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Message-ID: <56819579-def2-b045-f414-4de45188fe2e@oracle.com>
Date: Mon, 7 Jan 2019 22:07:34 +0800
From: Dongli Zhang <dongli.zhang@...cle.com>
To: Roger Pau Monné <roger.pau@...rix.com>,
Paul.Durrant@...rix.com
Cc: axboe@...nel.dk, konrad.wilk@...cle.com,
linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
xen-devel@...ts.xenproject.org
Subject: Re: [Xen-devel] [PATCH v4 2/2] xen/blkback: rework connect_ring() to
avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront
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@...cle.com>
>>> ---
>>> 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.
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@...ts.xenproject.org
>> https://lists.xenproject.org/mailman/listinfo/xen-devel
>>
>
> Dongli Zhang
>
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