When client on the host tries to connect(SOCK_STREAM, O_NONBLOCK) to the
server on the guest, there will be a panic on a ThunderX2 (armv8a server):

[  463.718844] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
[  463.718848] Mem abort info:
[  463.718849]   ESR = 0x96000044
[  463.718852]   EC = 0x25: DABT (current EL), IL = 32 bits
[  463.718853]   SET = 0, FnV = 0
[  463.718854]   EA = 0, S1PTW = 0
[  463.718855] Data abort info:
[  463.718856]   ISV = 0, ISS = 0x00000044
[  463.718857]   CM = 0, WnR = 1
[  463.718859] user pgtable: 4k pages, 48-bit VAs, pgdp=0000008f6f6e9000
[  463.718861] [0000000000000000] pgd=0000000000000000
[  463.718866] Internal error: Oops: 96000044 [#1] SMP
[...]
[  463.718977] CPU: 213 PID: 5040 Comm: vhost-5032 Tainted: G           O      5.7.0-rc7+ #139
[  463.718980] Hardware name: GIGABYTE R281-T91-00/MT91-FS1-00, BIOS F06 09/25/2018
[  463.718982] pstate: 60400009 (nZCv daif +PAN -UAO)
[  463.718995] pc : virtio_transport_recv_pkt+0x4c8/0xd40 [vmw_vsock_virtio_transport_common]
[  463.718999] lr : virtio_transport_recv_pkt+0x1fc/0xd40 [vmw_vsock_virtio_transport_common]
[  463.719000] sp : ffff80002dbe3c40
[...]
[  463.719025] Call trace:
[  463.719030]  virtio_transport_recv_pkt+0x4c8/0xd40 [vmw_vsock_virtio_transport_common]
[  463.719034]  vhost_vsock_handle_tx_kick+0x360/0x408 [vhost_vsock]
[  463.719041]  vhost_worker+0x100/0x1a0 [vhost]
[  463.719048]  kthread+0x128/0x130
[  463.719052]  ret_from_fork+0x10/0x18

The race condition is as follows:
Task1                                Task2
=====                                =====
__sock_release                       virtio_transport_recv_pkt
  __vsock_release                      vsock_find_bound_socket (found sk)
    lock_sock_nested
    vsock_remove_sock
    sock_orphan
      sk_set_socket(sk, NULL)
    sk->sk_shutdown = SHUTDOWN_MASK
    ...
    release_sock
                                    lock_sock
                                       virtio_transport_recv_connecting
                                         sk->sk_socket->state (panic!)

The root cause is that vsock_find_bound_socket can't hold the lock_sock,
so there is a small race window between vsock_find_bound_socket() and
lock_sock(). If __vsock_release() is running in another task,
sk->sk_socket will be set to NULL inadvertently.

This fixes it by checking sk->sk_shutdown(suggested by Stefano) after
lock_sock since sk->sk_shutdown is set to SHUTDOWN_MASK under the
protection of lock_sock_nested.
Signed-off-by: Jia He <justin.he@arm.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>