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Message-ID: <CAGXu5j+Re9RgEgh-j_71j=F_0ppx7CKpgs76yXbB=a6hRaH=cQ@mail.gmail.com>
Date: Fri, 24 Jun 2016 18:59:30 -0700
From: Kees Cook <keescook@...gle.com>
To: Jann Horn <jannh@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>,
Jann Horn <jann@...jh.net>
Subject: Re: [RFC] reference count hardening via kref: another attempt
On Fri, Jun 24, 2016 at 6:13 PM, Jann Horn <jannh@...gle.com> wrote:
> I would like to harden the kernel against reference count
> overflow bugs. The commit message of the patch contains
> a short analysis of code size impact, an explanation why I
> want reference count hardening to land in the kernel, and a
> description of the algorithm I'd want to use.
>
> The reason I'm writing a cover letter is that my patch, on
> its own, is pretty useless: My patch only adds hardening to
> struct kref, but nearly all reference counters are
> currently implemented using atomic_t, which is a generic
> atomic number type. For the patch to be useful, I'll have
> to go through the kernel and, for every atomic_t, decide
> whether it is a reference count and, if so, change it
> (together with all accesses to it) to a kref. According to
> a quick grep, there are currently about 2700 atomic_t
> struct members or variables in the kernel, so it's going to
> be a big amount of work, and the resulting patches will be
> gigantic.
>
> Therefore, before I actually spend lots of time on this,
> I'd like to know:
>
> - Does the reference count hardening in my patch look
> okay, and does it have good chances to get accepted
> when submitted for inclusion in the kernel at a later
> point in time?
>
> - If I manually go through the kernel and write a
> gigantic atomic_t -> struct kref patch (or a few
> patches coarsely grouped by subsystem), how high is
> the probability that it will get accepted?
My main concern is atomic_t will continue to get misused. While I have
no problem with wrap-checking kref, I think that we need to follow
grsecurity and introduce a new type (in grsec it is
"atomic_unchecked_t", but I think a more descriptive name would be
"atomic_wrap_t") and add them everywhere they're needed, and then
globally protect atomic_t wrapping. kref would gain the protections
automatically, though it would be arch-dependent...
-Kees
>
> (Note: I won't have much time for kernel work in the next
> four months or so - but afterwards, I could probably
> allocate time for getting this done.)
>
> Jann Horn (1):
> kref: pin objects with dangerously high reference count
>
> include/linux/kref.h | 39 +++++++++++++++++++++++++++++++++------
> init/Kconfig | 16 ++++++++++++++++
> kernel/Makefile | 2 +-
> kernel/kref.c | 17 +++++++++++++++++
> 4 files changed, 67 insertions(+), 7 deletions(-)
> create mode 100644 kernel/kref.c
>
> --
> 2.8.0.rc3.226.g39d4020
>
--
Kees Cook
Chrome OS & Brillo Security
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