CVE-2026-43324: 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 synchronizeirq() (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: dummyhcd: fix gpf in gadgetsetup") tried to fix this by moving the synchronizeirq() emulation code from dummystop() to dummypullup(), 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 dummypullup(), because that routine can be called for reasons other than an impending unbind. Therefore commits 7dc0c55e9f30 ("USB: UDC core: Add udcasynccallbacks gadget op") and 04145a03db9d ("USB: UDC: Implement udcasynccallbacks 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 synchronizeirq() occurs before the emulated interrupt-disable! That's no good, beause it means that more emulated interrupts can occur after the synchronizeirq() 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 synchronizeirq() emulation code yet again, to the dummyudcasynccallbacks() 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.