usercopy: Check valid lifetime via stack depth

Under CONFIG_HARDENED_USERCOPY=y, when exact stack frame boundary checking
is not available (i.e. everything except x86 with FRAME_POINTER), check
a stack object as being at least "current depth valid", in the sense
that any object within the stack region but not between start-of-stack
and current_stack_pointer should be considered unavailable (i.e. its
lifetime is from a call no longer present on the stack).

Additionally report usercopy bounds checking failures with an offset
from current_stack_pointer, which may assist with diagnosing failures.

The LKDTM USERCOPY_STACK_FRAME_TO and USERCOPY_STACK_FRAME_FROM tests
(once slightly adjusted in a separate patch) will pass again with
this fixed.

Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Kees Cook <keescook@chromium.org>

mm/usercopy.c

@@ -29,13 +29,20 @@
  * Returns:
  *	NOT_STACK: not at all on the stack
  *	GOOD_FRAME: fully within a valid stack frame
- *	GOOD_STACK: fully on the stack (when can't do frame-checking)
+ *	GOOD_STACK: within the current stack (when can't frame-check exactly)
  *	BAD_STACK: error condition (invalid stack position or bad stack frame)
  */
 static noinline int check_stack_object(const void *obj, unsigned long len)
 {
 	const void * const stack = task_stack_page(current);
 	const void * const stackend = stack + THREAD_SIZE;
+#ifndef CONFIG_STACK_GROWSUP
+	const void * const high = stackend;
+	const void * const low = (void *)current_stack_pointer;
+#else
+	const void * const high = (void *)current_stack_pointer;
+	const void * const low = stack;
+#endif
 	int ret;
 
 	/* Object is not on the stack at all. */
@@ -55,6 +62,12 @@ static noinline int check_stack_object(const void *obj, unsigned long len)
 	if (ret)
 		return ret;
 
+	/*
+	 * Reject: object not within current stack depth.
+	 */
+	if (obj < low || high < obj + len)
+		return BAD_STACK;
+
 	return GOOD_STACK;
 }
 
@@ -269,7 +282,13 @@ void __check_object_size(const void *ptr, unsigned long n, bool to_user)
 		 */
 		return;
 	default:
-		usercopy_abort("process stack", NULL, to_user, 0, n);
+		usercopy_abort("process stack", NULL, to_user,
+#ifndef CONFIG_STACK_GROWSUP
+				(void *)current_stack_pointer - ptr,
+#else
+				ptr - (void *)current_stack_pointer,
+#endif
+				n);
 	}
 
 	/* Check for bad heap object. */