kobj = container_of(kref, struct kobject, kref);
   728  #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
   729          unsigned long delay = HZ + HZ * (get_random_int() & 0x3);
   730          pr_info("kobject: '%s' (%p): %s, parent %p (delayed %ld)\n",
   731                   kobject_name(kobj), kobj, __func__, kobj->parent, delay);
   732          INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup);
                                                  ^^^^^^^^^^^^^^^^^^^^^^^

   733  
   734          schedule_delayed_work(&kobj->release, delay);
   735  #else
   736          kobject_cleanup(kobj);
   737  #endif
   738  }

This release will be done later and it references led_card->dev which is
now freed.

The Smatch check did work pretty decently.  These are all use after free
bugs if you have CONFIG_DEBUG_KOBJECT_RELEASE enabled.  (Line numbers
from Friday's linux-next).  I'm not going to bother fixing them because
they're only an issue for CONFIG_DEBUG_KOBJECT_RELEASE and not for
production but I will email people when more of these bugs are
introduced.

sound/core/control_led.c:688 snd_ctl_led_sysfs_add() warn: freeing device managed memory (UAF): 'led_card'
drivers/usb/gadget/function/f_mass_storage.c:2649 fsg_common_remove_lun() warn: freeing device managed memory (UAF): 'lun'
drivers/usb/gadget/function/f_mass_storage.c:2818 fsg_common_create_lun() warn: freeing device managed memory (UAF): 'lun'
drivers/usb/gadget/function/f_mass_storage.c:2881 fsg_common_release() warn: freeing device managed memory (UAF): 'lun'
drivers/w1/w1.c:810 w1_unref_slave() warn: freeing device managed memory (UAF): 'sl'
drivers/pci/endpoint/pci-epc-core.c:671 pci_epc_destroy() warn: freeing device managed memory (UAF): 'epc'
drivers/pci/endpoint/pci-epc-core.c:742 __pci_epc_create() warn: freeing device managed memory (UAF): 'epc'
drivers/infiniband/ulp/srp/ib_srp.c:3930 srp_add_port() warn: freeing device managed memory (UAF): 'host'
drivers/infiniband/ulp/srp/ib_srp.c:4058 srp_remove_one() warn: freeing device managed memory (UAF): 'host'
drivers/infiniband/ulp/rtrs/rtrs-clt.c:2695 alloc_clt() warn: freeing device managed memory (UAF): 'clt'
drivers/media/pci/solo6x10/solo6x10-core.c:156 free_solo_dev() warn: freeing device managed memory (UAF): 'solo_dev'
drivers/net/ethernet/netronome/nfp/nfpcore/nfp_cppcore.c:203 nfp_cpp_free() warn: freeing device managed memory (UAF): 'cpp'
drivers/net/ethernet/netronome/nfp/nfpcore/nfp_cppcore.c:1258 nfp_cpp_from_operations() warn: freeing device managed memory (UAF): 'cpp'
drivers/net/netdevsim/bus.c:354 nsim_bus_dev_del() warn: freeing device managed memory (UAF): 'nsim_bus_dev'
drivers/thermal/thermal_core.c:1002 __thermal_cooling_device_register() warn: freeing device managed memory (UAF): 'cdev'

regards,
dan carpenter