SCTP: fix race between sctp_bind_addr_free() and sctp_bind_addr_conflict()

During the sctp_close() call, we do not use rcu primitives to
destroy the address list attached to the endpoint.  At the same
time, we do the removal of addresses from this list before
attempting to remove the socket from the port hash

As a result, it is possible for another process to find the socket
in the port hash that is in the process of being closed.  It then
proceeds to traverse the address list to find the conflict, only
to have that address list suddenly disappear without rcu() critical
section.

Fix issue by closing address list removal inside RCU critical
section.

Race can result in a kernel crash with general protection fault or
kernel NULL pointer dereference:

kernel: general protection fault: 0000 [#1] SMP
kernel: RIP: 0010:[<ffffffffa02f3dde>]  [<ffffffffa02f3dde>] sctp_bind_addr_conflict+0x64/0x82 [sctp]
kernel: Call Trace:
kernel:  [<ffffffffa02f415f>] ? sctp_get_port_local+0x17b/0x2a3 [sctp]
kernel:  [<ffffffffa02f3d45>] ? sctp_bind_addr_match+0x33/0x68 [sctp]
kernel:  [<ffffffffa02f4416>] ? sctp_do_bind+0xd3/0x141 [sctp]
kernel:  [<ffffffffa02f5030>] ? sctp_bindx_add+0x4d/0x8e [sctp]
kernel:  [<ffffffffa02f5183>] ? sctp_setsockopt_bindx+0x112/0x4a4 [sctp]
kernel:  [<ffffffff81089e82>] ? generic_file_aio_write+0x7f/0x9b
kernel:  [<ffffffffa02f763e>] ? sctp_setsockopt+0x14f/0xfee [sctp]
kernel:  [<ffffffff810c11fb>] ? do_sync_write+0xab/0xeb
kernel:  [<ffffffff810e82ab>] ? fsnotify+0x239/0x282
kernel:  [<ffffffff810c2462>] ? alloc_file+0x18/0xb1
kernel:  [<ffffffff8134a0b1>] ? compat_sys_setsockopt+0x1a5/0x1d9
kernel:  [<ffffffff8134aaf1>] ? compat_sys_socketcall+0x143/0x1a4
kernel:  [<ffffffff810467dc>] ? sysenter_dispatch+0x7/0x32

Signed-off-by: Jacek Luczak <luczak.jacek@gmail.com>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
CC: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>