[net-next,v2] 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.

Reported-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/netdev/CAG48ez0MHBbENX5gCdHAUXZ7h7s20LnepBF-pa5M=7Bi-jZrEA@mail.gmail.com/
Reported-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/netdev/20220315113841.GA22337@pengutronix.de/
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org
Cc: Oliver Neukum <oneukum@suse.com>
---
 This seems to be Jakub's preferred way to solve the use-after-free.
 I've tagged the patch for net-next so that it can bake in linux-next
 for a few weeks, but it alternatively applies cleanly to net (in case
 maintainers are eager to get it into mainline).
 
 The patch supersedes the following prior attempts:
 
 usbnet: Fix use-after-free on disconnect
 https://lore.kernel.org/netdev/127121d9d933ebe3fc13f9f91cc33363d6a8a8ac.1649859147.git.lukas@wunner.de/

 net: linkwatch: ignore events for unregistered netdevs
 https://lore.kernel.org/netdev/18b3541e5372bc9b9fc733d422f4e698c089077c.1650177997.git.lukas@wunner.de/

 net: linkwatch: ignore events for unregistered netdevs (v2)
 https://lore.kernel.org/netdev/cover.1655024266.git.lukas@wunner.de/

 drivers/net/usb/usbnet.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

Hello:

This patch was applied to netdev/net-next.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 23 Jun 2022 14:50:59 +0200 you wrote:
> 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
> 
> [...]

Here is the summary with links:
  - [net-next,v2] usbnet: Fix linkwatch use-after-free on disconnect
    https://git.kernel.org/netdev/net-next/c/a69e617e533e

You are awesome, thank you!