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