Re: [Xen-devel] [PATCH v4 2/2] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

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

On Mon, Jan 07, 2019 at 01:35:59PM +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
>   * 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
> 
> Changed since v3:
>   * exit at the beginning if !nr_grefs
>   * change the if statements to avoid test (err != 1) twice
>   * initialize a 'blkif' stack variable (refer to PATCH 1/2)
> 
>  drivers/block/xen-blkback/xenbus.c | 76 
> +++++++++++++++++++++-----------------
>  1 file changed, 43 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/xenbus.c 
> b/drivers/block/xen-blkback/xenbus.c
> index a4aadac..a2acbc9 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]);
> +     nr_grefs = blkif->nr_ring_pages;
> +
> +     if (unlikely(!nr_grefs))
> +             return -EINVAL;

Is this even possible? AFAICT read_per_ring_refs will always be called
with blkif->nr_ring_pages != 0?

If so, I would consider turning this into a BUG_ON/WARN_ON.

> +
> +     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/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;
> +                     if (nr_grefs == 1)
> +                             break;
> +

You need to either set err to EINVAL before calling xenbus_dev_fatal,
or call xenbus_dev_fatal with EINVAL as the second parameter.

> +                     xenbus_dev_fatal(dev, err, "reading %s/%s",
> +                                      dir, ring_ref_name);
> +                     return -EINVAL;
>               }
> +     }
>  
> -             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;
> -                     }
> +     if (err != 1) {
> +             WARN_ON(nr_grefs != 1);
> +
> +             err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u",
> +                                &ring_ref[0]);
> +             if (err != 1) {
> +                     xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);

Second parameter should be EINVAL, or err should be set to EINVAL
before calling xenbus_dev_fatal.

Thanks, Roger.

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