vhost-net: fix use-after-free in vhost_net_flush

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 outstanding DMAs on memory change"
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.

Acked-by: Asias He <asias@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index f80d3dd41d8c..8ca5ac71b845 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -150,6 +150,11 @@ 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);
 }
 
@@ -948,7 +953,7 @@ 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);
@@ -966,7 +971,7 @@ err_used:
         rcu_assign_pointer(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: