On Mon, 2016-07-25 at 16:29 -0700, Laura Abbott wrote:

On 07/25/2016 02:42 PM, Rik van Riel wrote:

quoted

On Mon, 2016-07-25 at 12:16 -0700, Laura Abbott wrote:

quoted

On 07/20/2016 01:27 PM, Kees Cook wrote:

quoted

Under CONFIG_HARDENED_USERCOPY, this adds object size checking
to
the
SLUB allocator to catch any copies that may span objects.
Includes
a
redzone handling fix discovered by Michael Ellerman.

Based on code from PaX and grsecurity.

Signed-off-by: Kees Cook <redacted>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
---
 init/Kconfig |  1 +
 mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index 798c2020ee7c..1c4711819dfd 100644
--- a/init/Kconfig
+++ b/init/Kconfig

@@ -1765,6 +1765,7 @@ config SLAB

 config SLUB
 	bool "SLUB (Unqueued Allocator)"
+	select HAVE_HARDENED_USERCOPY_ALLOCATOR
 	help
 	   SLUB is a slab allocator that minimizes cache line
usage
 	   instead of managing queues of cached objects (SLAB
approach).

diff --git a/mm/slub.c b/mm/slub.c
index 825ff4505336..7dee3d9a5843 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t

flags, int node)
 EXPORT_SYMBOL(__kmalloc_node);
 #endif

+#ifdef CONFIG_HARDENED_USERCOPY
+/*
+ * Rejects objects that are incorrectly sized.
+ *
+ * Returns NULL if check passes, otherwise const char * to
name of
cache
+ * to indicate an error.
+ */
+const char *__check_heap_object(const void *ptr, unsigned long
n,
+				struct page *page)
+{
+	struct kmem_cache *s;
+	unsigned long offset;
+	size_t object_size;
+
+	/* Find object and usable object size. */
+	s = page->slab_cache;
+	object_size = slab_ksize(s);
+
+	/* Find offset within object. */
+	offset = (ptr - page_address(page)) % s->size;
+
+	/* Adjust for redzone and reject if within the
redzone. */
+	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
+		if (offset < s->red_left_pad)
+			return s->name;
+		offset -= s->red_left_pad;
+	}
+
+	/* Allow address range falling entirely within object
size. */
+	if (offset <= object_size && n <= object_size -
offset)
+		return NULL;
+
+	return s->name;
+}
+#endif /* CONFIG_HARDENED_USERCOPY */
+

I compared this against what check_valid_pointer does for
SLUB_DEBUG
checking. I was hoping we could utilize that function to avoid
duplication but a) __check_heap_object needs to allow accesses
anywhere
in the object, not just the beginning b) accessing page->objects
is racy without the addition of locking in SLUB_DEBUG.

Still, the ptr < page_address(page) check from
__check_heap_object
would
be good to add to avoid generating garbage large offsets and
trying
to
infer C math.

diff --git a/mm/slub.c b/mm/slub.c
index 7dee3d9..5370e4f 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void

*ptr, unsigned long n,
         s = page->slab_cache;
         object_size = slab_ksize(s);

+       if (ptr < page_address(page))
+               return s->name;
+
         /* Find offset within object. */
         offset = (ptr - page_address(page)) % s->size;

I don't get it, isn't that already guaranteed because we
look for the page that ptr is in, before __check_heap_object
is called?

Specifically, in patch 3/12:

+       page = virt_to_head_page(ptr);
+
+       /* Check slab allocator for flags and size. */
+       if (PageSlab(page))
+               return __check_heap_object(ptr, n, page);

How can that generate a ptr that is not inside the page?

What am I overlooking?  And, should it be in the changelog or
a comment? :)

I ran into the subtraction issue when the vmalloc detection wasn't
working on ARM64, somehow virt_to_head_page turned into a page
that happened to have PageSlab set. I agree if everything is working
properly this is redundant but given the type of feature this is, a
little bit of redundancy against a system running off into the weeds
or bad patches might be warranted.

That's fair.  I have no objection to the check, but would
like to see it documented, since it does look a little out
of place.

-- 

All Rights Reversed.