Comment 15 for bug 1924298

After spending some time I think I have narrowed down the bug fix to Linux kernel 5.10-rc6.

The bug reproed on Ubuntu with kernel 5.10-rc4 but not on 5.10-rc6.

Here is a diff of the kernel sources between those two versions: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/diff/?id=v5.10-rc6&id2=v5.10-rc4&dt=2

Of note are some changes to `net/ipv4/inet_hashtables.c`

Using `git blame` I think the most likely source of the bug fix was this commit by Ricardo Dias: https://github.com/torvalds/linux/commit/01770a166165738a6e05c3d911fb4609cc4eb416

The description for the commit describes a race condition which looks like it could cause the bug:

"When such event happens, the TCP stack code has a race condition that occurs between the momement a lookup is done to the established connections hashtable to check for the existence of a connection for the same client, and the moment that the child socket is added to the established connections hashtable. As a consequence, this race condition can lead to a situation where we add two child sockets to the established connections hashtable and deliver two sockets to the userspace program to the same client."

So for anyone who comes across this bug, the likely solution is to update your OS to a version which uses kernel 5.10 or greater.