CVE-2022-50220

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

usbnet: Fix linkwatch use-after-free on disconnect

usbnet uses the work usbnet_deferred_kevent() to perform tasks which may sleep. On disconnect, completion of the work was originally awaited in ->ndo_stop(). But in 2003, that was moved to ->disconnect() by historic commit "[PATCH] USB: usbnet, prevent exotic rtnl deadlock":

https://git.kernel.org/tglx/history/c/0f138bbfd83c

The change was made because back then, the kernel's workqueue implementation did not allow waiting for a single work. One had to wait for completion of all work by calling flush_scheduled_work(), and that could deadlock when waiting for usbnet_deferred_kevent() with rtnl_mutex held in ->ndo_stop().

The commit solved one problem but created another: It causes a use-after-free in USB Ethernet drivers aqc111.c, asix_devices.c, ax88179_178a.c, ch9200.c and smsc75xx.c:

• If the drivers receive a link change interrupt immediately before disconnect, they raise EVENT_LINK_RESET in their (non-sleepable) ->status() callback and schedule usbnet_deferred_kevent().
• usbnet_deferred_kevent() invokes the driver's ->link_reset() callback, which calls netif_carrier_{on,off}().
• That in turn schedules the work linkwatch_event().

Because usbnet_deferred_kevent() is awaited after unregister_netdev(), netif_carrier_{on,off}() may operate on an unregistered netdev and linkwatch_event() may run after free_netdev(), causing a use-after-free.

In 2010, usbnet was changed to only wait for a single instance of usbnet_deferred_kevent() instead of all work by commit 23f333a2bfaf ("drivers/net: don't use flush_scheduled_work()").

Unfortunately the commit neglected to move the wait back to ->ndo_stop(). Rectify that omission at long last.

The issue is a use-after-free (UAF) condition in the usbnet driver, triggered when a disconnect race overlaps with a pending linkwatch_event() scheduled via deferred kevent processing. This can lead to kernel crashes or system instability. It can happen only if a link reset event occurs just before device disconnection.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.