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

Al Viro
2018-06-07T18:05:04Z

On Thu, Jun 07, 2018 at 08:59:06PM +0300, Michael S. Tsirkin wrote:
> 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;

IDGI - what would your variant catch that kzalloc + 2 assignments won't?
Accesses to uninitialized ->node? Because that's the only difference in
what is and is not initialized between those variants...