BUG: TCP timewait sockets survive across namespace creation in net-next

[Leonard Crestez <cdleonard@gmail.com>]

Hello,

It appears that in recent net-next versions it is possible for sockets 
in the timewait state to survive across namespace add/del. Timewait 
sockets are inserted into a global hash and only the sock_net value is 
compared when they are enumerated from interfaces like /proc/net/tcp and 
inet_diag. Old TW sockets are not cleared after namespace delete and 
namespaces are allocated from a slab and thus their pointers get reused 
a lot, when that happens timewait sockets from an old namespace will 
show up in the new one.

This can be reproduced by establishing a TCP connection over a veth pair 
between two namespaces, closing and then recreating those namespaces. 
Old timewait sockets will be visible and it happens quite reliably, 
often on the first iteration. I can try to provide a script for this.

I can't point to specific bugs outside of tests that explicitly 
enumerate timewait sockets but letting sk_net be a dangling pointer 
seems very dangerous. It also violates the idea of network namespaces 
being independent and isolated.

This does not happen in 5.17, I bisected this behavior to commit 
0dad4087a86a ("tcp/dccp: get rid of inet_twsk_purge()")

--
Regards,
Leonard