Re: [PATCH] ALSA: control led: fix memory leak in snd_ctl_led_register
From: Dongliang Mu
Date: Tue Jun 01 2021 - 10:19:54 EST
On Tue, Jun 1, 2021 at 9:46 PM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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():
This is interesting. Let's dig a little deeper.
>
> 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 call chain of kobject_delayed_cleanup is kobject_delayed_cleanup
-> kobject_cleanup. From the comment, kobject_cleanup should only
clean the resources in the kobject, without touching the dev object.
To further confirm, I checked the implementation and found out there
seem no references to the dev object. Would you mind pointing out the
reference to dev object? Moreover, if kobject_cleanup touches the
resources out of kobject, shall we directly change this function other
than its callees?
#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
static void kobject_delayed_cleanup(struct work_struct *work)
{
kobject_cleanup(container_of(to_delayed_work(work),
struct kobject, release));
}
#endif
/*
* kobject_cleanup - free kobject resources.
* @kobj: object to cleanup
*/
static void kobject_cleanup(struct kobject *kobj)
{
struct kobject *parent = kobj->parent;
struct kobj_type *t = get_ktype(kobj);
const char *name = kobj->name;
pr_debug("kobject: '%s' (%p): %s, parent %p\n",
kobject_name(kobj), kobj, __func__, kobj->parent);
if (t && !t->release)
pr_debug("kobject: '%s' (%p): does not have a release() function, it
is broken and must be fixed. See
Documentation/core-api/kobject.rst.\n",
kobject_name(kobj), kobj);
/* remove from sysfs if the caller did not do it */
if (kobj->state_in_sysfs) {
pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n",
kobject_name(kobj), kobj);
__kobject_del(kobj);
} else {
/* avoid dropping the parent reference unnecessarily */
parent = NULL;
}
if (t && t->release) {
pr_debug("kobject: '%s' (%p): calling ktype release\n",
kobject_name(kobj), kobj);
t->release(kobj);
}
/* free name if we allocated it */
if (name) {
pr_debug("kobject: '%s': free name\n", name);
kfree_const(name);
}
kobject_put(parent);
}
>
> 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
>