Re: [PATCH v2 4/9] usb: dwc3: qcom: fix use-after-free on runtime-PM wakeup

[Manivannan Sadhasivam]

On Sat, Aug 06, 2022 at 06:08:51PM +0200, Johan Hovold wrote:
> On Sat, Aug 06, 2022 at 08:03:11PM +0530, Manivannan Sadhasivam wrote:
> > On Thu, Aug 04, 2022 at 05:09:56PM +0200, Johan Hovold wrote:
> > > The Qualcomm dwc3 runtime-PM implementation checks the xhci
> > > platform-device pointer in the wakeup-interrupt handler to determine
> > > whether the controller is in host mode and if so triggers a resume.
> > > 
> > > After a role switch in OTG mode the xhci platform-device would have been
> > > freed and the next wakeup from runtime suspend would access the freed
> > > memory.
> > > 
> > > Note that role switching is executed from a freezable workqueue, which
> > > guarantees that the pointer is stable during suspend.
> > > 
> > > Also note that runtime PM has been broken since commit 2664deb09306
> > > ("usb: dwc3: qcom: Honor wakeup enabled/disabled state"), which
> > > incidentally also prevents this issue from being triggered.
> > > 
> > > Fixes: a4333c3a6ba9 ("usb: dwc3: Add Qualcomm DWC3 glue driver")
> > > Cc: stable@xxxxxxxxxxxxxxx      # 4.18
> > > Signed-off-by: Johan Hovold <johan+linaro@xxxxxxxxxx>
> > 
> > It'd be good to mention the introduction of dwc3_qcom_is_host() function.
> > Initially I thought it is used in a single place, but going through the rest of
> > the patches reveals that it is used later on.
> 
> I think the helper is warranted on its own as it serves as documentation
> of the underlying assumptions that this code relies on.
> 

That's even better.

Thanks,
Mani

> > > +/* Only usable in contexts where the role can not change. */
> > > +static bool dwc3_qcom_is_host(struct dwc3_qcom *qcom)
> > > +{
> > > +	struct dwc3 *dwc = platform_get_drvdata(qcom->dwc3);
> > > +
> > > +	return dwc->xhci;
> > > +}
> > > +
> > >  static enum usb_device_speed dwc3_qcom_read_usb2_speed(struct dwc3_qcom *qcom)
> > >  {
> > >  	struct dwc3 *dwc = platform_get_drvdata(qcom->dwc3);
> > > @@ -460,7 +468,11 @@ static irqreturn_t qcom_dwc3_resume_irq(int irq, void *data)
> > >  	if (qcom->pm_suspended)
> > >  		return IRQ_HANDLED;
> > >  
> > > -	if (dwc->xhci)
> > > +	/*
> > > +	 * This is safe as role switching is done from a freezable workqueue
> > > +	 * and the wakeup interrupts are disabled as part of resume.
> > > +	 */
> > > +	if (dwc3_qcom_is_host(qcom))
> > >  		pm_runtime_resume(&dwc->xhci->dev);
> > >  
> > >  	return IRQ_HANDLED;
> > > diff --git a/drivers/usb/dwc3/host.c b/drivers/usb/dwc3/host.c
> > > index f56c30cf151e..f6f13e7f1ba1 100644
> > > --- a/drivers/usb/dwc3/host.c
> > > +++ b/drivers/usb/dwc3/host.c
> > > @@ -135,4 +135,5 @@ int dwc3_host_init(struct dwc3 *dwc)
> > >  void dwc3_host_exit(struct dwc3 *dwc)
> > >  {
> > >  	platform_device_unregister(dwc->xhci);
> > > +	dwc->xhci = NULL;
> > >  }
> > > -- 
> > > 2.35.1
> 
> Johan

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