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>
---
 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 a8b56c153330..c6a05de4f15f 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;
 	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);
-- 
2.26.2

On 13.05.2021 12:02, 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>
with one remark/question:

> @@ -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;

Without setting final_extra_ring_req to NULL just like is done for
extra_ring_req, ...

> @@ -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;

... are you sure all supported compilers will recognize the
conditional use and not warn about use of a possibly uninitialized
variable?

Jan

On 17.05.21 16:01, Jan Beulich wrote:
> On 13.05.2021 12:02, 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>
> with one remark/question:
> 
>> @@ -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;
> 
> Without setting final_extra_ring_req to NULL just like is done for
> extra_ring_req, ...
> 
>> @@ -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;
> 
> ... are you sure all supported compilers will recognize the
> conditional use and not warn about use of a possibly uninitialized
> variable?

Hmm, probably better safe than sorry. Will change it.


Juergen