Subject: Re: [nf-next PATCH 3/5] netfilter: avoid race with exp->master ct

From: Florian Westphal <[email-masked]>
Date: Thu, 27 Feb 2014 22:34:52 +0100

Jesper Dangaard Brouer <[email-masked]> wrote:
> Preparation for disconnecting the nf_conntrack_lock from the
> expectations code.  Once the nf_conntrack_lock is lifted, a race
> condition is exposed.
> 
> The expectations master conntrack exp->master, can race with
> delete operations, as the refcnt increment happens too late in
> init_conntrack().  Race is against other CPUs invoking
> ->destroy() (destroy_conntrack()), or nf_ct_delete() (via timeout
> or early_drop()).
> 
> Avoid this race in nf_ct_find_expectation() by using atomic_inc_not_zero(),
> and checking if nf_ct_is_dying() (path via nf_ct_delete()).
> 
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> Signed-off-by: Florian Westphal <[email-masked]>
> ---
> 
>  net/netfilter/nf_conntrack_core.c   |    2 +-
>  net/netfilter/nf_conntrack_expect.c |   16 +++++++++++++++-
>  2 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index ac85fd1..a822720 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -898,6 +898,7 @@ init_conntrack(struct net *net, struct nf_conn *tmpl,
>  			 ct, exp);
>  		/* Welcome, Mr. Bond.  We've been expecting you... */
>  		__set_bit(IPS_EXPECTED_BIT, &ct->status);
> +		/* exp->master safe, refcnt bumped in nf_ct_find_expectation */
>  		ct->master = exp->master;
>  		if (exp->helper) {
>  			help = nf_ct_helper_ext_add(ct, exp->helper,
> @@ -912,7 +913,6 @@ init_conntrack(struct net *net, struct nf_conn *tmpl,
>  #ifdef CONFIG_NF_CONNTRACK_SECMARK
>  		ct->secmark = exp->master->secmark;
>  #endif
> -		nf_conntrack_get(&ct->master->ct_general);
>  		NF_CT_STAT_INC(net, expect_new);
>  	} else {
>  		__nf_ct_try_assign_helper(ct, tmpl, GFP_ATOMIC);
> diff --git a/net/netfilter/nf_conntrack_expect.c b/net/netfilter/nf_conntrack_expect.c
> index 4fd1ca9..2c4ffdb 100644
> --- a/net/netfilter/nf_conntrack_expect.c
> +++ b/net/netfilter/nf_conntrack_expect.c
> @@ -147,13 +147,27 @@ nf_ct_find_expectation(struct net *net, u16 zone,
>  	if (!exp)
>  		return NULL;
>  
> +	/* Avoid race with other CPUs, that for exp->master ct, is
> +	 * about to invoke ->destroy(), or nf_ct_delete() via timeout
> +	 * or early_drop().
> +	 *
> +	 * The atomic_inc_not_zero() check tells:  If that fails, we
> +	 * know that the ct is being destroyed.  If it succeeds, we
> +	 * can be sure the ct cannot disappear underneath.
> +	 */
> +	if (unlikely(nf_ct_is_dying(exp->master) ||
> +		     !atomic_inc_not_zero(&exp->master->ct_general.use)))
> +		return NULL;
> +
>  	/* If master is not in hash table yet (ie. packet hasn't left
>  	   this machine yet), how can other end know about expected?
>  	   Hence these are not the droids you are looking for (if
>  	   master ct never got confirmed, we'd hold a reference to it
>  	   and weird things would happen to future packets). */
> -	if (!nf_ct_is_confirmed(exp->master))
> +	if (!nf_ct_is_confirmed(exp->master)) {
> +		atomic_dec(&exp->master->ct_general.use);
>  		return NULL;
> +	}

Not sure if this is safe.

What about:
CPU0: atomic_inc_not_zero()
CPU1: calls nf_conntrack_put()
CPU0: atomic_dec() -> zero refcnt without invocation of ->destroy

[ Cannot happen now because of nf_conntrack_lock ]

I'd suggest to test nf_ct_is_confirmed() first, it avoids the need to
undo the atomic_inc_not_zero.

You also need to deal with the "timer-deletion-fails" a bit later in the same
function:

        if (exp->flags & NF_CT_EXPECT_PERMANENT) {
                atomic_inc(&exp->use);
                return exp;
        } else if (del_timer(&exp->timeout)) {
                nf_ct_unlink_expect(exp);
                return exp;
        }
	// Problem: exp->master ref was bumped
	nf_ct_put(exp->master); // missing
        return NULL;