On 12/18/2018 11:13 PM, Roger Pau Monné wrote:

On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote:

quoted

Hi Roger,

On 12/18/2018 05:33 PM, Roger Pau Monné wrote:

quoted

On Tue, Dec 18, 2018 at 08:55:38AM +0800, Dongli Zhang wrote:

quoted

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 <redacted>
---
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".

quoted

 			err = -EINVAL;

There's no point in setting err here...

quoted

-			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 :).

However, for the "ring-page-order" case, the err used in xenbus_dev_fatal() is
not set as xenbus_read_unsigned() does not return any err?

For "ring-page-order", I would still need to set err = -EINVAL with extra one
line of code?

quoted

+       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!

Dongli Zhang