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

[Dongli Zhang]

On 12/18/2018 11:13 PM, Roger Pau Monné wrote:
> On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote:
>> Hi Roger,
>>
>> On 12/18/2018 05:33 PM, Roger Pau Monné wrote:
>>> 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).
>>
>> I would like to confirm if I would keep the err = -EINVAL in below because most
>> of the below code is copied from original implementation without modification.
>>
>> There is no err set by xenbus_read_unsigned().
> 
> Right, but instead of doing:
> 
> err = -EINVAL;
> return err;
> 
> You can just do:
> 
> return -EINVAL;
> 
> Which is one line shorter :).

However, for the "ring-page-order" case, the err used in xenbus_dev_fatal() is
not set as xenbus_read_unsigned() does not return any err?

For "ring-page-order", I would still need to set err = -EINVAL with extra one
line of code?

> 
>> +       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;
>>
>>
>> For the rest, I would do something like:
>>
>> +               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;
>> +               }
>>
>>
>> Thank you very much!
> 
> Thanks!
> 

Dongli Zhang