Re: [PATCH 2/2] USB: dwc3: fix use-after-free on core driver unbind

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Thu, Jun 08, 2023 at 03:49:58PM +0200, Johan Hovold wrote:
> On Thu, Jun 08, 2023 at 06:53:13PM +0530, Manivannan Sadhasivam wrote:
> > On Thu, Jun 08, 2023 at 03:09:27PM +0200, Johan Hovold wrote:
> > > On Thu, Jun 08, 2023 at 06:32:46PM +0530, Manivannan Sadhasivam wrote:
> > > > On Wed, Jun 07, 2023 at 12:05:40PM +0200, Johan Hovold wrote:
> > > > > Some dwc3 glue drivers are currently accessing the driver data of the
> > > > > child core device directly, which is clearly a bad idea as the child may
> > > > > not have probed yet or may have been unbound from its driver.
> > > > > 
> > > > > As a workaround until the glue drivers have been fixed, clear the driver
> > > > > data pointer before allowing the glue parent device to runtime suspend
> > > > > to prevent its driver from accessing data that has been freed during
> > > > > unbind.
> > > 
> > > > > @@ -1929,6 +1929,11 @@ static int dwc3_remove(struct platform_device *pdev)
> > > > >  	pm_runtime_disable(&pdev->dev);
> > > > >  	pm_runtime_dont_use_autosuspend(&pdev->dev);
> > > > >  	pm_runtime_put_noidle(&pdev->dev);
> > > > > +	/*
> > > > > +	 * HACK: Clear the driver data, which is currently accessed by parent
> > > > > +	 * glue drivers, before allowing the parent to suspend.
> > > > > +	 */
> > > > > +	platform_set_drvdata(pdev, NULL);
> > > > 
> > > > This is required because you have seen the glue driver going to runtime suspend
> > > > once the below pm_runtime_set_suspended() is completed?
> > > 
> > > This is based on analysis of the code. The parent (glue) can not suspend
> > > while the child (core) is in the active state, but once we set the
> > > suspended state that could happen.
> > 
> > I could see that the driver core is setting drvdata to NULL during
> > device_unbind_cleanup(), so not sure if this scenario could be met otherwise it
> > will be redundant.
> 
> If this was redundant I wouldn't have added it. ;)
> 
> The parent driver has no business accessing the driver data of the child
> in the first place, but it must absolutely not do so after the child has
> been unbound from its driver and the driver data is getting freed.
> 
> Relying on the clean up in driver core that resets this pointer does not
> work as that would still leave a window where the parent could access
> this stale data.
> 

Well, I agree with the small window here.

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

- Mani

> Johan

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