diff mbox series

[v2,2/3] xen/blkfront: don't take local copy of a request from the ring page

Message ID 20210708124345.10173-3-jgross@suse.com (mailing list archive)
State New, archived
Headers show
Series xen: harden blkfront against malicious backends | expand

Commit Message

Juergen Gross July 8, 2021, 12:43 p.m. UTC
In order to avoid a malicious backend being able to influence the local
copy of a request build the request locally first and then copy it to
the ring page instead of doing it the other way round as today.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
V2:
- init variable to avoid potential compiler warning (Jan Beulich)
---
 drivers/block/xen-blkfront.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

Comments

Roger Pau Monné July 9, 2021, 8:55 a.m. UTC | #1
On Thu, Jul 08, 2021 at 02:43:44PM +0200, Juergen Gross wrote:
> In order to avoid a malicious backend being able to influence the local
> copy of a request build the request locally first and then copy it to
> the ring page instead of doing it the other way round as today.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Roger Pau Monné <roger.pau@citrx.com>

Thanks!

One unrelated comment below.

> ---
> V2:
> - init variable to avoid potential compiler warning (Jan Beulich)
> ---
>  drivers/block/xen-blkfront.c | 25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 86356014d35e..80701860870a 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -546,7 +546,7 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo,
>  	rinfo->shadow[id].status = REQ_WAITING;
>  	rinfo->shadow[id].associated_id = NO_ASSOCIATED_ID;
>  
> -	(*ring_req)->u.rw.id = id;
> +	rinfo->shadow[id].req.u.rw.id = id;
>  
>  	return id;
>  }
> @@ -554,11 +554,12 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo,
>  static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_info *rinfo)
>  {
>  	struct blkfront_info *info = rinfo->dev_info;
> -	struct blkif_request *ring_req;
> +	struct blkif_request *ring_req, *final_ring_req;
>  	unsigned long id;
>  
>  	/* Fill out a communications ring structure. */
> -	id = blkif_ring_get_request(rinfo, req, &ring_req);
> +	id = blkif_ring_get_request(rinfo, req, &final_ring_req);
> +	ring_req = &rinfo->shadow[id].req;
>  
>  	ring_req->operation = BLKIF_OP_DISCARD;
>  	ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
> @@ -569,8 +570,8 @@ static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_inf
>  	else
>  		ring_req->u.discard.flag = 0;
>  
> -	/* Keep a private copy so we can reissue requests when recovering. */
> -	rinfo->shadow[id].req = *ring_req;
> +	/* Copy the request to the ring page. */
> +	*final_ring_req = *ring_req;
>  
>  	return 0;
>  }
> @@ -703,6 +704,7 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
>  {
>  	struct blkfront_info *info = rinfo->dev_info;
>  	struct blkif_request *ring_req, *extra_ring_req = NULL;
> +	struct blkif_request *final_ring_req, *final_extra_ring_req = NULL;
>  	unsigned long id, extra_id = NO_ASSOCIATED_ID;
>  	bool require_extra_req = false;
>  	int i;
> @@ -747,7 +749,8 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
>  	}
>  
>  	/* Fill out a communications ring structure. */
> -	id = blkif_ring_get_request(rinfo, req, &ring_req);
> +	id = blkif_ring_get_request(rinfo, req, &final_ring_req);
> +	ring_req = &rinfo->shadow[id].req;
>  
>  	num_sg = blk_rq_map_sg(req->q, req, rinfo->shadow[id].sg);
>  	num_grant = 0;
> @@ -798,7 +801,9 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
>  		ring_req->u.rw.nr_segments = num_grant;
>  		if (unlikely(require_extra_req)) {
>  			extra_id = blkif_ring_get_request(rinfo, req,
> -							  &extra_ring_req);
> +							  &final_extra_ring_req);
> +			extra_ring_req = &rinfo->shadow[extra_id].req;

I'm slightly confused about this extra request stuff because I cannot
find any check that asserts we have two empty slots on the ring before
getting here (I only see a RING_FULL check in blkif_queue_rq).

This is AFAIK only used on Arm when guest page size > 4KB.

