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



> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf
> Of Dongli Zhang
> Sent: 07 December 2018 04:18
> To: linux-kernel@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; linux-
> block@xxxxxxxxxxxxxxx
> Cc: axboe@xxxxxxxxx; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
> konrad.wilk@xxxxxxxxxx
> 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@xxxxxxxxxx>
> ---
>  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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