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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>
This commit is contained in:
Stefano Stabellini 2015-12-18 15:10:09 +00:00
parent f9e98e5d7a
commit 7ea11bf376
1 changed files with 6 additions and 4 deletions
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10
hw/display/xenfb.c
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@ -784,18 +784,20 @@ static void xenfb_invalidate(void *opaque)
static void xenfb_handle_events(struct XenFB *xenfb) 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; struct xenfb_page *page = xenfb->c.page;
prod = page->out_prod; prod = page->out_prod;
if (prod == page->out_cons) out_cons = page->out_cons;
if (prod == out_cons)
return; return;
xen_rmb(); /* ensure we see ring contents up to prod */ 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); union xenfb_out_event *event = &XENFB_OUT_RING_REF(page, cons);
uint8_t type = event->type;
int x, y, w, h; int x, y, w, h;
switch (event->type) { switch (type) {
case XENFB_TYPE_UPDATE: case XENFB_TYPE_UPDATE:
if (xenfb->up_count == UP_QUEUE) if (xenfb->up_count == UP_QUEUE)
xenfb->up_fullscreen = 1; xenfb->up_fullscreen = 1;