Re: KMSAN: uninit-value in _copy_to_iter (2)

Michael S. Tsirkin
2018-06-07T17:59:13Z

On Thu, Jun 07, 2018 at 06:43:55PM +0100, Al Viro wrote:
> On Thu, Jun 07, 2018 at 06:38:48PM +0300, Michael S. Tsirkin wrote:
> > #syz test: https://github.com/google/kmsan.git/master d2d741e5d1898dfde1a75ea3d29a9a3e2edf0617
> >
> > Subject: vhost: fix info leak
> >
> > Fixes: CVE-2018-1118
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index f0be5f35ab28..9beefa6ed1ce 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2345,6 +2345,9 @@ struct vhost_msg_node *vhost_new_msg(struct vhost_virtqueue *vq, int type)
> > struct vhost_msg_node *node = kmalloc(sizeof *node, GFP_KERNEL);
> > if (!node)
> > return NULL;
> > +
> > + /* Make sure all padding within the structure is initialized. */
> > + memset(&node->msg, 0, sizeof node->msg);
>
> Umm... Maybe kzalloc(), then? You have
>
> struct vhost_msg_node {
> struct vhost_msg msg;
> struct vhost_virtqueue *vq;
> struct list_head node;
> };
>
> and that's what, 68 bytes in msg, then either 4 bytes pointer or
> 4 bytes padding + 8 bytes pointer, then two pointers? How much
> does explicit partial memset() save you here?

Yes but 0 isn't a nop here so if this struct is used without
a sensible initialization, it will crash elsewhere.
I prefer KASAN to catch such uses.

> > node->vq = vq;
> > node->msg.type = type;
> > return node;