References: bsc#957988 From 0ffd4547665d2fec648ab2c9ff856c5d9db9b07c Mon Sep 17 00:00:00 2001 From: Stefano Stabellini Date: Fri, 20 Nov 2015 10:37:08 -0500 Subject: [PATCH 2/2] 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 --- hw/xenfb.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) Index: xen-4.6.0-testing/tools/qemu-xen-traditional-dir-remote/hw/xenfb.c =================================================================== --- xen-4.6.0-testing.orig/tools/qemu-xen-traditional-dir-remote/hw/xenfb.c +++ xen-4.6.0-testing/tools/qemu-xen-traditional-dir-remote/hw/xenfb.c @@ -827,18 +827,20 @@ static void xenfb_invalidate(void *opaqu 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;