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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 7 Jun 2018 20:59:06 +0300

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 <[email-masked]>
> > ---
> > 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;