Re: [PATCH] vhost: avoid buffer overflow in update_secure_len

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Wed, Nov 18, 2015 at 07:53:24AM +0000, Xie, Huawei wrote:
...
> >  	do {
> > +		if (vec_id >= BUF_VECTOR_MAX)
> > +			break;
> > +
> >  		next_desc = 0;
> >  		len += vq->desc[idx].len;
> >  		vq->buf_vec[vec_id].buf_addr = vq->desc[idx].addr;
> > @@ -519,6 +526,8 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t queue_id,
> >  					goto merge_rx_exit;
> >  				} else {
> >  					update_secure_len(vq, res_cur_idx, &secure_len, &vec_idx);
> > +					if (secure_len == 0)
> > +						goto merge_rx_exit;
> Why do we exit when secure_len is 0 rather than 1? :). Malicious guest

I confess it's not a proper fix. Making it return an error code, as Rich
suggested in early email, is better. It's generic enough, as we have to
check the vec_buf overflow here.

BTW, can we move the vec_buf outside `struct vhost_virtqueue'? It makes
the structure huge.

> could easily forge the desc len so that secure_len never reach pkt_len
> even it is not zero so that host enters into dead loop here.
> Generally speaking, we shouldn't fix for a specific issue,

Agreed.

> and the
> security checks should be as few as possible.

Idealy, yes.

> We need to consider
> refactor the code here for the generic fix.

What's your thougths?

	--yliu
> 
> >  					res_cur_idx++;
> >  				}
> >  			} while (pkt_len > secure_len);
> > @@ -631,6 +640,8 @@ rte_vhost_dequeue_burst(struct virtio_net *dev, uint16_t queue_id,
> >  		uint8_t alloc_err = 0;
> >  
> >  		desc = &vq->desc[head[entry_success]];
> > +		if (desc->len == 0)
> > +			break;
> >  
> >  		/* Discard first buffer as it is the virtio header */
> >  		if (desc->flags & VRING_DESC_F_NEXT) {
> > @@ -638,6 +649,8 @@ rte_vhost_dequeue_burst(struct virtio_net *dev, uint16_t queue_id,
> >  			vb_offset = 0;
> >  			vb_avail = desc->len;
> >  		} else {
> > +			if (desc->len < vq->vhost_hlen)
> > +				break;
> >  			vb_offset = vq->vhost_hlen;
> >  			vb_avail = desc->len - vb_offset;
> >  		}
> >
>