Messages in this thread | | | Date | Fri, 24 Feb 2012 23:13:01 +0400 | From | Vasiliy Kulikov <> | Subject | Re: [kernel-hardening] Re: Add overflow protection to kref |
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On Fri, Feb 24, 2012 at 14:05 -0500, Nick Bowler wrote: > On 2012-02-24 10:52 -0800, Kees Cook wrote: > > On Fri, Feb 24, 2012 at 10:37 AM, Greg KH <gregkh@linuxfoundation.org> wrote: > > > On Fri, Feb 24, 2012 at 12:58:35PM -0500, David Windsor wrote: > [...] > > >> 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? > > > > This is the same principle as the stack protector. When something has > > gone horribly wrong and cannot be sensibly recovered from, crash the > > machine. Wrapping the refcount would cause all kinds of problems, so > > that certainly seems worthy of a BUG(). > > But in this case, the principle does not apply because we can recover. > The reason we cannot recover from the stack protector case is because > the stack protector is reacting after the fact, which is not the case > here. Simply peg the reference count at the maximum value, neither > incrementing it nor decrementing it further.
...and simply loose one reference, which leads to use-after-free.
-- Vasiliy Kulikov http://www.openwall.com - bringing security into open computing environments -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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