linux-kernel - [PATCH 3.2 23/70] xen-blkback: only read request operation from shared ring once

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Message-ID: <lsq.1453087115.832287086@decadent.org.uk>
Date:	Mon, 18 Jan 2016 03:18:35 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"David Vrabel" <david.vrabel@...rix.com>,
	"Roger Pau Monné" <roger.pau@...rix.com>
Subject: [PATCH 3.2 23/70] xen-blkback: only read request operation from
 shared ring once

3.2.76-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Roger Pau Monné <roger.pau@...rix.com>

commit 1f13d75ccb806260079e0679d55d9253e370ec8a upstream.

A compiler may load a switch statement value multiple times, which could
be bad when the value is in memory shared with the frontend.

When converting a non-native request to a native one, ensure that
src->operation is only loaded once by using READ_ONCE().

This is part of XSA155.

Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
Signed-off-by: David Vrabel <david.vrabel@...rix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
[bwh: Backported to 3.2:
 - s/READ_ONCE/ACCESS_ONCE/
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/block/xen-blkback/common.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -238,11 +238,11 @@ static inline void blkif_get_x86_32_req(
 					struct blkif_x86_32_request *src)
 {
 	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST;
-	dst->operation = src->operation;
+	dst->operation = ACCESS_ONCE(src->operation);
 	dst->nr_segments = src->nr_segments;
 	dst->handle = src->handle;
 	dst->id = src->id;
-	switch (src->operation) {
+	switch (dst->operation) {
 	case BLKIF_OP_READ:
 	case BLKIF_OP_WRITE:
 	case BLKIF_OP_WRITE_BARRIER:
@@ -267,11 +267,11 @@ static inline void blkif_get_x86_64_req(
 					struct blkif_x86_64_request *src)
 {
 	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST;
-	dst->operation = src->operation;
+	dst->operation = ACCESS_ONCE(src->operation);
 	dst->nr_segments = src->nr_segments;
 	dst->handle = src->handle;
 	dst->id = src->id;
-	switch (src->operation) {
+	switch (dst->operation) {
 	case BLKIF_OP_READ:
 	case BLKIF_OP_WRITE:
 	case BLKIF_OP_WRITE_BARRIER: