RE: [PATCH v3 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> On 12/19/2018 09:23 PM, 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
> >   * use xenbus_read_unsigned() in connect_ring()
> >
> > Changed since v2:
> >   * simplify the condition check as "(err != 1 && nr_grefs > 1)"
> >   * avoid setting err as -EINVAL to remove extra one line of code
> >
> >  drivers/block/xen-blkback/xenbus.c | 74 +++++++++++++++++++++----------
> -------
> >  1 file changed, 41 insertions(+), 33 deletions(-)
> >
> > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-
> blkback/xenbus.c
> > index a4bc74e..dfea3a4 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 +936,36 @@ 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);
> > +	nr_grefs = blkif->nr_ring_pages;
> > +	WARN_ON(!nr_grefs);

Why not exit if !nr_grefs? There's nothing useful for this function to do in that case.

> > +
> > +	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 && nr_grefs > 1) {
> > +			xenbus_dev_fatal(dev, err, "reading %s/%s",
> > +					 dir, ring_ref_name);
> > +			return -EINVAL;
> > +		}
> > +
> > +		if (err != 1)
> > +			break;

Seems odd to test (err != 1) twice. I'd prefer:

if (err != 1) {
    if (nr_grefs == 1)
        break;

    <fatal error exit>
}

Either that or simply break if err != 1 and then...

> > +	}
> > +
> >  	if (err != 1) {

...add a check and fatal error exit here if nr_grefs != 1.

> > -		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u",
> &ring_ref[0]);
> > +		WARN_ON(nr_grefs != 1);
> > +
> > +		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];
> > -
> > -			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) {
> > -				err = -EINVAL;
> > -				xenbus_dev_fatal(dev, err, "reading %s/%s",
> > -						 dir, ring_ref_name);
> > -				return err;
> > -			}
> > +			return -EINVAL;
> >  		}
> >  	}
> > -	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 +1023,7 @@ 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;
> > +	unsigned int ring_page_order;
> >
> >  	pr_debug("%s %s\n", __func__, dev->otherend);
> >
> > @@ -1075,6 +1069,20 @@ static int connect_ring(struct backend_info *be)
> >  		 be->blkif->nr_rings, be->blkif->blk_protocol, protocol,
> >  		 pers_grants ? "persistent grants" : "");
> >
> > +	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;
> > +

That's one more be->blkif. I think it's overdue to initialize a 'blkif' stack variable in this function and use that.

  Paul

> >  	if (be->blkif->nr_rings == 1)
> >  		return read_per_ring_refs(&be->blkif->rings[0], dev-
> >otherend);
> >  	else {
> >