Subject: Re: [PATCH net-next 1/3] net: sctp: let sctp_destroy_sock destroy related members

From: Daniel Borkmann <[email-masked]>
Date: Fri, 07 Jun 2013 13:36:04 +0200

On 06/07/2013 12:54 PM, Neil Horman wrote:
> 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.

Agreed, you're right, thanks for pointing this out Neil! Is it *always* guaranteed
that at the time the endpoint is destroyed in a deferred way (e.g. exactly in such
a scenario you describe), the socket structure is still alive and not yet freed?
Either the ep->base.sk test in sctp_endpoint_destroy() would then be unnecessary
or, if necessary, we should move crypto_free_hash() and sctp_put_port() within this
body since they deref. socket members (but then that memory would be leaked in case
ep->base.sk is NULL). Probably, it might be best to add sth like this to explicitly
decouple it from the endpoint, which is then called when all refs are released from
the socket; then we could call this from __sk_free() via sk->sk_destruct():

static void sctp_sock_destruct(struct sock *sk)
{
	struct sctp_sock *sp = sctp_sk(sk);

	inet_sock_destruct(sk);

	/* Free up the HMAC transform. */
	crypto_free_hash(sp->hmac);

	/* Remove and free the port */
	if (sp->bind_hash)
		sctp_put_port(sk);
}