powerpc/sysfs: Disable writing to PURR in guest mode

commit d1211af3 upstream.

arch/powerpc/kernel/sysfs.c exports PURR with write permission.
This may be valid for kernel in phyp mode. But writing to
the file in guest mode causes crash due to a priviledge violation
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[bwh: Backported to 3.2:
 - Adjust context
 - CPUs are sysdev and we must use the sysdev API]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/powerpc/kernel/sysfs.c

@@ -18,6 +18,7 @@
 #include <asm/machdep.h>
 #include <asm/smp.h>
 #include <asm/pmc.h>
+#include <asm/firmware.h>
 
 #include "cacheinfo.h"
 
@@ -178,14 +179,24 @@ SYSFS_PMCSETUP(purr, SPRN_PURR);
 SYSFS_PMCSETUP(spurr, SPRN_SPURR);
 SYSFS_PMCSETUP(dscr, SPRN_DSCR);
 
+/*
+  Lets only enable read for phyp resources and
+  enable write when needed with a separate function.
+  Lets be conservative and default to pseries.
+*/
 static SYSDEV_ATTR(mmcra, 0600, show_mmcra, store_mmcra);
 static SYSDEV_ATTR(spurr, 0600, show_spurr, NULL);
 static SYSDEV_ATTR(dscr, 0600, show_dscr, store_dscr);
-static SYSDEV_ATTR(purr, 0600, show_purr, store_purr);
+static SYSDEV_ATTR(purr, 0400, show_purr, store_purr);
 
 unsigned long dscr_default = 0;
 EXPORT_SYMBOL(dscr_default);
 
+static void add_write_permission_dev_attr(struct sysdev_attribute *attr)
+{
+	attr->attr.mode |= 0200;
+}
+
 static ssize_t show_dscr_default(struct sysdev_class *class,
 		struct sysdev_class_attribute *attr, char *buf)
 {
@@ -394,8 +405,11 @@ static void __cpuinit register_cpu_online(unsigned int cpu)
 	if (cpu_has_feature(CPU_FTR_MMCRA))
 		sysdev_create_file(s, &attr_mmcra);
 
-	if (cpu_has_feature(CPU_FTR_PURR))
+	if (cpu_has_feature(CPU_FTR_PURR)) {
+		if (!firmware_has_feature(FW_FEATURE_LPAR))
+			add_write_permission_dev_attr(&attr_purr);
 		sysdev_create_file(s, &attr_purr);
+	}
 
 	if (cpu_has_feature(CPU_FTR_SPURR))
 		sysdev_create_file(s, &attr_spurr);