Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support

[Laura Abbott]

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;
 

With that, you can add

Reviwed-by: Laura Abbott <labbott@xxxxxxxxxx>

>  static size_t __ksize(const void *object)
>  {
>  	struct page *page;

Thanks,
Laura