Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Dec 18, 2018 at 08:55:38AM +0800, 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@xxxxxxxxxx>
> ---
> 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".

>                       err = -EINVAL;

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

> -                     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).

The rest LGTM, Thanks.

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