xen/blkif: avoid double access to any shared ring request fields

Commit f9e98e5d7a ("xen/blkif: Avoid double access to
src->nr_segments") didn't go far enough: src->operation is also being
used twice. And nothing was done to prevent the compiler from using the
source side of the copy done by blk_get_request() (granted that's very
unlikely).

Move the barrier()s up, and add another one to blk_get_request().

Note that for completing XSA-155, the barrier() getting added to
blk_get_request() would suffice, and hence the changes to xen_blkif.h
are more like just cleanup. And since, as said, the unpatched code
getting compiled to something vulnerable is very unlikely (and not
observed in practice), this isn't being viewed as a new security issue.

Signed-off-by: Jan Beulich <address@hidden>

--- a/hw/block/xen_blkif.h
+++ b/hw/block/xen_blkif.h
@@ -79,14 +79,14 @@ static inline void blkif_get_x86_32_req(
 	dst->handle = src->handle;
 	dst->id = src->id;
 	dst->sector_number = src->sector_number;
-	if (src->operation == BLKIF_OP_DISCARD) {
+	/* Prevent the compiler from using src->... instead. */
+	barrier();
+	if (dst->operation == BLKIF_OP_DISCARD) {
 		struct blkif_request_discard *s = (void *)src;
 		struct blkif_request_discard *d = (void *)dst;
 		d->nr_sectors = s->nr_sectors;
 		return;
 	}
-	/* prevent the compiler from optimizing the code and using src->nr_segments instead */
-	barrier();
 	if (n > dst->nr_segments)
 		n = dst->nr_segments;
 	for (i = 0; i < n; i++)
@@ -102,14 +102,14 @@ static inline void blkif_get_x86_64_req(
 	dst->handle = src->handle;
 	dst->id = src->id;
 	dst->sector_number = src->sector_number;
-	if (src->operation == BLKIF_OP_DISCARD) {
+	/* Prevent the compiler from using src->... instead. */
+	barrier();
+	if (dst->operation == BLKIF_OP_DISCARD) {
 		struct blkif_request_discard *s = (void *)src;
 		struct blkif_request_discard *d = (void *)dst;
 		d->nr_sectors = s->nr_sectors;
 		return;
 	}
-	/* prevent the compiler from optimizing the code and using src->nr_segments instead */
-	barrier();
 	if (n > dst->nr_segments)
 		n = dst->nr_segments;
 	for (i = 0; i < n; i++)
--- a/hw/block/xen_disk.c
+++ b/hw/block/xen_disk.c
@@ -679,6 +679,8 @@ static int blk_get_request(struct XenBlk
                              RING_GET_REQUEST(&blkdev->rings.x86_64_part, rc));
         break;
     }
+    /* Prevent the compiler from accessing the on-ring fields instead. */
+    barrier();
     return 0;
 }