xen-blkback: only read request operation from shared ring once

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.

CC: stable@vger.kernel.org
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 68e87a037b99..c929ae22764c 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -408,8 +408,8 @@ 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, j;
-        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:
@@ -456,8 +456,8 @@ 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, j;
-        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: