From f9e98e5d7a67367b862941e339a98b8322fa0cea Mon Sep 17 00:00:00 2001 From: Stefano Stabellini Date: Fri, 18 Dec 2015 15:09:58 +0000 Subject: [PATCH 1/2] xen/blkif: Avoid double access to src->nr_segments src is stored in shared memory and src->nr_segments is dereferenced twice at the end of the function. If a compiler decides to compile this into two separate memory accesses then the size limitation could be bypassed. Fix it by removing the double access to src->nr_segments. This is part of XSA-155. Signed-off-by: Stefano Stabellini --- hw/block/xen_blkif.h | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/hw/block/xen_blkif.h b/hw/block/xen_blkif.h index 711b692742..c68487cb31 100644 --- a/hw/block/xen_blkif.h +++ b/hw/block/xen_blkif.h @@ -85,8 +85,10 @@ static inline void blkif_get_x86_32_req(blkif_request_t *dst, blkif_x86_32_reque d->nr_sectors = s->nr_sectors; return; } - if (n > src->nr_segments) - n = src->nr_segments; + /* prevent the compiler from optimizing the code and using src->nr_segments instead */ + barrier(); + if (n > dst->nr_segments) + n = dst->nr_segments; for (i = 0; i < n; i++) dst->seg[i] = src->seg[i]; } @@ -106,8 +108,10 @@ static inline void blkif_get_x86_64_req(blkif_request_t *dst, blkif_x86_64_reque d->nr_sectors = s->nr_sectors; return; } - if (n > src->nr_segments) - n = src->nr_segments; + /* prevent the compiler from optimizing the code and using src->nr_segments instead */ + barrier(); + if (n > dst->nr_segments) + n = dst->nr_segments; for (i = 0; i < n; i++) dst->seg[i] = src->seg[i]; } From 7ea11bf376aea4bf8340eb363de9777c7f93e556 Mon Sep 17 00:00:00 2001 From: Stefano Stabellini Date: Fri, 18 Dec 2015 15:10:09 +0000 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/display/xenfb.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c index 5e324ef62d..4e2a27a3d6 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;