From 27942b0cb2327e93deb12326bbe7b36c81f9fa7b Mon Sep 17 00:00:00 2001
From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Date: Fri, 20 Nov 2015 10:56:00 -0500
Subject: [PATCH] blkif: Avoid double access to src->nr_segments

src is stored in shared memory and src->nr_segments is dereferenced
twice at the end of the function.  If a compiler decides to compile this
into two separate memory accesses then the size limitation could be
bypassed.

Fix it by removing the double access to src->nr_segments.

This is part of XSA-155.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 tools/qemu-xen-traditional/hw/xen_blkif.h | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/tools/qemu-xen-traditional/hw/xen_blkif.h b/tools/qemu-xen-traditional/hw/xen_blkif.h
index ca3a65b..eb29cb1 100644
--- a/tools/qemu-xen-traditional/hw/xen_blkif.h
+++ b/tools/qemu-xen-traditional/hw/xen_blkif.h
@@ -79,8 +79,10 @@ static inline void blkif_get_x86_32_req(blkif_request_t *dst, blkif_x86_32_reque
 	dst->handle = src->handle;
 	dst->id = src->id;
 	dst->sector_number = src->sector_number;
-	if (n > src->nr_segments)
-		n = src->nr_segments;
+	/* prevent the compiler from optimizing the code and using src->nr_segments instead */
+	xen_mb();
+	if (n > dst->nr_segments)
+		n = dst->nr_segments;
 	for (i = 0; i < n; i++)
 		dst->seg[i] = src->seg[i];
 }
@@ -94,8 +96,10 @@ static inline void blkif_get_x86_64_req(blkif_request_t *dst, blkif_x86_64_reque
 	dst->handle = src->handle;
 	dst->id = src->id;
 	dst->sector_number = src->sector_number;
-	if (n > src->nr_segments)
-		n = src->nr_segments;
+	/* prevent the compiler from optimizing the code and using src->nr_segments instead */
+	xen_mb();
+	if (n > dst->nr_segments)
+		n = dst->nr_segments;
 	for (i = 0; i < n; i++)
 		dst->seg[i] = src->seg[i];
 }
-- 
2.4.3