CVE-2026-43327USB: dummy-hcd: Fix locking/synchronization error

Official description

In the Linux kernel, the following vulnerability has been resolved: USB: dummy-hcd: Fix locking/synchronization error Syzbot testing was able to provoke an addressing exception and crash in the usb_gadget_udc_reset() routine in drivers/usb/gadgets/udc/core.c, resulting from the fact that the routine was called with a second ("driver") argument of NULL. The bad caller was set_link_state() in dummy_hcd.c, and the problem arose because of a race between a USB reset and driver unbind. These sorts of races were not supposed to be possible; commit 7dbd8f4cabd9 ("USB: dummy-hcd: Fix erroneous synchronization change"), along with a few followup commits, was written specifically to prevent them. As it turns out, there are (at least) two errors remaining in the code. Another patch will address the second error; this one is concerned with the first. The error responsible for the syzbot crash occurred because the stop_activity() routine will sometimes drop and then re-acquire the dum->lock spinlock. A call to stop_activity() occurs in set_link_state() when handling an emulated USB reset, after the test of dum->ints_enabled and before the increment of dum->callback_usage. This allowed another thread (doing a driver unbind) to sneak in and grab the spinlock, and then clear dum->ints_enabled and dum->driver. Normally this other thread would have to wait for dum->callback_usage to go down to 0 before it would clear dum->driver, but in this case it didn't have to wait since dum->callback_usage had not yet been incremented. The fix is to increment dum->callback_usage _before_ calling stop_activity() instead of after. Then the thread doing the unbind will not clear dum->driver until after the call to usb_gadget_udc_reset() safely returns and dum->callback_usage has been decremented again.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/6350c7dd33ab481ef41c931a238361490c32d15c
• https://git.kernel.org/stable/c/cc97fb5969177cccce2e23b31298df220fc7570d
• https://git.kernel.org/stable/c/218886b2ef2dea7627d3700ab0abaf4bf9d1161f
• https://git.kernel.org/stable/c/791966f85b439b261bf19865cf1c07c065ffb4b4
• https://git.kernel.org/stable/c/805b1833d6ed6da5086e610578a28e71bb54fbbb
• https://git.kernel.org/stable/c/efbd9441f1e769a7aae1813d497cec09cbdff031
• https://git.kernel.org/stable/c/69ab97a693251d6a6093e630060a3c744fd58524
• https://git.kernel.org/stable/c/616a63ff495df12863692ab3f9f7b84e3fa7a66d