Subject: Re: [PATCH] netfilter: fix a race in nf_ct_ext_create()

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 31 Aug 2010 08:51:37 -0700

On Tue, Aug 31, 2010 at 05:48:02PM +0200, Eric Dumazet wrote:
> As soon as rcu_read_unlock() is called, there is no guarantee current
> thread can safely derefence t pointer, rcu protected.
> 
> Fix is to copy t->alloc_size in a temporary variable.

Yow!!!  Good catch!!!

Reviewed-by: Paul E. McKenney <[email-masked]>

> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
>  net/netfilter/nf_conntrack_extend.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/nf_conntrack_extend.c b/net/netfilter/nf_conntrack_extend.c
> index 7dcf7a4..8d9e4c9 100644
> --- a/net/netfilter/nf_conntrack_extend.c
> +++ b/net/netfilter/nf_conntrack_extend.c
> @@ -48,15 +48,17 @@ nf_ct_ext_create(struct nf_ct_ext **ext, enum nf_ct_ext_id id, gfp_t gfp)
>  {
>  	unsigned int off, len;
>  	struct nf_ct_ext_type *t;
> +	size_t alloc_size;
> 
>  	rcu_read_lock();
>  	t = rcu_dereference(nf_ct_ext_types[id]);
>  	BUG_ON(t == NULL);
>  	off = ALIGN(sizeof(struct nf_ct_ext), t->align);
>  	len = off + t->len;
> +	alloc_size = t->alloc_size;
>  	rcu_read_unlock();
> 
> -	*ext = kzalloc(t->alloc_size, gfp);
> +	*ext = kzalloc(alloc_size, gfp);
>  	if (!*ext)
>  		return NULL;
> 
> 
> 
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