From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stefano Stabellini Subject: [PULL 1/2] xen/blkif: Avoid double access to src->nr_segments Date: Fri, 18 Dec 2015 15:17:56 +0000 Message-ID: <1450451877-25157-1-git-send-email-stefano.stabellini@eu.citrix.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: peter.maydell@linaro.org Cc: qemu-stable@nongnu.org, xen-devel@lists.xensource.com, qemu-devel@nongnu.org, stefano.stabellini@eu.citrix.com List-Id: xen-devel@lists.xenproject.org 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 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