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Date: Fri, 8 Jul 2016 12:07:54 -0400
From: Kees Cook <>
To: Christoph Lameter <>
Cc: Michael Ellerman <>, 
	"" <>, Jan Kara <>, 
	Catalin Marinas <>, Will Deacon <>, 
	Linux-MM <>, sparclinux <>,, Andrea Arcangeli <>, 
	linux-arch <>, "" <>, 
	Russell King <>, PaX Team <>, 
	Borislav Petkov <>, lin <>, 
	Mathias Krause <>, Fenghua Yu <>, 
	Rik van Riel <>, David Rientjes <>, Tony Luck <>, 
	Andy Lutomirski <>, Joonsoo Kim <>, 
	Dmitry Vyukov <>, Laura Abbott <>, 
	Brad Spengler <>, Ard Biesheuvel <>, 
	LKML <>, Pekka Enberg <>, 
	Case y Schauf ler <>, Andrew Morton <>, 
	"" <>, "David S. Miller" <>
Subject: Re: Re: [PATCH 9/9] mm: SLUB hardened usercopy support

On Fri, Jul 8, 2016 at 9:45 AM, Christoph Lameter <> wrote:
> On Fri, 8 Jul 2016, Michael Ellerman wrote:
>> > I wonder if this code should be using size_from_object() instead of s->size?

BTW, I can't reproduce this on x86 yet...

>> Hmm, not sure. Who's SLUB maintainer? :)
> Me.
> s->size is the size of the whole object including debugging info etc.
> ksize() gives you the actual usable size of an object.

Is check_valid_pointer() making sure the pointer is within the usable
size? It seemed like it was checking that it was within the slub
object (checks against s->size, wants it above base after moving
pointer to include redzone, etc).

I think a potential problem with Michael's fix is that the ptr in
__check_heap_object() may not point at the _start_ of the usable
object, so doing the red zone shift isn't quite right.

This finds the ptr's offset within the slub object (since s->size is
the slub object size):

        offset = (ptr - page_address(page)) % s->size;

But this looks at object_size and doesn't take into account actual size:

        if (offset <= s->object_size && n <= s->object_size - offset)
                return NULL;

I think offset needs to be adjusted by the size of padding, which the
restore_red_left() call had the same effect, but may not cover all
padding conditions? I'm not sure.

Should it be:

        /* Find offset within slab object. */
        offset = (ptr - page_address(page)) % s->size;

        /* Adjust offset for meta data and padding. */
        offset -= s->size - s->object_size;

        /* Make sure offset and size are within bounds of the
allocation size. */
        if (offset <= s->object_size && n <= s->object_size - offset)
                return NULL;



Kees Cook
Chrome OS & Brillo Security

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