netdev - Re: [PATCH net-next 1/3] net: sctp: let sctp_destroy

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Date:	Fri, 7 Jun 2013 06:54:19 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-sctp@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net: sctp: let sctp_destroy_sock destroy
 related members

On Fri, Jun 07, 2013 at 10:35:04AM +0200, Daniel Borkmann wrote:
> Release socket related data during sctp_destroy_sock() after we
> called sctp_endpoint_free(), but still before we leave the socket
> destruction callback. The socket's hmac and bind_hash are only
> used in socket.c and actually not in endpointola.c, so also let
> it collect the garbage there, no need to go this detour via
> endpoints.
> 
> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
> ---
>  net/sctp/endpointola.c | 7 -------
>  net/sctp/socket.c      | 9 +++++++++
>  2 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/net/sctp/endpointola.c b/net/sctp/endpointola.c
> index 5fbd7bc..ecfba70 100644
> --- a/net/sctp/endpointola.c
> +++ b/net/sctp/endpointola.c
> @@ -248,9 +248,6 @@ static void sctp_endpoint_destroy(struct sctp_endpoint *ep)
>  {
>  	SCTP_ASSERT(ep->base.dead, "Endpoint is not dead", return);
>  
> -	/* Free up the HMAC transform. */
> -	crypto_free_hash(sctp_sk(ep->base.sk)->hmac);
> -
>  	/* Free the digest buffer */
>  	kfree(ep->digest);
>  
> @@ -270,10 +267,6 @@ static void sctp_endpoint_destroy(struct sctp_endpoint *ep)
>  
>  	memset(ep->secret_key, 0, sizeof(ep->secret_key));
>  
> -	/* Remove and free the port */
> -	if (sctp_sk(ep->base.sk)->bind_hash)
> -		sctp_put_port(ep->base.sk);
> -
>  	/* Give up our hold on the sock. */
>  	if (ep->base.sk)
>  		sock_put(ep->base.sk);
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index f631c5f..3267534 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -4007,7 +4007,16 @@ SCTP_STATIC void sctp_destroy_sock(struct sock *sk)
>  		sp->do_auto_asconf = 0;
>  		list_del(&sp->auto_asconf_list);
>  	}
> +
>  	sctp_endpoint_free(sp->ep);
> +
> +	/* Free up the HMAC transform. */
> +	crypto_free_hash(sp->hmac);
> +
> +	/* Remove and free the port */
> +	if (sp->bind_hash)
> +		sctp_put_port(sk);
> +
>  	local_bh_disable();
>  	percpu_counter_dec(&sctp_sockets_allocated);
>  	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
> -- 

I'm not sure this is safe.  Comment in sk_common_release indicates that the
network can still find the socket in the receive path.  What if we receive a
cookie chunk while the socket is being torn down?  We would wind up using the
hmac to unpack it potentially after you just freed it.  I think you need to wait
until you drop the last reference to the endpoint, not whenever you destroy the
local socket.  Note that sctp_endpoint_free doesn't actually free anything, it
just removes it from the hash list so it can't be found again, and drops a
refcount.  If a parallel recieve op has already found it, hmac may still be
used.

Neil

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