CVE-2021-47111

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

xen-netback: take a reference to the RX task thread

Do this in order to prevent the task from being freed if the thread

returns (which can be triggered by the frontend) before the call to

kthread_stop done as part of the backend tear down. Not taking the

reference will lead to a use-after-free in that scenario. Such

reference was taken before but dropped as part of the rework done in

2ac061ce97f4.

Reintroduce the reference taking and add a comment this time

explaining why it's needed.

This is XSA-374 / CVE-2021-28691.