CVE-2021-47248: Concurrent Execution using Shared Resource with Improper Synchronization

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

udp: fix race between close() and udp_abort()

Kaustubh reported and diagnosed a panic in udp_lib_lookup(). The root cause is udp_abort() racing with close(). Both racing functions acquire the socket lock, but udp{v6}_destroy_sock() release it before performing destructive actions.

We can't easily extend the socket lock scope to avoid the race, instead use the SOCK_DEAD flag to prevent udp_abort from doing any action when the critical race happens.

Diagnosed-and-tested-by: Kaustubh Pandey <[email protected]>