Re: linux 5.15-rc4: refcount underflow when unloading gpio-mockup

[Kent Gibson <warthog618@gmail.com>]

On Mon, Oct 04, 2021 at 10:56:00PM +0300, Andy Shevchenko wrote:
> On Mon, Oct 4, 2021 at 8:51 PM Kent Gibson <warthog618@gmail.com> wrote:
> >
> > On Mon, Oct 04, 2021 at 10:17:54PM +0800, Kent Gibson wrote:
> > > On Mon, Oct 04, 2021 at 04:20:55PM +0300, Heikki Krogerus wrote:
> > > > On Mon, Oct 04, 2021 at 08:47:01PM +0800, Kent Gibson wrote:
> > > > > On Mon, Oct 04, 2021 at 03:30:43PM +0300, Heikki Krogerus wrote:
> > > > > > On Mon, Oct 04, 2021 at 08:19:42PM +0800, Kent Gibson wrote:
> > > > > > > On Mon, Oct 04, 2021 at 11:44:17AM +0200, Greg Kroah-Hartman wrote:
> > > > > > > > On Mon, Oct 04, 2021 at 05:34:16PM +0800, Kent Gibson wrote:
> > > > > > > > > Hi,
> > > > > > > > >
> >
> > [snip]
> >
> > > > > Looking at the offending patch, it effectively replaces a call to
> > > > > device_add_properties() with one to
> > > > > device_create_managed_software_node(), and those two functions appear
> > > > > quite different - at least at first glance.
> > > > > Is that correct?
> > > >
> > > > The only real difference between the two functions is that
> > > > device_create_managed_software_node() marks the software node it
> > > > creates (and it does it exactly the same way as
> > > > device_add_properties()) as "managed" with a specific flag.
> > > >
> > >
> >
> > That managed flag makes all the difference.
> >
> > I've tried to find a fix along the same lines as Laurentiu Tudor's
> > 5aeb05b27f8 software node: balance refcount for managed software nodes
> > but haven't found anything that works.
> >
> > What does work for me is to revert the call to
> > device_create_managed_software_node() to a call to
> > device_add_properties():
> >
> > diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> > index 652531f67135..2f30bdb94fab 100644
> > --- a/drivers/base/platform.c
> > +++ b/drivers/base/platform.c
> > @@ -826,8 +826,7 @@ struct platform_device *platform_device_register_full(
> >                 goto err;
> >
> >         if (pdevinfo->properties) {
> > -               ret = device_create_managed_software_node(&pdev->dev,
> > -                                                         pdevinfo->properties, NULL);
> > +               ret = device_add_properties(&pdev->dev, pdevinfo->properties);
> >                 if (ret)
> >                         goto err;
> >         }
> >
> > That obviously wont work with your latest series that removes
> > device_add_properties(), but that is the simplest, and only, solution
> > that I've found so far.
> 
> Here is debug prints of what happens
> 
> # modprobe gpio-mockup gpio_mockup_ranges=-1,10
> [  212.850993]  (null): device_create_managed_software_node <<< 0
> [  212.858177] platform gpio-mockup.0: software_node_notify 0 <<<
> [  212.865264] platform gpio-mockup.0: software_node_notify 1 <<< 1
> [  212.874159] gpio-mockup gpio-mockup.0: no of_node; not parsing pinctrl DT
> [  212.882962] gpiochip_find_base: found new base at 840
> [  212.889873] gpio gpiochip3: software_node_notify 0 <<<
> [  212.896164] gpio gpiochip3: software_node_notify 1 <<< 1
> [  212.905099] gpio gpiochip3: (gpio-mockup-A): added GPIO chardev (254:3)
> [  212.913313] gpio gpiochip840: software_node_notify 0 <<<
> [  212.920676] gpio gpiochip3: registered GPIOs 840 to 849 on gpio-mockup-A
> [  212.935601] modprobe (185) used greatest stack depth: 12744 bytes left
> 
> As I read it the software node is created for gpio-mockup device and
> then _shared_ with the GPIO device, since it's managed it's gone when
> GPIO device gone followed by UAF when mockup (platform) device itself
> gone. I.o.w. this software node mustn't be managed because it covers
> the lifetime of two different objects. The correct fix is to create
> manually software node and assign it to the pdev and manually free in
> gpio_mockup_unregister_pdevs()/
> 
> Tl;DR: it's a bug in gpio-mockup.
> 

So the bug is in the behaviour of gpio_mockup_register_chip()?
That is out of my court, so I'll leave it to you and Bart to sort out.

Cheers,
Kent.