ARM: 8813/1: Make aligned 2-byte getuser()/putuser() atomic on ARMv6+

getuser() and putuser() (and there underscored variants) use two
strb[t]/ldrb[t] instructions when they are asked to get/put 16-bits.
This means that the read/write is not atomic even when performed to a
16-bit-aligned address.

This leads to problems with vhost: vhost uses __getuser() to read the
vring's 16-bit avail.index field, and if it happens to observe a partial
update of the index, wrong descriptors will be used which will lead to a
breakdown of the virtio communication.  A similar problem exists for
__putuser() which is used to write to the vring's used.index field.

The reason these functions use strb[t]/ldrb[t] is because strht/ldrht
instructions did not exist until ARMv6T2/ARMv7.  So we should be easily
able to fix this on ARMv7.  Also, since all ARMv6 processors also don't
actually use the unprivileged instructions anymore for uaccess (since
CONFIG_CPU_USE_DOMAINS is not used) we can easily fix them too.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/lib/getuser.S

@@ -42,6 +42,12 @@ _ASM_NOKPROBE(__get_user_1)
 
 ENTRY(__get_user_2)
 	check_uaccess r0, 2, r1, r2, __get_user_bad
+#if __LINUX_ARM_ARCH__ >= 6
+
+2: TUSER(ldrh)	r2, [r0]
+
+#else
+
 #ifdef CONFIG_CPU_USE_DOMAINS
 rb	.req	ip
 2:	ldrbt	r2, [r0], #1
@@ -56,6 +62,9 @@ rb	.req	r0
 #else
 	orr	r2, rb, r2, lsl #8
 #endif
+
+#endif /* __LINUX_ARM_ARCH__ >= 6 */
+
 	mov	r0, #0
 	ret	lr
 ENDPROC(__get_user_2)
@@ -145,7 +154,9 @@ _ASM_NOKPROBE(__get_user_bad8)
 .pushsection __ex_table, "a"
 	.long	1b, __get_user_bad
 	.long	2b, __get_user_bad
+#if __LINUX_ARM_ARCH__ < 6
 	.long	3b, __get_user_bad
+#endif
 	.long	4b, __get_user_bad
 	.long	5b, __get_user_bad8
 	.long	6b, __get_user_bad8