Re: [PATCH 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Roger Pau MonnÃ]

On Fri, Dec 07, 2018 at 12:18:04PM +0800, Dongli Zhang wrote:
> The xenstore 'ring-page-order' is used globally for each blkback queue and
> therefore should be read from xenstore only once. However, it is obtained
> in read_per_ring_refs() which might be called multiple times during the
> initialization of each blkback queue.
> 
> If the blkfront is malicious and the 'ring-page-order' is set in different
> value by blkfront every time before blkback reads it, this may end up at
> the "WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));" in
> xen_blkif_disconnect() when frontend is destroyed.
> 
> This patch reworks connect_ring() to read xenstore 'ring-page-order' only
> once.
> 
> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> ---
>  drivers/block/xen-blkback/xenbus.c | 49 ++++++++++++++++++++++++--------------
>  1 file changed, 31 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index a4bc74e..4a8ce20 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -919,14 +919,15 @@ static void connect(struct backend_info *be)
>  /*
>   * Each ring may have multi pages, depends on "ring-page-order".
>   */
> -static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
> +static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir,
> +			      bool use_ring_page_order)

Why not change the order of read_per_ring_refs so that it tries to
fetch the grant references from "ring-refXX" first and then fallback
to reading the legacy grant reference if nr_ring_pages == 1?

This will avoid having to pass an extra parameter to it.

>  {
>  	unsigned int ring_ref[XENBUS_MAX_RING_GRANTS];
>  	struct pending_req *req, *n;
>  	int err, i, j;
>  	struct xen_blkif *blkif = ring->blkif;
>  	struct xenbus_device *dev = blkif->be->dev;
> -	unsigned int ring_page_order, nr_grefs, evtchn;
> +	unsigned int nr_grefs, evtchn;
>  
>  	err = xenbus_scanf(XBT_NIL, dir, "event-channel", "%u",
>  			  &evtchn);
> @@ -936,28 +937,18 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
>  		return err;
>  	}
>  
> -	err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
> -			  &ring_page_order);
> -	if (err != 1) {
> +	nr_grefs = blkif->nr_ring_pages;
> +
> +	if (!use_ring_page_order) {
>  		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u", &ring_ref[0]);
>  		if (err != 1) {
>  			err = -EINVAL;
>  			xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
>  			return err;
>  		}
> -		nr_grefs = 1;
>  	} else {
>  		unsigned int i;
>  
> -		if (ring_page_order > xen_blkif_max_ring_order) {
> -			err = -EINVAL;
> -			xenbus_dev_fatal(dev, err, "%s/request %d ring page order exceed max:%d",
> -					 dir, ring_page_order,
> -					 xen_blkif_max_ring_order);
> -			return err;
> -		}
> -
> -		nr_grefs = 1 << ring_page_order;
>  		for (i = 0; i < nr_grefs; i++) {
>  			char ring_ref_name[RINGREF_NAME_LEN];
>  
> @@ -972,7 +963,6 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
>  			}
>  		}
>  	}
> -	blkif->nr_ring_pages = nr_grefs;
>  
>  	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
>  		req = kzalloc(sizeof(*req), GFP_KERNEL);
> @@ -1030,6 +1020,8 @@ static int connect_ring(struct backend_info *be)
>  	size_t xspathsize;
>  	const size_t xenstore_path_ext_size = 11; /* sufficient for "/queue-NNN" */
>  	unsigned int requested_num_queues = 0;
> +	bool use_ring_page_order = false;
> +	unsigned int ring_page_order;
>  
>  	pr_debug("%s %s\n", __func__, dev->otherend);
>  
> @@ -1075,8 +1067,28 @@ static int connect_ring(struct backend_info *be)
>  		 be->blkif->nr_rings, be->blkif->blk_protocol, protocol,
>  		 pers_grants ? "persistent grants" : "");
>  
> +	err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
> +			   &ring_page_order);

You can use xenbus_read_unsigned IMO, which will simplify the logic
below a little bit.

Thanks, Roger.