virtio_pci: fix use after free on release

commit 2989be09 upstream.

KASan detected a use-after-free error in virtio-pci remove code. In
virtio_pci_remove(), vp_dev is still used after being freed in
unregister_virtio_device() (in virtio_pci_release_dev() more
precisely).

To fix, keep a reference until cleanup is done.

Fixes: 63bd62a0 ("virtio_pci: defer kfree until release callback")
Reported-by: Jerome Marchand <jmarchan@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Jerome Marchand <jmarchan@redhat.com>
[ luis: backported to 3.16:
  - file rename: virtio_pci_legacy.c -> virtio_pci.c ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/virtio/virtio_pci.c

@@ -756,6 +756,7 @@ static int virtio_pci_probe(struct pci_dev *pci_dev,
 static void virtio_pci_remove(struct pci_dev *pci_dev)
 {
 	struct virtio_pci_device *vp_dev = pci_get_drvdata(pci_dev);
+	struct device *dev = get_device(&vp_dev->vdev.dev);
 
 	unregister_virtio_device(&vp_dev->vdev);
 
@@ -763,6 +764,7 @@ static void virtio_pci_remove(struct pci_dev *pci_dev)
 	pci_iounmap(pci_dev, vp_dev->ioaddr);
 	pci_release_regions(pci_dev);
 	pci_disable_device(pci_dev);
+	put_device(dev);
 }
 
 #ifdef CONFIG_PM_SLEEP