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Message-ID: <CAGXu5jKxw3RxWNKLX4XVCwJ6x_zA=_RwiU9jLDm2+VRO79G7+w@mail.gmail.com>
Date: Fri, 8 Jul 2016 16:48:38 -0400
From: Kees Cook <keescook@...omium.org>
To: Christoph Lameter <cl@...ux.com>
Cc: Michael Ellerman <mpe@...erman.id.au>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Jan Kara <jack@...e.cz>,
Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>,
Linux-MM <linux-mm@...ck.org>, sparclinux <sparclinux@...r.kernel.org>,
linux-ia64@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>,
linux-arch <linux-arch@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>,
Russell King <linux@...linux.org.uk>, PaX Team <pageexec@...email.hu>,
Borislav Petkov <bp@...e.de>, Mathias Krause <minipli@...glemail.com>, Fenghua Yu <fenghua.yu@...el.com>,
Rik van Riel <riel@...hat.com>, David Rientjes <rientjes@...gle.com>, Tony Luck <tony.luck@...el.com>,
Andy Lutomirski <luto@...nel.org>, Joonsoo Kim <iamjoonsoo.kim@....com>,
Dmitry Vyukov <dvyukov@...gle.com>, Laura Abbott <labbott@...oraproject.org>,
Brad Spengler <spender@...ecurity.net>, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
LKML <linux-kernel@...r.kernel.org>, Pekka Enberg <penberg@...nel.org>,
Case y Schauf ler <casey@...aufler-ca.com>, Andrew Morton <akpm@...ux-foundation.org>,
"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, "David S. Miller" <davem@...emloft.net>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Re: [PATCH 9/9] mm: SLUB hardened usercopy support
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;
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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