[PATCH 4.4 27/52] xen/blkfront: dont take local copy of a request from the ring page

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Juergen Gross <jgross@suse.com>

commit 8f5a695d99000fc3aa73934d7ced33cfc64dcdab upstream.

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@citrix.com>
Link: https://lore.kernel.org/r/20210730103854.12681-3-jgross@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/block/xen-blkfront.c |   38 ++++++++++++++++++++++++++------------
 1 file changed, 26 insertions(+), 12 deletions(-)

--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -456,16 +456,31 @@ static int blkif_ioctl(struct block_devi
 	return 0;
 }
 
+static unsigned long blkif_ring_get_request(struct blkfront_info *info,
+					    struct request *req,
+					    struct blkif_request **ring_req)
+{
+	unsigned long id;
+
+	*ring_req = RING_GET_REQUEST(&info->ring, info->ring.req_prod_pvt);
+	info->ring.req_prod_pvt++;
+
+	id = get_id_from_freelist(info);
+	info->shadow[id].request = req;
+	info->shadow[id].req.u.rw.id = id;
+
+	return id;
+}
+
 static int blkif_queue_discard_req(struct request *req)
 {
 	struct blkfront_info *info = req->rq_disk->private_data;
-	struct blkif_request *ring_req;
+	struct blkif_request *ring_req, *final_ring_req;
 	unsigned long id;
 
 	/* Fill out a communications ring structure. */
-	ring_req = RING_GET_REQUEST(&info->ring, info->ring.req_prod_pvt);
-	id = get_id_from_freelist(info);
-	info->shadow[id].request = req;
+	id = blkif_ring_get_request(info, req, &final_ring_req);
+	ring_req = &info->shadow[id].req;
 
 	ring_req->operation = BLKIF_OP_DISCARD;
 	ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
@@ -478,8 +493,8 @@ static int blkif_queue_discard_req(struc
 
 	info->ring.req_prod_pvt++;
 
-	/* Keep a private copy so we can reissue requests when recovering. */
-	info->shadow[id].req = *ring_req;
+	/* Copy the request to the ring page. */
+	*final_ring_req = *ring_req;
 
 	return 0;
 }
@@ -569,7 +584,7 @@ static void blkif_setup_rw_req_grant(uns
 static int blkif_queue_rw_req(struct request *req)
 {
 	struct blkfront_info *info = req->rq_disk->private_data;
-	struct blkif_request *ring_req;
+	struct blkif_request *ring_req, *final_ring_req;
 	unsigned long id;
 	int i;
 	struct setup_rw_req setup = {
@@ -613,9 +628,8 @@ static int blkif_queue_rw_req(struct req
 		new_persistent_gnts = 0;
 
 	/* Fill out a communications ring structure. */
-	ring_req = RING_GET_REQUEST(&info->ring, info->ring.req_prod_pvt);
-	id = get_id_from_freelist(info);
-	info->shadow[id].request = req;
+	id = blkif_ring_get_request(info, req, &final_ring_req);
+	ring_req = &info->shadow[id].req;
 
 	BUG_ON(info->max_indirect_segments == 0 &&
 	       GREFS(req->nr_phys_segments) > BLKIF_MAX_SEGMENTS_PER_REQUEST);
@@ -696,8 +710,8 @@ static int blkif_queue_rw_req(struct req
 
 	info->ring.req_prod_pvt++;
 
-	/* Keep a private copy so we can reissue requests when recovering. */
-	info->shadow[id].req = *ring_req;
+	/* Copy request(s) to the ring page. */
+	*final_ring_req = *ring_req;
 
 	if (new_persistent_gnts)
 		gnttab_free_grant_references(setup.gref_head);