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Date: Mon, 11 Jul 2016 15:08:29 +0900
From: Joonsoo Kim <iamjoonsoo.kim@....com>
To: Kees Cook <keescook@...omium.org>
Cc: Christoph Lameter <cl@...ux.com>, Michael Ellerman <mpe@...erman.id.au>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
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"David S. Miller" <davem@...emloft.net>,
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Subject: Re: Re: [PATCH 9/9] mm: SLUB hardened usercopy
support
On Fri, Jul 08, 2016 at 04:48:38PM -0400, Kees Cook wrote:
> On Fri, Jul 8, 2016 at 1:41 PM, Kees Cook <keescook@...omium.org> wrote:
> > On Fri, Jul 8, 2016 at 12:20 PM, Christoph Lameter <cl@...ux.com> wrote:
> >> On Fri, 8 Jul 2016, Kees Cook wrote:
> >>
> >>> 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).
> >>
> >> check_valid_pointer verifies that a pointer is pointing to the start of an
> >> object. It is used to verify the internal points that SLUB used and
> >> should not be modified to do anything different.
> >
> > Yup, no worries -- I won't touch it. :) I just wanted to verify my
> > understanding.
> >
> > And after playing a bit more, I see that the only thing to the left is
> > padding and redzone. SLUB layout, from what I saw:
> >
> > offset: what's there
> > -------
> > start: padding, redzone
> > red_left_pad: object itself
> > inuse: rest of metadata
> > size: start of next slub object
> >
> > (and object_size == inuse - red_left_pad)
> >
> > i.e. a pointer must be between red_left_pad and inuse, which is the
> > same as pointer - ref_left_pad being less than object_size.
> >
> > So, as found already, the position in the usercopy check needs to be
> > bumped down by red_left_pad, which is what Michael's fix does, so I'll
> > include it in the next version.
>
> Actually, after some offline chats, I think this is better, since it
> makes sure the ptr doesn't end up somewhere weird before we start the
> calculations. This leaves the pointer as-is, but explicitly handles
> the redzone on the offset instead, with no wrapping, etc:
>
> /* Find offset within object. */
> offset = (ptr - page_address(page)) % s->size;
>
> + /* Adjust for redzone and reject if within the redzone. */
> + if (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 <= s->object_size && n <= s->object_size - offset)
> return NULL;
>
As Christoph saids, please use slab_ksize() rather than
s->object_size.
Otherwise, looks good to me.
Thanks.
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