usbnet: Fix linkwatch use-after-free on disconnect

Overview

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.

Affected

• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < d2d6b530d89b0a912148018027386aa049f0a309
• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < e2a521a7dcc463c5017b4426ca0804e151faeff7
• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < 7f77dcbc030c2faa6d8e8a594985eeb34018409e
• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < 8b4588b8b00b299be16a35be67b331d8fdba03f3
• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < 135199a2edd459d2b123144efcd7f9bcd95128e4
• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < 635fd8953e4309b54ca6a81bed1d4a87668694f4
• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < d49bb8cf9bfaa06aa527eb30f1a52a071da2e32f
• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < db3b738ae5f726204876f4303c49cfdf4311403f
• Linux >= 23f333a2bfafba80339315b724808982a9de57d9 < a69e617e533edddf3fa3123149900f36e0a6dc74
• Linux 2.6.38

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.