USB: dummy-hcd: Fix interrupt synchronization error

In the Linux kernel, the following vulnerability has been resolved:

USB: dummy-hcd: Fix interrupt synchronization error

This fixes an error in synchronization in the dummy-hcd driver. The

error has a somewhat involved history. The synchronization mechanism

was introduced by commit 7dbd8f4cabd9 ("USB: dummy-hcd: Fix erroneous

synchronization change"), which added an emulated "interrupts enabled"

flag together with code emulating synchronize_irq() (it waits until

all current handler callbacks have returned).

But the emulated interrupt-disable occurred too late, after the driver

containing the handler callback routines had been told that it was

unbound and no more callbacks would occur. Commit 4a5d797a9f9c ("usb:

gadget: dummy_hcd: fix gpf in gadget_setup") tried to fix this by

moving the synchronize_irq() emulation code from dummy_stop() to

dummy_pullup(), which runs before the unbind callback.

There still were races, though, because the emulated interrupt-disable

still occurred too late. It couldn't be moved to dummy_pullup(),

because that routine can be called for reasons other than an impending

unbind. Therefore commits 7dc0c55e9f30 ("USB: UDC core: Add

udc_async_callbacks gadget op") and 04145a03db9d ("USB: UDC: Implement

udc_async_callbacks in dummy-hcd") added an API allowing the UDC core

to tell dummy-hcd exactly when emulated interrupts and their callbacks

should be disabled.

That brings us to the current state of things, which is still wrong

because the emulated synchronize_irq() occurs before the emulated

interrupt-disable! That's no good, beause it means that more emulated

interrupts can occur after the synchronize_irq() emulation has run,

leading to the possibility that a callback handler may be running when

the gadget driver is unbound.

To fix this, we have to move the synchronize_irq() emulation code yet

again, to the dummy_udc_async_callbacks() routine, which takes care of

enabling and disabling emulated interrupt requests. The

synchronization will now run immediately after emulated interrupts are

disabled, which is where it belongs.