net-packet: fix race in packet_set_ring on PACKET_RESERVE

net-packet: fix race in packet_set_ring on PACKET_RESERVE

PACKET_RESERVE reserves headroom in memory mapped packet ring frames.
The value po->tp_reserve must is verified to be safe in packet_set_ring

  if (unlikely(req->tp_frame_size < po->tp_hdrlen + po->tp_reserve))

and the setsockopt fails once a ring is set.

  if (po->rx_ring.pg_vec || po->tx_ring.pg_vec)
          return -EBUSY;

This operation does not take the socket lock. This leads to a race
similar to the one with PACKET_VERSION fixed in commit 84ac7260236a
("packet: fix race condition in packet_set_ring").

Fix this issue in the same manner: take the socket lock, which as of
that patch is held for the duration of packet_set_ring.

This bug was discovered with syzkaller.

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
CVE-2017-1000111

(backported from email submission)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>