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

David Windsor dwindsor at gmail.com
Fri Feb 24 17:58:35 UTC 2012


>> 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.


Signed-off-by: David Windsor <dwindsor at 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;
-   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);


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