[S390] mm: add devmem_is_allowed() for STRICT_DEVMEM checking

Provide the devmem_is_allowed() routine to restrict access to
kernel memory from userspace.
Set the CONFIG_STRICT_DEVMEM config option to switch on checking.
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig.debug

@@ -6,6 +6,18 @@ config TRACE_IRQFLAGS_SUPPORT
 
 source "lib/Kconfig.debug"
 
+config STRICT_DEVMEM
+	def_bool y
+	prompt "Filter access to /dev/mem"
+	---help---
+	  This option restricts access to /dev/mem.  If this option is
+	  disabled, you allow userspace access to all memory, including
+	  kernel and userspace memory. Accidental memory access is likely
+	  to be disastrous.
+	  Memory access is required for experts who want to debug the kernel.
+
+	  If you are unsure, say Y.
+
 config DEBUG_STRICT_USER_COPY_CHECKS
 	bool "Strict user copy size checks"
 	---help---

arch/s390/include/asm/page.h

@@ -130,6 +130,11 @@ struct page;
 void arch_free_page(struct page *page, int order);
 void arch_alloc_page(struct page *page, int order);
 
+static inline int devmem_is_allowed(unsigned long pfn)
+{
+	return 0;
+}
+
 #define HAVE_ARCH_FREE_PAGE
 #define HAVE_ARCH_ALLOC_PAGE