Re: [kernel-hardening] Re: Add overflow protection to kref

[Greg KH <gregkh@linuxfoundation.org>]

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