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SubjectRe: [kernel-hardening] Re: Add overflow protection to kref
On Fri, Feb 24, 2012 at 12:58:35PM -0500, David Windsor wrote:
> <snip>
>
> >> Greg, I'm not sure why you're opposed to adding this checking...
> >> it's pretty clear that buggy error paths that forget to do a put are
> >> pretty common and will continue to be common in new code, and
> >> making them harder to exploit seems pretty sane to me.
> >>
> >> What's the downside?
> >
> > The downside is that there has not even been a patch sent for any of
> > this.  Combine that with a lack of understanding about reference
> > counting and atomic_t usages in the kernel, and the whole thing is ripe
> > for misunderstanding and confusion.
> >
> > greg k-h
>
> This approach to adding overflow protection to kref uses
> atomic_add_unless to increment the refcounter only if it is not
> already at INT_MAX. This
> leaks the internal representation of atomic_t, which is defined as an
> int in linux/types.h, into kref.
>
> If we can agree on an approach to adding overflow protection, if it is
> indeed desired, we can then discuss adding a Kconfig option and/or a
> sysctl for this protection.
>
> Thanks,
> David
>
>
> Signed-off-by: David Windsor <dwindsor@gmail.com>
> ---
> include/linux/kref.h | 6 +++++-
> 1 files changed, 5 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/kref.h b/include/linux/kref.h
> index 9c07dce..fc0756a 100644
> --- a/include/linux/kref.h
> +++ b/include/linux/kref.h
> @@ -38,8 +38,12 @@ static inline void kref_init(struct kref *kref)
> */
> static inline void kref_get(struct kref *kref)
> {
> + int rc = 0;
> WARN_ON(!atomic_read(&kref->refcount));
> - atomic_inc(&kref->refcount);
> + smp_mb__before_atomic_inc();
> + rc = atomic_add_unless(&kref->refcount, 1, INT_MAX);
> + smp_mb__after_atomic_inc();
> + BUG_ON(!rc);

So you are guaranteeing to crash a machine here if this fails? And you
were trying to say this is a "security" based fix?

And people wonder why I no longer have any hair...

greg k-h
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