Re: [kernel-hardening] Re: [RFC] reference count hardening via kref: another attempt

[Greg KH]

On Fri, Jun 24, 2016 at 06:59:30PM -0700, Kees Cook wrote:
> On Fri, Jun 24, 2016 at 6:13 PM, Jann Horn <jannh@xxxxxxxxxx> 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...

I think that's the better thing to do as well, not all reference counts
use kref, and in doing that type of conversion, you will find lots of
places that _should_ be using a kref instead of an atomic_t.  Or they
don't even need to be an atomic_t due to author confusion about what an
atomic variable even does (something I see a lot of in out-of-tree or
newly submitted drivers...)

thanks,

greg k-h