Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote:
> Hi Roger,
> 
> On 12/18/2018 05:33 PM, Roger Pau Monné wrote:
> > On Tue, Dec 18, 2018 at 08:55:38AM +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@oracle.com>
> >> ---
> >> Changed since v1:
> >>   * change the order of xenstore read in read_per_ring_refs(suggested by Roger Pau Monne)
> >>   * use xenbus_read_unsigned() in connect_ring() (suggested by Roger Pau Monne)
> >>
> >>  drivers/block/xen-blkback/xenbus.c | 70 ++++++++++++++++++++++----------------
> >>  1 file changed, 40 insertions(+), 30 deletions(-)
> >>
> >> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> >> index a4bc74e..7178f0f 100644
> >> --- a/drivers/block/xen-blkback/xenbus.c
> >> +++ b/drivers/block/xen-blkback/xenbus.c
> >> @@ -926,7 +926,7 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
> >>  	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);
> >> @@ -936,43 +936,38 @@ 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) {
> >> -		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u", &ring_ref[0]);
> >> -		if (err != 1) {
> >> +	nr_grefs = blkif->nr_ring_pages;
> >> +	WARN_ON(!nr_grefs);
> >> +
> >> +	for (i = 0; i < nr_grefs; i++) {
> >> +		char ring_ref_name[RINGREF_NAME_LEN];
> >> +
> >> +		snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> >> +		err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
> >> +				   "%u", &ring_ref[i]);
> >> +
> >> +		if (err != 1 && (i || (!i && nr_grefs > 1))) {
> > 
> > AFAICT the above condition can be simplified as "err != 1 &&
> > nr_grefs".
> > 
> >>  			err = -EINVAL;
> > 
> > There's no point in setting err here...
> > 
> >> -			xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
> >> +			xenbus_dev_fatal(dev, err, "reading %s/%s",
> >> +					 dir, ring_ref_name);
> >>  			return err;
> > 
> > ...since you can just return -EINVAL (same applies to the other
> > instance below).
> 
> I would like to confirm if I would keep the err = -EINVAL in below because most
> of the below code is copied from original implementation without modification.
> 
> There is no err set by xenbus_read_unsigned().

Right, but instead of doing:

err = -EINVAL;
return err;

You can just do:

return -EINVAL;

Which is one line shorter :).

> +       ring_page_order = xenbus_read_unsigned(dev->otherend,
> +                                              "ring-page-order", 0);
> +
> +       if (ring_page_order > xen_blkif_max_ring_order) {
> +               err = -EINVAL;
> +               xenbus_dev_fatal(dev, err,
> +                                "requested ring page order %d exceed max:%d",
> +                                ring_page_order,
> +                                xen_blkif_max_ring_order);
> +               return err;
> +       }
> +
> +       be->blkif->nr_ring_pages = 1 << ring_page_order;
> 
> 
> For the rest, I would do something like:
> 
> +               err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
> +                                  "%u", &ring_ref[i]);
> +
> +               if (err != 1 && nr_grefs > 1) {
> +                       xenbus_dev_fatal(dev, err, "reading %s/%s",
> +                                        dir, ring_ref_name);
> +                       return -EINVAL;
> +               }
> 
> 
> Thank you very much!

Thanks!