l2tp: Fix UDP socket reference count bugs in the pppol2tp driver

l2tp: Fix UDP socket reference count bugs in the pppol2tp driver

This patch fixes UDP socket refcnt bugs in the pppol2tp driver.

A bug can cause a kernel stack trace when a tunnel socket is closed.

A way to reproduce the issue is to prepare the UDP socket for L2TP (by
opening a tunnel pppol2tp socket) and then close it before any L2TP
sessions are added to it. The sequence is

Create UDP socket
Create tunnel pppol2tp socket to prepare UDP socket for L2TP
  pppol2tp_connect: session_id=0, peer_session_id=0
L2TP SCCRP control frame received (tunnel_id==0)
  pppol2tp_recv_core: sock_hold()
  pppol2tp_recv_core: sock_put
L2TP ZLB control frame received (tunnel_id=nnn)
  pppol2tp_recv_core: sock_hold()
  pppol2tp_recv_core: sock_put
Close tunnel management socket
  pppol2tp_release: session_id=0, peer_session_id=0
Close UDP socket
  udp_lib_close: BUG

The addition of sock_hold() in pppol2tp_connect() solves the problem.

For data frames, two sock_put() calls were added to plug a refcnt leak
per received data frame. The ref that is grabbed at the top of
pppol2tp_recv_core() must always be released, but this wasn't done for
accepted data frames or data frames discarded because of bad UDP
checksums. This leak meant that any UDP socket that had passed L2TP
data traffic (i.e. L2TP data frames, not just L2TP control frames)
using pppol2tp would not be released by the kernel.

WARNING: at include/net/sock.h:435 udp_lib_unhash+0x117/0x120()
Pid: 1086, comm: openl2tpd Not tainted 2.6.33-rc1 #8
Call Trace:
 [<c119e9b7>] ? udp_lib_unhash+0x117/0x120
 [<c101b871>] ? warn_slowpath_common+0x71/0xd0
 [<c119e9b7>] ? udp_lib_unhash+0x117/0x120
 [<c101b8e3>] ? warn_slowpath_null+0x13/0x20
 [<c119e9b7>] ? udp_lib_unhash+0x117/0x120
 [<c11598a7>] ? sk_common_release+0x17/0x90
 [<c11a5e33>] ? inet_release+0x33/0x60
 [<c11577b0>] ? sock_release+0x10/0x60
 [<c115780f>] ? sock_close+0xf/0x30
 [<c106e542>] ? __fput+0x52/0x150
 [<c106b68e>] ? filp_close+0x3e/0x70
 [<c101d2e2>] ? put_files_struct+0x62/0xb0
 [<c101eaf7>] ? do_exit+0x5e7/0x650
 [<c1081623>] ? mntput_no_expire+0x13/0x70
 [<c106b68e>] ? filp_close+0x3e/0x70
 [<c101eb8a>] ? do_group_exit+0x2a/0x70
 [<c101ebe1>] ? sys_exit_group+0x11/0x20
 [<c10029b0>] ? sysenter_do_call+0x12/0x26

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>