Re: [PATCH 1/9] mm: Hardened usercopy

On Wed, 6 Jul 2016, Kees Cook wrote:

+
+#if defined(CONFIG_FRAME_POINTER) && defined(CONFIG_X86)
+	const void *frame = NULL;
+	const void *oldframe;
+#endif

That's ugly

+
+	/* Object is not on the stack at all. */
+	if (obj + len <= stack || stackend <= obj)
+		return 0;
+
+	/*
+	 * Reject: object partially overlaps the stack (passing the
+	 * the check above means at least one end is within the stack,
+	 * so if this check fails, the other end is outside the stack).
+	 */
+	if (obj < stack || stackend < obj + len)
+		return -1;
+
+#if defined(CONFIG_FRAME_POINTER) && defined(CONFIG_X86)
+	oldframe = __builtin_frame_address(1);
+	if (oldframe)
+		frame = __builtin_frame_address(2);
+	/*
+	 * low ----------------------------------------------> high
+	 * [saved bp][saved ip][args][local vars][saved bp][saved ip]
+	 *		     ^----------------^
+	 *             allow copies only within here
+	 */
+	while (stack <= frame && frame < stackend) {
+		/*
+		 * If obj + len extends past the last frame, this
+		 * check won't pass and the next frame will be 0,
+		 * causing us to bail out and correctly report
+		 * the copy as invalid.
+		 */
+		if (obj + len <= frame)
+			return obj >= oldframe + 2 * sizeof(void *) ? 2 : -1;
+		oldframe = frame;
+		frame = *(const void * const *)frame;
+	}
+	return -1;
+#else
+	return 1;
+#endif

I'd rather make that a weak function returning 1 which can be replaced by
x86 for CONFIG_FRAME_POINTER=y. That also allows other architectures to
implement their specific frame checks.

Thanks,

	tglx