[Xen-devel] [PULL 2/2] xenfb: avoid reading twice the same fields from the shared page

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

Reading twice the same field could give the guest an attack of
opportunity. In the case of event->type, gcc could compile the switch
statement into a jump table, effectively ending up reading the type
field multiple times.

This is part of XSA-155.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
---
 hw/display/xenfb.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
index 5e324ef..4e2a27a 100644
--- a/hw/display/xenfb.c
+++ b/hw/display/xenfb.c
@@ -784,18 +784,20 @@ 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;
-- 
1.7.10.4


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