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

DISCUSSION Johan Hovold 1 participant 1 message Jun 20, 2018 USB

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Johan Hovold

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Quoted 29 lines · Rafael J. Wysocki
Quoted 28 lines · CEST Johan Hovold
Quoted 26 lines · Tony Lindgren

Hi,

Adding Rafael and linux-pm to Cc as well.

Quoted 21 lines · Felipe Balbi

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)

Quoted 6 lines
Quoted 4 lines

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).

Quoted 11 lines
Quoted 2 lines

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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