tun: fix use after free for ptr_ring

commit b196d88aba8ac72b775137854121097f4c4c6862 upstream.

We used to initialize ptr_ring during TUNSETIFF, this is because its
size depends on the tx_queue_len of netdevice. And we try to clean it
up when socket were detached from netdevice. A race were spotted when
trying to do uninit during a read which will lead a use after free for
pointer ring. Solving this by always initialize a zero size ptr_ring
in open() and do resizing during TUNSETIFF, and then we can safely do
cleanup during close(). With this, there's no need for the workaround
that was introduced by commit 4df0bfc79904 ("tun: fix a memory leak
for tfile->tx_array").

Backport Note :-
Comparison with the upstream patch:
[1] A "semantic revert" of the changes made in
    4df0bfc799("tun: fix a memory leak for tfile->tx_array").
        4df0bfc799 was applied upstream, and then skb array was changed
	to use ptr_ring. The upstream patch then removes the changes introduced
	by 4df0bfc799. This backport does the same; "revert" the changes
	made by 4df0bfc799.
[2] xdp_rxq_info_unreg() being called in relevant locations
        As xdp_rxq_info related patches are not present in 4.14, these
	changes are not needed in the backport.
[3] An instance of ptr_ring_init needs to be replaced by skb_array_init
	Inside tun_attach()
[4] ptr_ring_cleanup needs to be replaced by skb_array_cleanup
	Inside tun_chr_close()

Note that the backport for 7063efd33b ("tuntap: fix use after free during release")
needs to be applied on top of this patch.

Reported-by:  <syzbot+e8b902c3c3fadf0a9dba@syzkaller.appspotmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Fixes: 1576d986 ("tun: switch to use skb array for tx")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Zubin Mithra <zsm@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>