Affected: Linux

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

smb: client: Fix use-after-free of network namespace.

Recently, we got a customer report that CIFS triggers oops while reconnecting to a server. [0]

The workload runs on Kubernetes, and some pods mount CIFS servers in non-root network namespaces. The problem rarely happened, but it was always while the pod was dying.

The root cause is wrong reference counting for network namespace.

CIFS uses kernel sockets, which do not hold refcnt of the netns that the socket belongs to. That means CIFS must ensure the socket is always freed before its netns; otherwise, use-after-free happens.

The repro steps are roughly:

1. mount CIFS in a non-root netns 2. drop packets from the netns 3. destroy the netns 4. unmount CIFS

We can reproduce the issue quickly with the script [1] below and see the splat [2] if CONFIG_NET_NS_REFCNT_TRACKER is enabled.

When the socket is TCP, it is hard to guarantee the netns lifetime without holding refcnt due to async timers.

Let's hold netns refcnt for each socket as done for SMC in commit 9744d2bf1976 ("smc: Fix use-after-free in tcp_write_timer_handler().").

Note that we need to move put_net() from cifs_put_tcp_session() to clean_demultiplex_info(); otherwise, __sock_create() still could touch a freed netns while cifsd tries to reconnect from cifs_demultiplex_thread().

Also, maybe_get_net() cannot be put just before __sock_create() because the code is not under RCU and…