[PATCH 3.19.y-ckt 108/160] xen: Add RING_COPY_REQUEST()

[Kamal Mostafa]

3.19.8-ckt13 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: David Vrabel <david.vrabel at citrix.com>

commit 454d5d882c7e412b840e3c99010fe81a9862f6fb upstream.

Using RING_GET_REQUEST() on a shared ring is easy to use incorrectly
(i.e., by not considering that the other end may alter the data in the
shared ring while it is being inspected).  Safe usage of a request
generally requires taking a local copy.

Provide a RING_COPY_REQUEST() macro to use instead of
RING_GET_REQUEST() and an open-coded memcpy().  This takes care of
ensuring that the copy is done correctly regardless of any possible
compiler optimizations.

Use a volatile source to prevent the compiler from reordering or
omitting the copy.

This is part of XSA155.

Signed-off-by: David Vrabel <david.vrabel at citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 include/xen/interface/io/ring.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/xen/interface/io/ring.h b/include/xen/interface/io/ring.h
index 7d28aff..7dc685b 100644
--- a/include/xen/interface/io/ring.h
+++ b/include/xen/interface/io/ring.h
@@ -181,6 +181,20 @@ struct __name##_back_ring {						\
 #define RING_GET_REQUEST(_r, _idx)					\
     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
 
+/*
+ * Get a local copy of a request.
+ *
+ * Use this in preference to RING_GET_REQUEST() so all processing is
+ * done on a local copy that cannot be modified by the other end.
+ *
+ * Note that https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 may cause this
+ * to be ineffective where _req is a struct which consists of only bitfields.
+ */
+#define RING_COPY_REQUEST(_r, _idx, _req) do {				\
+	/* Use volatile to force the copy into _req. */			\
+	*(_req) = *(volatile typeof(_req))RING_GET_REQUEST(_r, _idx);	\
+} while (0)
+
 #define RING_GET_RESPONSE(_r, _idx)					\
     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
 
-- 
1.9.1