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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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