lkdtm: adjust usercopy tests to bypass const checks

The hardened usercopy is now consistently avoiding checks against const
sizes, since we really only want to perform runtime bounds checking
on lengths that weren't known at build time. To test the hardened usercopy
code, we must force the length arguments to be seen as non-const.
Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/misc/lkdtm_usercopy.c

@@ -9,7 +9,15 @@
 #include <linux/uaccess.h>
 #include <asm/cacheflush.h>
 
-static size_t cache_size = 1024;
+/*
+ * Many of the tests here end up using const sizes, but those would
+ * normally be ignored by hardened usercopy, so force the compiler
+ * into choosing the non-const path to make sure we trigger the
+ * hardened usercopy checks by added "unconst" to all the const copies,
+ * and making sure "cache_size" isn't optimized into a const.
+ */
+static volatile size_t unconst = 0;
+static volatile size_t cache_size = 1024;
 static struct kmem_cache *bad_cache;
 
 static const unsigned char test_text[] = "This is a test.\n";
@@ -67,14 +75,14 @@ static noinline void do_usercopy_stack(bool to_user, bool bad_frame)
 	if (to_user) {
 		pr_info("attempting good copy_to_user of local stack\n");
 		if (copy_to_user((void __user *)user_addr, good_stack,
-				 sizeof(good_stack))) {
+				 unconst + sizeof(good_stack))) {
 			pr_warn("copy_to_user failed unexpectedly?!\n");
 			goto free_user;
 		}
 
 		pr_info("attempting bad copy_to_user of distant stack\n");
 		if (copy_to_user((void __user *)user_addr, bad_stack,
-				 sizeof(good_stack))) {
+				 unconst + sizeof(good_stack))) {
 			pr_warn("copy_to_user failed, but lacked Oops\n");
 			goto free_user;
 		}
@@ -88,14 +96,14 @@ static noinline void do_usercopy_stack(bool to_user, bool bad_frame)
 
 		pr_info("attempting good copy_from_user of local stack\n");
 		if (copy_from_user(good_stack, (void __user *)user_addr,
-				   sizeof(good_stack))) {
+				   unconst + sizeof(good_stack))) {
 			pr_warn("copy_from_user failed unexpectedly?!\n");
 			goto free_user;
 		}
 
 		pr_info("attempting bad copy_from_user of distant stack\n");
 		if (copy_from_user(bad_stack, (void __user *)user_addr,
-				   sizeof(good_stack))) {
+				   unconst + sizeof(good_stack))) {
 			pr_warn("copy_from_user failed, but lacked Oops\n");
 			goto free_user;
 		}
@@ -109,7 +117,7 @@ static void do_usercopy_heap_size(bool to_user)
 {
 	unsigned long user_addr;
 	unsigned char *one, *two;
-	const size_t size = 1024;
+	size_t size = unconst + 1024;
 
 	one = kmalloc(size, GFP_KERNEL);
 	two = kmalloc(size, GFP_KERNEL);
@@ -285,13 +293,14 @@ void lkdtm_USERCOPY_KERNEL(void)
 
 	pr_info("attempting good copy_to_user from kernel rodata\n");
 	if (copy_to_user((void __user *)user_addr, test_text,
-			 sizeof(test_text))) {
+			 unconst + sizeof(test_text))) {
 		pr_warn("copy_to_user failed unexpectedly?!\n");
 		goto free_user;
 	}
 
 	pr_info("attempting bad copy_to_user from kernel text\n");
-	if (copy_to_user((void __user *)user_addr, vm_mmap, PAGE_SIZE)) {
+	if (copy_to_user((void __user *)user_addr, vm_mmap,
+			 unconst + PAGE_SIZE)) {
 		pr_warn("copy_to_user failed, but lacked Oops\n");
 		goto free_user;
 	}