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

From: Rik van Riel <[email-masked]>
Date: Mon, 25 Jul 2016 17:42:03 -0400

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 <[email-masked]>
> > Tested-by: Michael Ellerman <[email-masked]>
> > ---
> >  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? :)

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