Re: [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@citrix.com>]

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@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
> 
> 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.