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

[Andy Shevchenko]

On Tue, Oct 05, 2021 at 08:40:35AM +0800, Kent Gibson wrote:
> 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@xxxxxxxxx> wrote:

...

> > 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()?

Not really. The utter root cause is the former API (device_add_properties()
et al) which Heikki is getting rid of in particular because of this issue,
i.e. when users blindly call it without fully understanding the picture of
the object lifetimes.

> That is out of my court, so I'll leave it to you and Bart to sort out.

I'll see what I can do about.

-- 
With Best Regards,
Andy Shevchenko