[PATCH 4.14 60/61] Fix double fget() in vhost_net_set_backend()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Al Viro <viro@zeniv.linux.org.uk>

commit fb4554c2232e44d595920f4d5c66cf8f7d13f9bc upstream.

Descriptor table is a shared resource; two fget() on the same descriptor
may return different struct file references.  get_tap_ptr_ring() is
called after we'd found (and pinned) the socket we'll be using and it
tries to find the private tun/tap data structures associated with it.
Redoing the lookup by the same file descriptor we'd used to get the
socket is racy - we need to same struct file.

Thanks to Jason for spotting a braino in the original variant of patch -
I'd missed the use of fd == -1 for disabling backend, and in that case
we can end up with sock == NULL and sock != oldsock.

Cc: stable@kernel.org
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[4.14: Account for get_tap_skb_array() instead of get_tap_ptr_ring()]
Signed-off-by: Samuel Mendoza-Jonas <samjonas@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/vhost/net.c |   15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -1047,13 +1047,9 @@ err:
 	return ERR_PTR(r);
 }
 
-static struct skb_array *get_tap_skb_array(int fd)
+static struct skb_array *get_tap_skb_array(struct file *file)
 {
 	struct skb_array *array;
-	struct file *file = fget(fd);
-
-	if (!file)
-		return NULL;
 	array = tun_get_skb_array(file);
 	if (!IS_ERR(array))
 		goto out;
@@ -1062,7 +1058,6 @@ static struct skb_array *get_tap_skb_arr
 		goto out;
 	array = NULL;
 out:
-	fput(file);
 	return array;
 }
 
@@ -1143,8 +1138,12 @@ static long vhost_net_set_backend(struct
 		vhost_net_disable_vq(n, vq);
 		vq->private_data = sock;
 		vhost_net_buf_unproduce(nvq);
-		if (index == VHOST_NET_VQ_RX)
-			nvq->rx_array = get_tap_skb_array(fd);
+		if (index == VHOST_NET_VQ_RX) {
+			if (sock)
+				nvq->rx_array = get_tap_skb_array(sock->file);
+			else
+				nvq->rx_array = NULL;
+		}
 		r = vhost_vq_init_access(vq);
 		if (r)
 			goto err_used;