Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC

[Will Deacon]

On Thu, Nov 10, 2016 at 09:37:49PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 10, 2016 at 10:24:35PM +0200, Elena Reshetova wrote:
> > This series brings the PaX/Grsecurity PAX_REFCOUNT
> > feature support to the upstream kernel. All credit for the
> > feature goes to the feature authors.
> > 
> > The name of the upstream feature is HARDENED_ATOMIC
> > and it is configured using CONFIG_HARDENED_ATOMIC and
> > HAVE_ARCH_HARDENED_ATOMIC.
> > 
> > This series only adds x86 support; other architectures are expected
> > to add similar support gradually.
> > 
> > More information about the feature can be found in the following
> > commit messages.
> 
> No, this should be here. As it stands this is completely without
> content.
> 
> In any case, NAK on this approach. Its the wrong way around.
> 
> _IF_ you want to do a non-wrapping variant, it must not be the default.
> 
> Since you need to audit every single atomic_t user in the kernel anyway,
> it doesn't matter. But changing atomic_t to non-wrap by default is not
> robust, if you forgot one, you can then trivially dos the kernel.

Completely agreed.

Whilst I understand that you're addressing an important and commonly
exploited vulnerability, this really needs to be opt-in rather than
opt-out given the prevalence of atomic_t users in the kernel. Having a
"hardened" kernel that does the wrong thing is useless.

> That said, I still don't much like this.
> 
> I would much rather you make kref useful and use that. It still means
> you get to audit all refcounts in the kernel, but hey, you had to do
> that anyway.

What needs to happen to kref to make it useful? Like many others, I've
been guilty of using atomic_t for refcounts in the past.

Will