[PATCH v6 1/2] xen/blkback: add stack variable 'blkif' in connect_ring()

[PATCH v6 1/2] xen/blkback: add stack variable 'blkif' in connect_ring()

[Dongli Zhang]

As 'be->blkif' is used for many times in connect_ring(), the stack variable
'blkif' is added to substitute 'be-blkif'.

Suggested-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
---
 drivers/block/xen-blkback/xenbus.c | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index a4bc74e..a4aadac 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -1023,6 +1023,7 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 static int connect_ring(struct backend_info *be)
 {
 	struct xenbus_device *dev = be->dev;
+	struct xen_blkif *blkif = be->blkif;
 	unsigned int pers_grants;
 	char protocol[64] = "";
 	int err, i;
@@ -1033,25 +1034,25 @@ static int connect_ring(struct backend_info *be)
 
 	pr_debug("%s %s\n", __func__, dev->otherend);
 
-	be->blkif->blk_protocol = BLKIF_PROTOCOL_DEFAULT;
+	blkif->blk_protocol = BLKIF_PROTOCOL_DEFAULT;
 	err = xenbus_scanf(XBT_NIL, dev->otherend, "protocol",
 			   "%63s", protocol);
 	if (err <= 0)
 		strcpy(protocol, "unspecified, assuming default");
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_NATIVE))
-		be->blkif->blk_protocol = BLKIF_PROTOCOL_NATIVE;
+		blkif->blk_protocol = BLKIF_PROTOCOL_NATIVE;
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_32))
-		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_32;
+		blkif->blk_protocol = BLKIF_PROTOCOL_X86_32;
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_64))
-		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_64;
+		blkif->blk_protocol = BLKIF_PROTOCOL_X86_64;
 	else {
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		return -ENOSYS;
 	}
 	pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent",
 					   0);
-	be->blkif->vbd.feature_gnt_persistent = pers_grants;
-	be->blkif->vbd.overflow_max_grants = 0;
+	blkif->vbd.feature_gnt_persistent = pers_grants;
+	blkif->vbd.overflow_max_grants = 0;
 
 	/*
 	 * Read the number of hardware queues from frontend.
@@ -1067,16 +1068,16 @@ static int connect_ring(struct backend_info *be)
 				requested_num_queues, xenblk_max_queues);
 		return -ENOSYS;
 	}
-	be->blkif->nr_rings = requested_num_queues;
-	if (xen_blkif_alloc_rings(be->blkif))
+	blkif->nr_rings = requested_num_queues;
+	if (xen_blkif_alloc_rings(blkif))
 		return -ENOMEM;
 
 	pr_info("%s: using %d queues, protocol %d (%s) %s\n", dev->nodename,
-		 be->blkif->nr_rings, be->blkif->blk_protocol, protocol,
+		 blkif->nr_rings, blkif->blk_protocol, protocol,
 		 pers_grants ? "persistent grants" : "");
 
-	if (be->blkif->nr_rings == 1)
-		return read_per_ring_refs(&be->blkif->rings[0], dev->otherend);
+	if (blkif->nr_rings == 1)
+		return read_per_ring_refs(&blkif->rings[0], dev->otherend);
 	else {
 		xspathsize = strlen(dev->otherend) + xenstore_path_ext_size;
 		xspath = kmalloc(xspathsize, GFP_KERNEL);
@@ -1085,10 +1086,10 @@ static int connect_ring(struct backend_info *be)
 			return -ENOMEM;
 		}
 
-		for (i = 0; i < be->blkif->nr_rings; i++) {
+		for (i = 0; i < blkif->nr_rings; i++) {
 			memset(xspath, 0, xspathsize);
 			snprintf(xspath, xspathsize, "%s/queue-%u", dev->otherend, i);
-			err = read_per_ring_refs(&be->blkif->rings[i], xspath);
+			err = read_per_ring_refs(&blkif->rings[i], xspath);
 			if (err) {
 				kfree(xspath);
 				return err;
-- 
2.7.4

[PATCH v6 2/2] 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
  * 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)

Changed since v4:
  * use BUG_ON() when (nr_grefs == 0) to reminder the developer
  * set err = -EINVAL before xenbus_dev_fatal()

Changed since v5:
  * use WARN_ON() instead of BUG_ON()

 drivers/block/xen-blkback/xenbus.c | 72 +++++++++++++++++++++++---------------
 1 file changed, 43 insertions(+), 29 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index a4aadac..0878e55 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 +936,42 @@ 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)) {
+		WARN_ON(true);
+		return -EINVAL;
+	}
+
+	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) {
+			if (nr_grefs == 1)
+				break;
+
 			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 (err != 1) {
+		WARN_ON(nr_grefs != 1);
 
-		if (ring_page_order > xen_blkif_max_ring_order) {
+		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);
@@ -1031,6 +1030,7 @@ 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);
 
@@ -1076,6 +1076,20 @@ static int connect_ring(struct backend_info *be)
 		 blkif->nr_rings, 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;
+	}
+
+	blkif->nr_ring_pages = 1 << ring_page_order;
+
 	if (blkif->nr_rings == 1)
 		return read_per_ring_refs(&blkif->rings[0], dev->otherend);
 	else {
-- 
2.7.4

Re: [Xen-devel] [PATCH v6 2/2] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Roger Pau Monné]

On Tue, Jan 15, 2019 at 12:41:44AM +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>

LGTM:

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks!

Re: [Xen-devel] [PATCH v6 2/2] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Konrad Rzeszutek Wilk]

On Tue, Jan 15, 2019 at 09:20:36AM +0100, Roger Pau Monné wrote:
> On Tue, Jan 15, 2019 at 12:41:44AM +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>
> 
> LGTM:
> 
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Applied.

Will push out to Jens in a couple of days. Thank you!
> 
> Thanks!

Re: [Xen-devel] [PATCH v6 2/2] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Dongli Zhang]

Hi Konrad,

On 1/17/19 11:29 PM, Konrad Rzeszutek Wilk wrote:
> On Tue, Jan 15, 2019 at 09:20:36AM +0100, Roger Pau Monné wrote:
>> On Tue, Jan 15, 2019 at 12:41:44AM +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>
>>
>> LGTM:
>>
>> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
> 
> Applied.
> 
> Will push out to Jens in a couple of days. Thank you!
>>
>> Thanks!

I only see the PATCH 1/2 (xen/blkback: add stack variable 'blkif' in
connect_ring()) on the top of below branch for Jens, would you please apply this
one (PATCH 2/2) as well?

https://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git/log/?h=linux-next

1/2 is only a prerequisite for 2/2.

Thank you very much!

Dongli Zhang