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 <dongli.zhang@xxxxxxxxxx>]

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().

+       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!

Dongi Zhang

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