Commit d75e7f63 authored by Paul Durrant's avatar Paul Durrant Committed by Juergen Gross
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xen-blkback: fix compatibility bug with single page rings 



Prior to commit 4a8c31a1 ("xen/blkback: rework connect_ring() to avoid
inconsistent xenstore 'ring-page-order' set by malicious blkfront"), the
behaviour of xen-blkback when connecting to a frontend was:

- read 'ring-page-order'
- if not present then expect a single page ring specified by 'ring-ref'
- else expect a ring specified by 'ring-refX' where X is between 0 and
  1 << ring-page-order

This was correct behaviour, but was broken by the afforementioned commit to
become:

- read 'ring-page-order'
- if not present then expect a single page ring (i.e. ring-page-order = 0)
- expect a ring specified by 'ring-refX' where X is between 0 and
  1 << ring-page-order
- if that didn't work then see if there's a single page ring specified by
  'ring-ref'

This incorrect behaviour works most of the time but fails when a frontend
that sets 'ring-page-order' is unloaded and replaced by one that does not
because, instead of reading 'ring-ref', xen-blkback will read the stale
'ring-ref0' left around by the previous frontend will try to map the wrong
grant reference.

This patch restores the original behaviour.

Fixes: 4a8c31a1 ("xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront")
Signed-off-by: default avatarPaul Durrant <pdurrant@amazon.com>
Reviewed-by: default avatarDongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: default avatar"Roger Pau Monné" <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20210202175659.18452-1-paul@xen.org


Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
parent 01325044

drivers/block/xen-blkback/common.h

+1 −0
Original line number Diff line number Diff line
@@ -313,6 +313,7 @@ struct xen_blkif { 


	struct work_struct	free_work;

	unsigned int 		nr_ring_pages;

	bool			multi_ref;

	/* All rings for this device. */

	struct xen_blkif_ring	*rings;

	unsigned int		nr_rings;

drivers/block/xen-blkback/xenbus.c

+16 −22
Original line number Diff line number Diff line
@@ -998,14 +998,17 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir) 
	for (i = 0; i < nr_grefs; i++) {

		char ring_ref_name[RINGREF_NAME_LEN];



		if (blkif->multi_ref)

			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);

		else {

			WARN_ON(i != 0);

			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref");

		}



		err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,

				   "%u", &ring_ref[i]);



		if (err != 1) {

			if (nr_grefs == 1)

				break;



			err = -EINVAL;

			xenbus_dev_fatal(dev, err, "reading %s/%s",

					 dir, ring_ref_name);
@@ -1013,18 +1016,6 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir) 
		}

	}



	if (err != 1) {

		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;

		}

	}



	err = -ENOMEM;

	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {

		req = kzalloc(sizeof(*req), GFP_KERNEL);
@@ -1129,10 +1120,15 @@ static int connect_ring(struct backend_info *be) 
		 blkif->nr_rings, blkif->blk_protocol, protocol,

		 blkif->vbd.feature_gnt_persistent ? "persistent grants" : "");



	ring_page_order = xenbus_read_unsigned(dev->otherend,

					       "ring-page-order", 0);



	if (ring_page_order > xen_blkif_max_ring_order) {

	err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",

			   &ring_page_order);

	if (err != 1) {

		blkif->nr_ring_pages = 1;

		blkif->multi_ref = false;

	} else if (ring_page_order <= xen_blkif_max_ring_order) {

		blkif->nr_ring_pages = 1 << ring_page_order;

		blkif->multi_ref = true;

	} else {

		err = -EINVAL;

		xenbus_dev_fatal(dev, err,

				 "requested ring page order %d exceed max:%d",
@@ -1141,8 +1137,6 @@ static int connect_ring(struct backend_info *be) 
		return err;

	}



	blkif->nr_ring_pages = 1 << ring_page_order;



	if (blkif->nr_rings == 1)

		return read_per_ring_refs(&blkif->rings[0], dev->otherend);

	else {