lkdtm/usercopy: Expand size of "out of frame" object

To be sufficiently out of range for the usercopy test to see the lifetime
mismatch, expand the size of the "bad" buffer, which will let it be
beyond current_stack_pointer regardless of stack growth direction.
Paired with the recent addition of stack depth checking under
CONFIG_HARDENED_USERCOPY=y, this will correctly start tripping again.
Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/lkml/762faf1b-0443-5ddf-4430-44a20cf2ec4d@collabora.com/Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/misc/lkdtm/usercopy.c

@@ -30,12 +30,12 @@ static const unsigned char test_text[] = "This is a test.\n";
  */
 static noinline unsigned char *trick_compiler(unsigned char *stack)
 {
-	return stack + 0;
+	return stack + unconst;
 }
 
 static noinline unsigned char *do_usercopy_stack_callee(int value)
 {
-	unsigned char buf[32];
+	unsigned char buf[128];
 	int i;
 
 	/* Exercise stack to avoid everything living in registers. */
@@ -43,7 +43,12 @@ static noinline unsigned char *do_usercopy_stack_callee(int value)
 		buf[i] = value & 0xff;
 	}
 
-	return trick_compiler(buf);
+	/*
+	 * Put the target buffer in the middle of stack allocation
+	 * so that we don't step on future stack users regardless
+	 * of stack growth direction.
+	 */
+	return trick_compiler(&buf[(128/2)-32]);
 }
 
 static noinline void do_usercopy_stack(bool to_user, bool bad_frame)
@@ -66,6 +71,12 @@ static noinline void do_usercopy_stack(bool to_user, bool bad_frame)
 		bad_stack -= sizeof(unsigned long);
 	}
 
+#ifdef ARCH_HAS_CURRENT_STACK_POINTER
+	pr_info("stack     : %px\n", (void *)current_stack_pointer);
+#endif
+	pr_info("good_stack: %px-%px\n", good_stack, good_stack + sizeof(good_stack));
+	pr_info("bad_stack : %px-%px\n", bad_stack, bad_stack + sizeof(good_stack));
+
 	user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
 			    PROT_READ | PROT_WRITE | PROT_EXEC,
 			    MAP_ANONYMOUS | MAP_PRIVATE, 0);