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 <dongli.zhang@xxxxxxxxxx>]

On 01/07/2019 10:05 PM, Dongli Zhang wrote:
> 
> 
> On 01/07/2019 08:01 PM, Roger Pau Monné 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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>> https://lists.xenproject.org/mailman/listinfo/xen-devel
>>
> 
> Dongli Zhang
> 

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