xen/disk: don't leak stack data via response ring

Rather than constructing a local structure instance on the stack, fill
the fields directly on the shared ring, just like other (Linux)
backends do. Build on the fact that all response structure flavors are
actually identical (aside from alignment and padding at the end).

This is XSA-216.

Reported by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>

hw/block/xen_disk.c

@@ -769,31 +769,30 @@ static int blk_send_response_one(struct ioreq *ioreq)
     struct XenBlkDev  *blkdev = ioreq->blkdev;
     int               send_notify   = 0;
     int               have_requests = 0;
-    blkif_response_t  resp;
-    void              *dst;
-
-    resp.id        = ioreq->req.id;
-    resp.operation = ioreq->req.operation;
-    resp.status    = ioreq->status;
+    blkif_response_t  *resp;
 
     /* Place on the response ring for the relevant domain. */
     switch (blkdev->protocol) {
     case BLKIF_PROTOCOL_NATIVE:
-        dst = RING_GET_RESPONSE(&blkdev->rings.native, blkdev->rings.native.rsp_prod_pvt);
+        resp = (blkif_response_t *) RING_GET_RESPONSE(&blkdev->rings.native,
+                                 blkdev->rings.native.rsp_prod_pvt);
         break;
     case BLKIF_PROTOCOL_X86_32:
-        dst = RING_GET_RESPONSE(&blkdev->rings.x86_32_part,
-                                blkdev->rings.x86_32_part.rsp_prod_pvt);
+        resp = (blkif_response_t *) RING_GET_RESPONSE(&blkdev->rings.x86_32_part,
+                                 blkdev->rings.x86_32_part.rsp_prod_pvt);
         break;
     case BLKIF_PROTOCOL_X86_64:
-        dst = RING_GET_RESPONSE(&blkdev->rings.x86_64_part,
-                                blkdev->rings.x86_64_part.rsp_prod_pvt);
+        resp = (blkif_response_t *) RING_GET_RESPONSE(&blkdev->rings.x86_64_part,
+                                 blkdev->rings.x86_64_part.rsp_prod_pvt);
         break;
     default:
-        dst = NULL;
         return 0;
     }
-    memcpy(dst, &resp, sizeof(resp));
+
+    resp->id        = ioreq->req.id;
+    resp->operation = ioreq->req.operation;
+    resp->status    = ioreq->status;
+
     blkdev->rings.common.rsp_prod_pvt++;
 
     RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&blkdev->rings.common, send_notify);