[PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Dongli Zhang]

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(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))) {
 			err = -EINVAL;
-			xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
+			xenbus_dev_fatal(dev, err, "reading %s/%s",
+					 dir, ring_ref_name);
 			return err;
 		}
-		nr_grefs = 1;
-	} else {
-		unsigned int i;
 
-		if (ring_page_order > xen_blkif_max_ring_order) {
+		if (err != 1)
+			break;
+	}
+
+	if (err != 1) {
+		WARN_ON(nr_grefs != 1);
+
+		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u",
+				   &ring_ref[0]);
+		if (err != 1) {
 			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);
+			xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
 			return err;
 		}
-
-		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;
-			}
-		}
 	}
-	blkif->nr_ring_pages = nr_grefs;
 
 	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
 		req = kzalloc(sizeof(*req), GFP_KERNEL);
@@ -1030,6 +1025,7 @@ static int connect_ring(struct backend_info *be)
 	size_t xspathsize;
 	const size_t xenstore_path_ext_size = 11; /* sufficient for "/queue-NNN" */
 	unsigned int requested_num_queues = 0;
+	unsigned int ring_page_order;
 
 	pr_debug("%s %s\n", __func__, dev->otherend);
 
@@ -1075,6 +1071,20 @@ static int connect_ring(struct backend_info *be)
 		 be->blkif->nr_rings, be->blkif->blk_protocol, protocol,
 		 pers_grants ? "persistent grants" : "");
 
+	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;
+
 	if (be->blkif->nr_rings == 1)
 		return read_per_ring_refs(&be->blkif->rings[0], dev->otherend);
 	else {
-- 
2.7.4

Re: [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Roger Pau Monné]

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@oracle.com>
> ---
> 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).

The rest LGTM, Thanks.

Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Roger Pau Monné]

On Tue, Dec 18, 2018 at 10:33:00AM +0100, Roger Pau Monné wrote:
> On Tue, Dec 18, 2018 at 08:55:38AM +0800, Dongli Zhang wrote:
> > +	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".

Sorry, this should be "err != 1 && nr_grefs > 1", since it's not order
but rather the number of grefs.

Roger.

Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Dongli Zhang]

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@oracle.com>
>> ---
>> 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

Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Roger Pau Monné]

On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote:
> 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@oracle.com>
> >> ---
> >> 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().

Right, but instead of doing:

err = -EINVAL;
return err;

You can just do:

return -EINVAL;

Which is one line shorter :).

> +       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!

Thanks!

Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Dongli Zhang]

On 12/18/2018 11:13 PM, Roger Pau Monné wrote:
> On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote:
>> 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@oracle.com>
>>>> ---
>>>> 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().
> 
> Right, but instead of doing:
> 
> err = -EINVAL;
> return err;
> 
> You can just do:
> 
> return -EINVAL;
> 
> Which is one line shorter :).

However, for the "ring-page-order" case, the err used in xenbus_dev_fatal() is
not set as xenbus_read_unsigned() does not return any err?

For "ring-page-order", I would still need to set err = -EINVAL with extra one
line of code?

> 
>> +       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!
> 
> Thanks!
> 

Dongli Zhang

Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Roger Pau Monné]

On Tue, Dec 18, 2018 at 11:29:16PM +0800, Dongli Zhang wrote:
> 
> 
> On 12/18/2018 11:13 PM, Roger Pau Monné wrote:
> > On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote:
> >> 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@oracle.com>
> >>>> ---
> >>>> 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().
> > 
> > Right, but instead of doing:
> > 
> > err = -EINVAL;
> > return err;
> > 
> > You can just do:
> > 
> > return -EINVAL;
> > 
> > Which is one line shorter :).
> 
> However, for the "ring-page-order" case, the err used in xenbus_dev_fatal() is
> not set as xenbus_read_unsigned() does not return any err?
> 
> For "ring-page-order", I would still need to set err = -EINVAL with extra one
> line of code?

Given this, I don't have a strong opinion, so do as you please.

Thanks, Roger.