Fix double fget() in vhost_net_set_backend()

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>

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 792ab5f..297b5db 100644 (file)
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -1450,13 +1450,9 @@ 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;
@@ -1465,7 +1461,6 @@ static struct ptr_ring *get_tap_ptr_ring(int fd)
                goto out;
        ring = NULL;
 out:
-       fput(file);
        return ring;
 }
 
@@ -1552,8 +1547,12 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
                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;