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From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
To: peter.maydell@linaro.org
Cc: qemu-stable@nongnu.org, xen-devel@lists.xensource.com,
	qemu-devel@nongnu.org, stefano.stabellini@eu.citrix.com
Subject: [Qemu-devel] [PULL 1/2] xen/blkif: Avoid double access to src->nr_segments
Date: Fri, 18 Dec 2015 15:17:56 +0000	[thread overview]
Message-ID: <1450451877-25157-1-git-send-email-stefano.stabellini@eu.citrix.com> (raw)
In-Reply-To: <alpine.DEB.2.02.1512181513460.17516@kaball.uk.xensource.com>

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

WARNING: multiple messages have this Message-ID (diff)
From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
To: peter.maydell@linaro.org
Cc: qemu-stable@nongnu.org, xen-devel@lists.xensource.com,
	qemu-devel@nongnu.org, stefano.stabellini@eu.citrix.com
Subject: [PULL 1/2] xen/blkif: Avoid double access to src->nr_segments
Date: Fri, 18 Dec 2015 15:17:56 +0000	[thread overview]
Message-ID: <1450451877-25157-1-git-send-email-stefano.stabellini@eu.citrix.com> (raw)
In-Reply-To: <alpine.DEB.2.02.1512181513460.17516@kaball.uk.xensource.com>

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

  reply	other threads:[~2015-12-18 15:18 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-12-18 15:17 [Qemu-devel] [PULL 0/2] xsa155 Stefano Stabellini
2015-12-18 15:17 ` Stefano Stabellini
2015-12-18 15:17 ` Stefano Stabellini [this message]
2015-12-18 15:17   ` [PULL 1/2] xen/blkif: Avoid double access to src->nr_segments Stefano Stabellini
2015-12-18 15:17 ` [Qemu-devel] [PULL 2/2] xenfb: avoid reading twice the same fields from the shared page Stefano Stabellini
2015-12-18 15:17   ` Stefano Stabellini
2015-12-18 15:33 ` [Qemu-devel] [PULL 0/2] xsa155 Stefano Stabellini
2015-12-18 15:33   ` Stefano Stabellini
2015-12-18 16:04 ` [Qemu-devel] " Peter Maydell
2015-12-18 16:04   ` Peter Maydell

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