Re: [kernel-hardening] Re: [PATCH 9/9] mm: SLUB hardened usercopy support

Kees Cook [off-list ref] writes:

On Thu, Jul 7, 2016 at 12:35 AM, Michael Ellerman [off-list ref] wrote:

quoted

I gave this a quick spin on powerpc, it blew up immediately :)

Wheee :) This series is rather easy to test: blows up REALLY quickly
if it's wrong. ;)

Better than subtle race conditions which is the usual :)

quoted

diff --git a/mm/slub.c b/mm/slub.c
index 0c8ace04f075..66191ea4545a 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -3630,6 +3630,9 @@ const char *__check_heap_object(const void *ptr, unsigned long n,
        /* Find object. */
        s = page->slab_cache;

+       /* Subtract red zone if enabled */
+       ptr = restore_red_left(s, ptr);
+

Ah, interesting. Just to make sure: you've built with
CONFIG_SLUB_DEBUG and either CONFIG_SLUB_DEBUG_ON or booted with
either slub_debug or slub_debug=z ?

Yeah built with CONFIG_SLUB_DEBUG_ON, and booted with and without slub_debug
options.

Thanks for the slub fix!

I wonder if this code should be using size_from_object() instead of s->size?

Hmm, not sure. Who's SLUB maintainer? :)

I was modelling it on the logic in check_valid_pointer(), which also does the
restore_red_left(), and then checks for % s->size:

static inline int check_valid_pointer(struct kmem_cache *s,
				struct page *page, void *object)
{
	void *base;

	if (!object)
		return 1;

	base = page_address(page);
	object = restore_red_left(s, object);
	if (object < base || object >= base + page->objects * s->size ||
		(object - base) % s->size) {
		return 0;
	}

	return 1;
}

cheers
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