[PATCH 4.19 13/44] 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>
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
@@ -1209,13 +1209,9 @@ err:
 	return ERR_PTR(r);
 }
 
-static struct ptr_ring *get_tap_ptr_ring(int fd)
+static struct ptr_ring *get_tap_ptr_ring(struct file *file)
 {
 	struct ptr_ring *ring;
-	struct file *file = fget(fd);
-
-	if (!file)
-		return NULL;
 	ring = tun_get_tx_ring(file);
 	if (!IS_ERR(ring))
 		goto out;
@@ -1224,7 +1220,6 @@ static struct ptr_ring *get_tap_ptr_ring
 		goto out;
 	ring = NULL;
 out:
-	fput(file);
 	return ring;
 }
 
@@ -1311,8 +1306,12 @@ static long vhost_net_set_backend(struct
 		r = vhost_net_enable_vq(n, vq);
 		if (r)
 			goto err_used;
-		if (index == VHOST_NET_VQ_RX)
-			nvq->rx_ring = get_tap_ptr_ring(fd);
+		if (index == VHOST_NET_VQ_RX) {
+			if (sock)
+				nvq->rx_ring = get_tap_ptr_ring(sock->file);
+			else
+				nvq->rx_ring = NULL;
+		}
 
 		oldubufs = nvq->ubufs;
 		nvq->ubufs = ubufs;