Hi Roger,
On 01/07/2019 11:27 PM, Roger Pau Monné wrote:
> On Mon, Jan 07, 2019 at 10:07:34PM +0800, Dongli Zhang wrote:
>>
>>
>> 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).
>
> Given that this function will never be called with nr_ring_pages == 0
> I would be fine with just using a BUG_ON, getting here with
> nr_ring_pages == 0 would imply memory corruption or some other severe
> issue has happened, and there's no possible recovery.
>
> If you want to instead keep the return, please use plain WARN instead
> of WARN_ON.
>
> Thanks, Roger.
>
Is there any reason using WARN than WARN_ON? Because of the message printed by
WARN? something like below?
941 if (unlikely(!nr_grefs)) {
942 WARN(!nr_grefs,
943 "read_per_ring_refs() should be called with
blkif->nr_ring_pages != 0");
944 return -EINVAL;
945 }
Thank you very much.
Dongli Zhang
