Patch "s390/ctl_reg: make __ctl_load a full memory barrier" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

s390/ctl_reg: make __ctl_load a full memory barrier

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     s390-ctl_reg-make-__ctl_load-a-full-memory-barrier.patch

and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Heiko Carstens <[email protected]>
Date: Wed, 28 Dec 2016 11:33:48 +0100
Subject: s390/ctl_reg: make __ctl_load a full memory barrier

From: Heiko Carstens <[email protected]>

[ Upstream commit e991c24d68b8c0ba297eeb7af80b1e398e98c33f ]

We have quite a lot of code that depends on the order of the __ctl_load inline assemby and subsequent memory accesses, like e.g. disabling lowcore protection and the writing to lowcore.

Since the __ctl_load macro does not have memory barrier semantics, nor any other dependencies the compiler is, theoretically, free to shuffle code around. Or in other words: storing to lowcore could happen before lowcore protection is disabled.

In order to avoid this class of potential bugs simply add a full memory barrier to the __ctl_load macro.

---
 arch/s390/include/asm/ctl_reg.h |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

@@ -15,7 +15,9 @@
 	BUILD_BUG_ON(sizeof(addrtype) != (high - low + 1) * sizeof(long));\
 	asm volatile(							\
 		"	lctlg	%1,%2,%0\n"				\
-		: : "Q" (*(addrtype *)(&array)), "i" (low), "i" (high));\
+		:							\
+		: "Q" (*(addrtype *)(&array)), "i" (low), "i" (high)	\
+		: "memory");						\
 }
 
 #define __ctl_store(array, low, high) {					\

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/s390-ctl_reg-make-__ctl_load-a-full-memory-barrier.patch