Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support
From: Laura Abbott
Date: Mon Jul 25 2016 - 19:30:26 EST
On 07/25/2016 02:42 PM, Rik van Riel wrote:
On Mon, 2016-07-25 at 12:16 -0700, Laura Abbott wrote:
On 07/20/2016 01:27 PM, Kees Cook wrote:
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 <keescook@xxxxxxxxxxxx>
Tested-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
---
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.
I'm not super attached to the check if other maintainers think it
is redundant. Updating the __check_heap_object header comment
with a note of what we are assuming could work
Thanks,
Laura