[PATCH net] vhost-net: fix use-after-free in vhost_net_flush

["Michael S. Tsirkin" <mst@redhat.com>]

vhost_net_ubuf_put_and_wait has a confusing name:
it will actually also free it's argument.
Thus since commit 1280c27f8e29acf4af2da914e80ec27c3dbd5c01
vhost_net_flush tries to use the argument after passing it
to vhost_net_ubuf_put_and_wait, this results
in use after free.
To fix, don't free the argument in vhost_net_ubuf_put_and_wait,
add an new API for callers that want to free ubufs.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Dave, this is needed for stable as well, but
the code has moved a bit since then.
I'll post a patch tweaked to apply against 3.9 and 3.8 separately.

 drivers/vhost/net.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 5c77d6a..534adb0 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -149,6 +149,11 @@ static void vhost_net_ubuf_put_and_wait(struct vhost_net_ubuf_ref *ubufs)
 {
 	kref_put(&ubufs->kref, vhost_net_zerocopy_done_signal);
 	wait_event(ubufs->wait, !atomic_read(&ubufs->kref.refcount));
+}
+
+static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs)
+{
+	vhost_net_ubuf_put_and_wait(ubufs);
 	kfree(ubufs);
 }
 
@@ -1073,7 +1078,7 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 	mutex_unlock(&vq->mutex);
 
 	if (oldubufs) {
-		vhost_net_ubuf_put_and_wait(oldubufs);
+		vhost_net_ubuf_put_wait_and_free(oldubufs);
 		mutex_lock(&vq->mutex);
 		vhost_zerocopy_signal_used(n, vq);
 		mutex_unlock(&vq->mutex);
@@ -1091,7 +1096,7 @@ err_used:
 	vq->private_data = oldsock;
 	vhost_net_enable_vq(n, vq);
 	if (ubufs)
-		vhost_net_ubuf_put_and_wait(ubufs);
+		vhost_net_ubuf_put_wait_and_free(ubufs);
 err_ubufs:
 	fput(sock->file);
 err_vq:
-- 
MST