usb: dwc3: of-simple: fix use-after-free on remove

[Johan Hovold <johan@kernel.org>]

On Wed, Jun 20, 2018 at 02:54:10PM +0200, Rafael J. Wysocki wrote:
> On Wednesday, June 20, 2018 2:23:46 PM CEST Johan Hovold wrote:
> > On Wed, Jun 20, 2018 at 02:16:59AM -0700, Tony Lindgren wrote:
> > > Hi,
> > > 
> > > Adding Rafael and linux-pm to Cc as well.
> > > 
> > > * Felipe Balbi <balbi@kernel.org> [180619 01:23]:
> > > > This is a direct consequence of not paying attention to the order of
> > > > things. If driver were to assume that pm_domain->activate() would do the
> > > > right thing for the device -- meaning that probe would run with an
> > > > active device --, then we wouldn't need that pm_runtime_get() call on
> > > > probe at all. Rather we would follow the sequence:
> > > > 
> > > > 	pm_runtime_forbid()
> > > > 	pm_runtime_set_active()
> > > > 	pm_runtime_enable()
> > > > 
> > > > 	/* do your probe routine */
> > > > 
> > > > 	pm_runtime_put_noidle()
> > > > 
> > > > Then you remove you would need to call pm_runtime_get_noresume() to
> > > > balance out the pm_runtime_put_noidle() there.
> > 
> > > > (If you need to know why the pm_runtime_put_noidle(), remember that
> > > > pm_runtime_set_active() increments the usage counter, so
> > > > pm_runtime_put_noidle is basically allowing pm_runtime to happen as soon
> > > > as userspace writes "auto" to /sys/..../power/control)
> > 
> > That's not correct; pm_runtime_set_active() only increments the usage
> > counter of a parent (under some circumstances), so unless you have bus
> > code incrementing the usage counter before probe, the above
> > pm_runtime_put_noidle() would actually introduce an imbalance.
> 
> No, it wouldn't.  It balances the incrementation in pm_runtime_forbid().

Right, but even if you take the whole sequence, which included
pm_runtime_forbid(), consider what happens when pm_runtime_allow() is
later called through sysfs (see below).

> > And note that that's also the case even if you meant to say that
> > *pm_runtime_forbid()* increments the usage counter (which it does).
> 
> Why is it?
> 
> Surely, after
> 
> pm_runtime_forbid(dev);
> pm_runtime_put_noidle(dev);
> 
> the runtime PM usage counter of dev will be the same as before, won't it?

Sure, but the imbalance, or rather inconsistent state, has already been
introduced.

Consider the following sequence of events:

					usage count
					0
probe()
	pm_runtime_forbid()		1
	pm_runtime_set_active()
	pm_runtime_enable()		
	pm_runtime_put_noidle()		0

Here nothing is preventing the device from runtime suspending, despite
runtime PM being forbidden. In fact, it will typically be suspended due
to the pm_request_idle() in driver_probe_device(). If later we have:

echo auto > power/control
	pm_runtime_allow()		-1

then the device remains suspended, but we get a negative usage count.
This does not seem to play well with neither runtime pm (consider a
synchronous get followed by a put, which will fail to again suspend the
device) or, for example, system suspend, which tries to prevent runtime
pm by incrementing the usage count.

And if runtime pm is later again forbidden:

echo on > power/control
	pm_runtime_forbid()		0

then the device will not be resumed.

Johan
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