CVE-2022-49501

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

usbnet: Run unregister_netdev() before unbind() again

Commit 2c9d6c2b871d (usbnet: run unbind() before unregister_netdev()) sought to fix a use-after-free on disconnect of USB Ethernet adapters.

It turns out that a different fix is necessary to address the issue: https://lore.kernel.org/netdev/18b3541e5372bc9b9fc733d422f4e698c089077c.1650177997.git.lukas@wunner.de/

So the commit was not necessary.

The commit made binding and unbinding of USB Ethernet asymmetrical: Before, usbnet_probe() first invoked the ->bind() callback and then register_netdev(). usbnet_disconnect() mirrored that by first invoking unregister_netdev() and then ->unbind().

Since the commit, the order in usbnet_disconnect() is reversed and no longer mirrors usbnet_probe().

One consequence is that a PHY disconnected (and stopped) in ->unbind() is afterwards stopped once more by unregister_netdev() as it closes the netdev before unregistering. That necessitates a contortion in ->stop() because the PHY may only be stopped if it hasnt already been disconnected.

Reverting the commit allows making the call to phy_stop() unconditional in ->stop().

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/6d5deb242874d924beccf7eb3cef04c1c3b0da79
• https://git.kernel.org/stable/c/969a1b3ea3cb7d58a16fe12fd1b04bfc0ea40509
• https://git.kernel.org/stable/c/d1408f6b4dd78fb1b9e26bcf64477984e5f85409
• https://git.kernel.org/stable/c/fbda837107f9bd4ec658d2aa88c6856dba606f06