linux-kernel - [PATCH 3.10 036/319] xen-blkback: only read request operation from shared ring once

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Date:   Sun,  5 Feb 2017 20:20:23 +0100
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Roger Pau Monné <roger.pau@...rix.com>,
        David Vrabel <david.vrabel@...rix.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "Jan Beulich" <JBeulich@...e.com>, Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 036/319] xen-blkback: only read request operation from shared ring once

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>
Cc: "Jan Beulich" <JBeulich@...e.com>
Signed-off-by: Willy Tarreau <w@....eu>
---
 drivers/block/xen-blkback/common.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 60103e2..6774cba 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -269,8 +269,8 @@ static inline void blkif_get_x86_32_req(struct blkif_request *dst,
 					struct blkif_x86_32_request *src)
 {
 	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST;
-	dst->operation = src->operation;
-	switch (src->operation) {
+	dst->operation = READ_ONCE(src->operation);
+	switch (dst->operation) {
 	case BLKIF_OP_READ:
 	case BLKIF_OP_WRITE:
 	case BLKIF_OP_WRITE_BARRIER:
@@ -305,8 +305,8 @@ static inline void blkif_get_x86_64_req(struct blkif_request *dst,
 					struct blkif_x86_64_request *src)
 {
 	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST;
-	dst->operation = src->operation;
-	switch (src->operation) {
+	dst->operation = READ_ONCE(src->operation);
+	switch (dst->operation) {
 	case BLKIF_OP_READ:
 	case BLKIF_OP_WRITE:
 	case BLKIF_OP_WRITE_BARRIER:
-- 
2.8.0.rc2.1.gbe9624a