xenfb: avoid reading twice the same fields from the shared page

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@eu.citrix.com>

diff --git a/hw/xenfb.c b/hw/xenfb.c
index 442a63a320cdf9d251881652c239cabd8cff17fd..6251353efa7173a5c71fc86af9011b6b63bb1410 100644 (file)
--- a/hw/xenfb.c
+++ b/hw/xenfb.c
@@ -798,18 +798,20 @@ 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;