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Date: Mon, 3 Oct 2016 17:26:14 -0400
From: David Windsor <dwindsor@...il.com>
To: Kees Cook <keescook@...omium.org>
Cc: Elena Reshetova <elena.reshetova@...el.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Hans Liljestrand <ishkamiel@...il.com>
Subject: Re: [RFC PATCH 01/13] Add architecture independent hardened atomic base
On Mon, Oct 3, 2016 at 5:10 PM, Kees Cook <keescook@...omium.org> wrote:
> > +}
> > +#endif
> > diff --git a/security/Kconfig b/security/Kconfig
> > index 118f454..dc39f7e 100644
> > --- a/security/Kconfig
> > +++ b/security/Kconfig
> > @@ -158,6 +158,21 @@ config HARDENED_USERCOPY_PAGESPAN
> > been removed. This config is intended to be used only while
> > trying to find such users.
> >
> > +config HAVE_ARCH_HARDENED_ATOMIC
> > + bool
> > + help
> > + The architecture supports CONFIG_HARDENED_ATOMIC by
> > + providing additional checks on counter overflows for atomic_t
> > +
> > +config HARDENED_ATOMIC
> > + bool "Prevent reference counter overflow in atomic_t"
> > + depends on HAVE_ARCH_HARDENED_ATOMIC
>
> Oh, this should select BUG too.
>
> > + help
> > + This option prevents reference counters in atomic_t from
> > + overflow. This allows to avoid the
> > + situation when counter overflow leads to an exploitable
> > + use-after-free situation.
>
> I think the Kconfig help text could be clarified up a bit (and needs
> some minor formatting adjustments). Perhaps something like:
>
> config HAVE_ARCH_HARDENED_ATOMIC
> ...
> The architecture supports CONFIG_HARDENED_ATOMIC by
> providing trapping on atomic_t wraps, with a call to
> hardened_atomic_overflow().
>
> config HARDENED_ATOMIC
> ...
> This option catches counter wrapping in atomic_t, which
> can turn refcounting over/underflow bugs into resource
> consumption bugs instead of exploitable user-after-free flaws.
>
Sorry to be pedantic, but this feature doesn't actually protect
against underflowing atomic_t, and you actually meant "use-after-free"
flaws.
Do we want to mention something about the negligible performance
impact, as is done in the original PAX_REFCOUNT feature:
Since this has a negligible performance impact, you should enable
this feature.
Not sure if we eventually envision this feature being on-by-default;
if not, it may be useful to include this text.
> > +
> > source security/selinux/Kconfig
> > source security/smack/Kconfig
> > source security/tomoyo/Kconfig
> > --
> > 2.7.4
> >
>
> -Kees
>
> --
> Kees Cook
> Nexus Security
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