From 90b6c3f92ab346ec0d6fbe2552cfb649e3ea0f28 Mon Sep 17 00:00:00 2001
From: "Joshua M. Clulow" <jmc@joyent.com>
Date: Fri, 5 Feb 2016 23:07:30 +0000
Subject: [PATCH] HVM-842 QEMU might succumb to compiler enthusiasm and read
 twice (CVE-2015-8550)

---
 hw/xen_blkif.h | 20 ++++++++++++++++----
 hw/xenfb.c     | 10 ++++++----
 2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/hw/xen_blkif.h b/hw/xen_blkif.h
index c0f4136..a0c85c4 100644
--- a/hw/xen_blkif.h
+++ b/hw/xen_blkif.h
@@ -79,8 +79,14 @@ 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;
+
+	/*
+	 * Ensure we use a consistent view of "nr_segments", which is shared
+	 * with the guest:
+	 */
+	barrier();
+	if (n > dst->nr_segments)
+		n = dst->nr_segments;
 	for (i = 0; i < n; i++)
 		dst->seg[i] = src->seg[i];
 }
@@ -94,8 +100,14 @@ 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;
+
+	/*
+	 * Ensure we use a consistent view of "nr_segments", which is shared
+	 * with the guest:
+	 */
+	barrier();
+	if (n > dst->nr_segments)
+		n = dst->nr_segments;
 	for (i = 0; i < n; i++)
 		dst->seg[i] = src->seg[i];
 }
diff --git a/hw/xenfb.c b/hw/xenfb.c
index da5297b..762237e 100644
--- a/hw/xenfb.c
+++ b/hw/xenfb.c
@@ -787,18 +787,20 @@ static void xenfb_invalidate(void *opaque)
 
 static void xenfb_handle_events(struct XenFB *xenfb)
 {
-    uint32_t prod, cons;
+    uint32_t prod, cons, out_cons;
     struct xenfb_page *page = xenfb->c.page;
 
     prod = page->out_prod;
-    if (prod == page->out_cons)
+    out_cons = page->out_cons;
+    if (prod == out_cons)
 	return;
     xen_rmb();		/* ensure we see ring contents up to prod */
-    for (cons = page->out_cons; cons != prod; cons++) {
+    for (cons = out_cons; cons != prod; cons++) {
 	union xenfb_out_event *event = &XENFB_OUT_RING_REF(page, cons);
+	uint8_t type = event->type;
 	int x, y, w, h;
 
-	switch (event->type) {
+	switch (type) {
 	case XENFB_TYPE_UPDATE:
 	    if (xenfb->up_count == UP_QUEUE)
 		xenfb->up_fullscreen = 1;