From: Dongli Zhang <dongli.zhang@oracle.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>, Paul.Durrant@citrix.com
Cc: axboe@kernel.dk, konrad.wilk@oracle.com,
linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
xen-devel@lists.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
Date: Mon, 7 Jan 2019 22:05:46 +0800 [thread overview]
Message-ID: <35c7d495-2d6c-5fec-abf8-c5aef55cf866@oracle.com> (raw)
In-Reply-To: <20190107120107.euf7mrq7gk6bmibz@mac>
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@oracle.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.
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@lists.xenproject.org
> https://lists.xenproject.org/mailman/listinfo/xen-devel
>
Dongli Zhang
next prev parent reply other threads:[~2019-01-07 14:06 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-07 5:35 [PATCH v4 1/2] xen/blkback: add stack variable 'blkif' in connect_ring() Dongli Zhang
2019-01-07 5:35 ` [PATCH v4 2/2] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront Dongli Zhang
2019-01-07 9:18 ` Paul Durrant
2019-01-07 12:01 ` Roger Pau Monné
2019-01-07 14:05 ` Dongli Zhang [this message]
2019-01-07 14:07 ` [Xen-devel] " Dongli Zhang
2019-01-07 15:27 ` Roger Pau Monné
2019-01-08 9:52 ` Dongli Zhang
2019-01-11 14:57 ` Roger Pau Monné
2019-01-07 9:11 ` [PATCH v4 1/2] xen/blkback: add stack variable 'blkif' in connect_ring() Paul Durrant
2019-01-07 11:53 ` Roger Pau Monné
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