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

[Dongli Zhang]

On 01/07/2019 10:05 PM, Dongli Zhang wrote:
> 
> 
> On 01/07/2019 08:01 PM, Roger Pau Monnà wrote:
>> 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.
> 
> It used to be "WARN_ON(!nr_grefs);" in the v3 of the patch.
> 
> I would turn it into WARN_ON if it is fine with both Paul and you.

To clarify, I would use WARN_ON() before exit with -EINVAL (when
blkif->nr_ring_pages is 0).

Dongli Zhang

> 
> I prefer WARN_ON because it would remind the developers in the future that
> read_per_ring_refs() should be used only when blkif->nr_ring_pages != 0.
> 
>>
>>> +
>>> +	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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>>
> 
> Dongli Zhang
>