[Qemu-devel] [PULL 0/2] xsa155

[Qemu-devel] [PULL 0/2] xsa155

[Stefano Stabellini]

The following changes since commit 18f49881cf8359e89396aac12f5d3cf3f8a632ba:

  configure: Fix shell syntax to placate OpenBSD's pdksh (2015-12-18 13:32:49 +0000)

are available in the git repository at:

  git://xenbits.xen.org/people/sstabellini/qemu-dm.git tags/xsa155

for you to fetch changes up to 7ea11bf376aea4bf8340eb363de9777c7f93e556:

  xenfb: avoid reading twice the same fields from the shared page (2015-12-18 15:10:09 +0000)


It would be great if they were backported to the QEMU stable trees too.


----------------------------------------------------------------
XSA-155 fixes

----------------------------------------------------------------
Stefano Stabellini (2):
      xen/blkif: Avoid double access to src->nr_segments
      xenfb: avoid reading twice the same fields from the shared page

 hw/block/xen_blkif.h |   12 ++++++++----
 hw/display/xenfb.c   |   10 ++++++----
 2 files changed, 14 insertions(+), 8 deletions(-)

[Qemu-devel] [PULL 1/2] xen/blkif: Avoid double access to src->nr_segments

[Stefano Stabellini]

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 <stefano.stabellini@eu.citrix.com>
---
 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 711b692..c68487cb 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];
 }
-- 
1.7.10.4

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

[Stefano Stabellini]

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>
---
 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

Re: [Qemu-devel] [PULL 0/2] xsa155

[Stefano Stabellini]

Michael, could you please backport these two patches? They should apply
fine.

On Fri, 18 Dec 2015, Stefano Stabellini wrote:
> The following changes since commit 18f49881cf8359e89396aac12f5d3cf3f8a632ba:
> 
>   configure: Fix shell syntax to placate OpenBSD's pdksh (2015-12-18 13:32:49 +0000)
> 
> are available in the git repository at:
> 
>   git://xenbits.xen.org/people/sstabellini/qemu-dm.git tags/xsa155
> 
> for you to fetch changes up to 7ea11bf376aea4bf8340eb363de9777c7f93e556:
> 
>   xenfb: avoid reading twice the same fields from the shared page (2015-12-18 15:10:09 +0000)
> 
> 
> It would be great if they were backported to the QEMU stable trees too.
> 
> 
> ----------------------------------------------------------------
> XSA-155 fixes
> 
> ----------------------------------------------------------------
> Stefano Stabellini (2):
>       xen/blkif: Avoid double access to src->nr_segments
>       xenfb: avoid reading twice the same fields from the shared page
> 
>  hw/block/xen_blkif.h |   12 ++++++++----
>  hw/display/xenfb.c   |   10 ++++++----
>  2 files changed, 14 insertions(+), 8 deletions(-)
> 

Re: [Qemu-devel] [PULL 0/2] xsa155

[Peter Maydell]

On 18 December 2015 at 15:17, Stefano Stabellini
<stefano.stabellini@eu.citrix.com> wrote:
> The following changes since commit 18f49881cf8359e89396aac12f5d3cf3f8a632ba:
>
>   configure: Fix shell syntax to placate OpenBSD's pdksh (2015-12-18 13:32:49 +0000)
>
> are available in the git repository at:
>
>   git://xenbits.xen.org/people/sstabellini/qemu-dm.git tags/xsa155
>
> for you to fetch changes up to 7ea11bf376aea4bf8340eb363de9777c7f93e556:
>
>   xenfb: avoid reading twice the same fields from the shared page (2015-12-18 15:10:09 +0000)
>
>
> It would be great if they were backported to the QEMU stable trees too.
>
>
> ----------------------------------------------------------------
> XSA-155 fixes
>
> ----------------------------------------------------------------
> Stefano Stabellini (2):
>       xen/blkif: Avoid double access to src->nr_segments
>       xenfb: avoid reading twice the same fields from the shared page

Applied, thanks.

-- PMM