From: Pavel Emelyanov <xemul@openvz.org>
Date: Wed, 17 Oct 2007 13:45:54 +0400

> The race can result in that some sock will get an sk_filter
> pointer set to kfree-d memory. Look
> 
> CPU1:                            CPU2:
> sk_clone():                      sk_attach_filter():
>   new_sk = sk_alloc(...);
>   sock_copy(new_sk, sk); 
>   /* copies the filter ptr */
>   ...
>   filter = new_sk->sk_filter;
>   if (filter)
>                                      old_fp = sk->sk_filter;
>                                      ...
>                                      sk_filter_release(old_fp);
>                                        if (atomic_dec_and_test(&old_fp->refcnt))
>      atomic_inc(&filter->refcnt);
>                                           /* true */
>                                           call_rcu(&fp->rcu, kfree);
> 
> that's it - after a quiescent state pass the new_sk will have 
> a pointer on kfree-d filter.
> 
> The same problem exists for detaching filter (SO_DETACH_FILTER).
> 
> The proposed fix consists of 3 preparation patches and the fix itself.
> 
> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

Looks good, applied.

Thanks for fixing this bug Pavel!