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

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@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[ luis: backported to 3.16:
  - replaced READ_ONCE() by ACCESS_ONCE() ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index f65b807e32361c861266a6d0d4320bfc7be5fd4e..ef64f59921a7cf815efaf3c4ae104dc49122d5f4 100644 (file)
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -391,8 +391,8 @@ 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 = ACCESS_ONCE(src->operation);
+       switch (dst->operation) {
        case BLKIF_OP_READ:
        case BLKIF_OP_WRITE:
        case BLKIF_OP_WRITE_BARRIER:
@@ -439,8 +439,8 @@ 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 = ACCESS_ONCE(src->operation);
+       switch (dst->operation) {
        case BLKIF_OP_READ:
        case BLKIF_OP_WRITE:
        case BLKIF_OP_WRITE_BARRIER: