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SubjectRE: [Xen-devel] [PATCH 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront
Date
> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@lists.xenproject.org] On Behalf
> Of Dongli Zhang
> Sent: 07 December 2018 04:18
> To: linux-kernel@vger.kernel.org; xen-devel@lists.xenproject.org; linux-
> block@vger.kernel.org
> Cc: axboe@kernel.dk; Roger Pau Monne <roger.pau@citrix.com>;
> konrad.wilk@oracle.com
> Subject: [Xen-devel] [PATCH 1/1] xen/blkback: rework connect_ring() to
> avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront
>
> 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.

That is certainly sub-optimal.

>
> 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.

I can't actually see what useful function blkif->nr_ring_pages actually performs any more. Perhaps you could actually get rid of it?

>
> This patch reworks connect_ring() to read xenstore 'ring-page-order' only
> once.

That is certainly a good thing :-)

Paul

>
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> ---
> 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)
> {
> 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);
> +
> + if (err != 1) {
> + be->blkif->nr_ring_pages = 1;
> + } else {
> + 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;
> + }
> +
> + use_ring_page_order = true;
> + be->blkif->nr_ring_pages = 1 << ring_page_order;
> + }
> +
> if (be->blkif->nr_rings == 1)
> - return read_per_ring_refs(&be->blkif->rings[0], dev-
> >otherend);
> + return read_per_ring_refs(&be->blkif->rings[0], dev->otherend,
> + use_ring_page_order);
> else {
> xspathsize = strlen(dev->otherend) + xenstore_path_ext_size;
> xspath = kmalloc(xspathsize, GFP_KERNEL);
> @@ -1088,7 +1100,8 @@ static int connect_ring(struct backend_info *be)
> for (i = 0; i < be->blkif->nr_rings; i++) {
> memset(xspath, 0, xspathsize);
> snprintf(xspath, xspathsize, "%s/queue-%u", dev-
> >otherend, i);
> - err = read_per_ring_refs(&be->blkif->rings[i], xspath);
> + err = read_per_ring_refs(&be->blkif->rings[i], xspath,
> + use_ring_page_order);
> if (err) {
> kfree(xspath);
> return err;
> --
> 2.7.4
>
>
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> Xen-devel mailing list
> Xen-devel@lists.xenproject.org
> https://lists.xenproject.org/mailman/listinfo/xen-devel
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