Re: [PATCH 0/8] counter: Remove struct counter_device::priv

[Lars-Peter Clausen]

On 12/21/21 12:35 PM, Uwe Kleine-König wrote:
> Hello Lars,
>
> On Tue, Dec 21, 2021 at 12:12:12PM +0100, Lars-Peter Clausen wrote:
> > On 12/21/21 11:45 AM, Uwe Kleine-König wrote:
> > > similar to patch
> > > https://lore.kernel.org/r/4bde7cbd9e43a5909208102094444219d3154466.1640072891.git.vilhelm.gray@xxxxxxxxx
> > > the usage of struct counter_device::priv can be replaced by
> > > container_of which improves type safety and code size.
> > >
> > > This series depends on above patch, converts the remaining drivers and
> > > finally drops struct counter_device::priv.
> > Not sure if this is such a good idea. struct counter_device should not be
> > embedded in the drivers state struct in the first place.
> Just to mention it: My patch series didn't change this, this was already
> broken before.
I know, but this series has to be reverted when the framework is fixed.
> > struct counter_device contains a struct device, which is a reference counted
> > object. But by embedding it in the driver state struct the life time of both
> > the struct counter_device and and struct device are bound to the life time
> > of the driver state struct.
> >
> > Which means the struct device memory can get freed before the last reference
> > is dropped, which leads to a use-after-free and undefined behavior.
> Well, the driver struct is allocated using devm_kzalloc for all drivers.

devm_kzalloc() doesn't make a difference. The managed memory is freed 
when the parent device is unbound/removed. There may very well be 
reference to the counter_device at this point.

> So I think it's not *very* urgent to fix. Still you're right, this
> should be addressed.

Yes and no, this can trivially be used for privilege escalation, but 
then again on systems with a counter_device probably everything runs as 
root anyway.

>   
> > The framework should be changed to rather then embedding the struct
> > counter_device in the state struct to just have a pointer to it. With the
> > struct counter_device having its own allocation that will be freed when the
> > last reference to the struct device is dropped.
> My favourite would be to implement a counter_device_alloc /
> counter_device_add approach, similar to what spi_alloc_controller and
> alloc_etherdev do. The downside is that this isn't typesafe either :-\