s390/kaslr: randomize module base load address

Randomize the load address of modules in the kernel to make KASLR effective
for modules.
This is the s390 variant of commit e2b32e67 ("x86, kaslr: randomize
module base load address").
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/module.c

@@ -26,6 +26,7 @@
 #include <asm/facility.h>
 #include <asm/ftrace.lds.h>
 #include <asm/set_memory.h>
+#include <asm/setup.h>
 
 #if 0
 #define DEBUGP printk
@@ -35,6 +36,24 @@
 
 #define PLT_ENTRY_SIZE 22
 
+static unsigned long get_module_load_offset(void)
+{
+	static DEFINE_MUTEX(module_kaslr_mutex);
+	static unsigned long module_load_offset;
+
+	if (!kaslr_enabled())
+		return 0;
+	/*
+	 * Calculate the module_load_offset the first time this code
+	 * is called. Once calculated it stays the same until reboot.
+	 */
+	mutex_lock(&module_kaslr_mutex);
+	if (!module_load_offset)
+		module_load_offset = get_random_u32_inclusive(1, 1024) * PAGE_SIZE;
+	mutex_unlock(&module_kaslr_mutex);
+	return module_load_offset;
+}
+
 void *module_alloc(unsigned long size)
 {
 	gfp_t gfp_mask = GFP_KERNEL;
@@ -42,7 +61,8 @@ void *module_alloc(unsigned long size)
 
 	if (PAGE_ALIGN(size) > MODULES_LEN)
 		return NULL;
-	p = __vmalloc_node_range(size, MODULE_ALIGN, MODULES_VADDR, MODULES_END,
+	p = __vmalloc_node_range(size, MODULE_ALIGN,
+				 MODULES_VADDR + get_module_load_offset(), MODULES_END,
 				 gfp_mask, PAGE_KERNEL_EXEC, VM_DEFER_KMEMLEAK, NUMA_NO_NODE,
 				 __builtin_return_address(0));
 	if (p && (kasan_alloc_module_shadow(p, size, gfp_mask) < 0)) {