CVE-2025-37835

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

smb: client: Fix netns refcount imbalance causing leaks and use-after-free

Commit ef7134c7fc48 ("smb: client: Fix use-after-free of network namespace.") attempted to fix a netns use-after-free issue by manually adjusting reference counts via sk->sk_net_refcnt and sock_inuse_add().

However, a later commit e9f2517a3e18 ("smb: client: fix TCP timers deadlock after rmmod") pointed out that the approach of manually setting sk->sk_net_refcnt in the first commit was technically incorrect, as sk->sk_net_refcnt should only be set for user sockets. It led to issues like TCP timers not being cleared properly on close. The second commit moved to a model of just holding an extra netns reference for server->ssocket using get_net(), and dropping it when the server is torn down.

But there remain some gaps in the get_net()/put_net() balancing added by these commits. The incomplete reference handling in these fixes results in two issues:

1. Netns refcount leaks[1]

The problem process is as follows:

mount.cifs                        cifsd

cifs_do_mount
  cifs_mount
    cifs_mount_get_session
      cifs_get_tcp_session
        get_net()  /* First get net. */
        ip_connect
          generic_ip_connect /* Try port 445 */
            get_net()
            ->connect() /* Failed */
            put_net()
          generic_ip_connect /* Try port 139 */
            get_net() /* Missing matching put_net() for this get_net().*/
      cifs_get_smb_ses
        cifs_negotiate_protocol
          smb2_negotiate
            SMB2_negotiate
              cifs_send_recv
                wait_for_response
                                 cifs_demultiplex_thread
                                   cifs_read_from_socket
                                     cifs_readv_from_socket
                                       cifs_reconnect
                                         cifs_abort_connection
                                           sock_release();
                                           server->ssocket = NULL;
                                           /* Missing put_net() here. */
                                           generic_ip_connect
                                             get_net()
                                             ->connect() /* Failed */
                                             put_net()
                                             sock_release();
                                             server->ssocket = NULL;
          free_rsp_buf
    ...
                                   clean_demultiplex_info
                                     /* It's only called once here. */
                                     put_net()