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

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

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