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

On Thu, Jul 7, 2016 at 3:42 AM, Thomas Gleixner [off-list ref] wrote:

On Wed, 6 Jul 2016, Kees Cook wrote:

quoted

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

That's ugly

Yeah, I'd like to have this be controlled by a specific CONFIG, like I
invented for the linear mapping, but I wasn't sure what was the best
approach.

quoted

+
+     /* 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.

Yeah, though I prefer CONFIG-controlled stuff over weak functions, but
I agree, something like arch_check_stack_frame(...) or similar. I'll
build something for this on the next revision.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security