Roger.
Juergen Gross July 9, 2021, 1:54 p.m. UTC | #2
On 09.07.21 10:55, Roger Pau Monné wrote:
> On Thu, Jul 08, 2021 at 02:43:44PM +0200, Juergen Gross wrote:
>> In order to avoid a malicious backend being able to influence the local
>> copy of a request build the request locally first and then copy it to
>> the ring page instead of doing it the other way round as today.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> 
> Acked-by: Roger Pau Monné <roger.pau@citrx.com>
> 
> Thanks!
> 
> One unrelated comment below.
> 

...

>> @@ -798,7 +801,9 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
>>   		ring_req->u.rw.nr_segments = num_grant;
>>   		if (unlikely(require_extra_req)) {
>>   			extra_id = blkif_ring_get_request(rinfo, req,
>> -							  &extra_ring_req);
>> +							  &final_extra_ring_req);
>> +			extra_ring_req = &rinfo->shadow[extra_id].req;
> 
> I'm slightly confused about this extra request stuff because I cannot
> find any check that asserts we have two empty slots on the ring before
> getting here (I only see a RING_FULL check in blkif_queue_rq).
> 
> This is AFAIK only used on Arm when guest page size > 4KB.

I agree that this is a bug and should be fixed.


Juergen
diff mbox series

Patch

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 86356014d35e..80701860870a 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -546,7 +546,7 @@  static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo,
 	rinfo->shadow[id].status = REQ_WAITING;
 	rinfo->shadow[id].associated_id = NO_ASSOCIATED_ID;
 
-	(*ring_req)->u.rw.id = id;
+	rinfo->shadow[id].req.u.rw.id = id;
 
 	return id;
 }
@@ -554,11 +554,12 @@  static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo,
 static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_info *rinfo)
 {
 	struct blkfront_info *info = rinfo->dev_info;
-	struct blkif_request *ring_req;
+	struct blkif_request *ring_req, *final_ring_req;
 	unsigned long id;
 
 	/* Fill out a communications ring structure. */
-	id = blkif_ring_get_request(rinfo, req, &ring_req);
+	id = blkif_ring_get_request(rinfo, req, &final_ring_req);
+	ring_req = &rinfo->shadow[id].req;
 
 	ring_req->operation = BLKIF_OP_DISCARD;
 	ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
@@ -569,8 +570,8 @@  static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_inf
 	else
 		ring_req->u.discard.flag = 0;
 
-	/* Keep a private copy so we can reissue requests when recovering. */
-	rinfo->shadow[id].req = *ring_req;
+	/* Copy the request to the ring page. */
+	*final_ring_req = *ring_req;
 
 	return 0;
 }
@@ -703,6 +704,7 @@  static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 {
 	struct blkfront_info *info = rinfo->dev_info;
 	struct blkif_request *ring_req, *extra_ring_req = NULL;
+	struct blkif_request *final_ring_req, *final_extra_ring_req = NULL;
 	unsigned long id, extra_id = NO_ASSOCIATED_ID;
 	bool require_extra_req = false;
 	int i;
@@ -747,7 +749,8 @@  static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 	}
 
 	/* Fill out a communications ring structure. */
-	id = blkif_ring_get_request(rinfo, req, &ring_req);
+	id = blkif_ring_get_request(rinfo, req, &final_ring_req);
+	ring_req = &rinfo->shadow[id].req;
 
 	num_sg = blk_rq_map_sg(req->q, req, rinfo->shadow[id].sg);
 	num_grant = 0;
@@ -798,7 +801,9 @@  static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 		ring_req->u.rw.nr_segments = num_grant;
 		if (unlikely(require_extra_req)) {
 			extra_id = blkif_ring_get_request(rinfo, req,
-							  &extra_ring_req);
+							  &final_extra_ring_req);
+			extra_ring_req = &rinfo->shadow[extra_id].req;
+
 			/*
 			 * Only the first request contains the scatter-gather
 			 * list.
@@ -840,10 +845,10 @@  static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 	if (setup.segments)
 		kunmap_atomic(setup.segments);
 
-	/* Keep a private copy so we can reissue requests when recovering. */
-	rinfo->shadow[id].req = *ring_req;
+	/* Copy request(s) to the ring page. */
+	*final_ring_req = *ring_req;
 	if (unlikely(require_extra_req))
-		rinfo->shadow[extra_id].req = *extra_ring_req;
+		*final_extra_ring_req = *extra_ring_req;
 
 	if (new_persistent_gnts)
 		gnttab_free_grant_references(setup.gref_head);