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
