sctp: deny peeloff operation on asocs with threads sleeping on it

commit 2dcab5984841 ("sctp: avoid BUG_ON on sctp_wait_for_sndbuf")
attempted to avoid a BUG_ON call when the association being used for a
sendmsg() is blocked waiting for more sndbuf and another thread did a
peeloff operation on such asoc, moving it to another socket.

As Ben Hutchings noticed, then in such case it would return without
locking back the socket and would cause two unlocks in a row.

Further analysis also revealed that it could allow a double free if the
application managed to peeloff the asoc that is created during the
sendmsg call, because then sctp_sendmsg() would try to free the asoc
that was created only for that call.

This patch takes another approach. It will deny the peeloff operation
if there is a thread sleeping on the asoc, so this situation doesn't
exist anymore. This avoids the issues described above and also honors
the syscalls that are already being handled (it can be multiple sendmsg
calls).

Joint work with Xin Long.

Fixes: 2dcab5984841 ("sctp: avoid BUG_ON on sctp_wait_for_sndbuf")
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
static int __sctp_connect(struct sock *sk,
			  struct sockaddr *kaddrs,
			  int addrs_size,
			  sctp_assoc_t *assoc_id)
{
	struct net *net = sock_net(sk);
	struct sctp_sock *sp;
	struct sctp_endpoint *ep;
	struct sctp_association *asoc = NULL;
	struct sctp_association *asoc2;
	struct sctp_transport *transport;
	union sctp_addr to;
	sctp_scope_t scope;
	long timeo;
	int err = 0;
	int addrcnt = 0;
	int walk_size = 0;
	union sctp_addr *sa_addr = NULL;
	void *addr_buf;
	unsigned short port;
	unsigned int f_flags = 0;

	sp = sctp_sk(sk);
	ep = sp->ep;

	/* connect() cannot be done on a socket that is already in ESTABLISHED
	 * state - UDP-style peeled off socket or a TCP-style socket that
	 * is already connected.
	 * It cannot be done even on a TCP-style listening socket.
	 */
	if (sctp_sstate(sk, ESTABLISHED) || sctp_sstate(sk, CLOSING) ||
	    (sctp_style(sk, TCP) && sctp_sstate(sk, LISTENING))) {
		err = -EISCONN;
		goto out_free;
	}

	/* Walk through the addrs buffer and count the number of addresses. */
	addr_buf = kaddrs;
	while (walk_size < addrs_size) {
		struct sctp_af *af;

		if (walk_size + sizeof(sa_family_t) > addrs_size) {
			err = -EINVAL;
			goto out_free;
		}

		sa_addr = addr_buf;
		af = sctp_get_af_specific(sa_addr->sa.sa_family);

		/* If the address family is not supported or if this address
		 * causes the address buffer to overflow return EINVAL.
		 */
		if (!af || (walk_size + af->sockaddr_len) > addrs_size) {
			err = -EINVAL;
			goto out_free;
		}

		port = ntohs(sa_addr->v4.sin_port);

		/* Save current address so we can work with it */
		memcpy(&to, sa_addr, af->sockaddr_len);

		err = sctp_verify_addr(sk, &to, af->sockaddr_len);
		if (err)
			goto out_free;

		/* Make sure the destination port is correctly set
		 * in all addresses.
		 */
		if (asoc && asoc->peer.port && asoc->peer.port != port) {
			err = -EINVAL;
			goto out_free;
		}

		/* Check if there already is a matching association on the
		 * endpoint (other than the one created here).
		 */
		asoc2 = sctp_endpoint_lookup_assoc(ep, &to, &transport);
		if (asoc2 && asoc2 != asoc) {
			if (asoc2->state >= SCTP_STATE_ESTABLISHED)
				err = -EISCONN;
			else
				err = -EALREADY;
			goto out_free;
		}

		/* If we could not find a matching association on the endpoint,
		 * make sure that there is no peeled-off association matching
		 * the peer address even on another socket.
		 */
		if (sctp_endpoint_is_peeled_off(ep, &to)) {
			err = -EADDRNOTAVAIL;
			goto out_free;
		}

		if (!asoc) {
			/* If a bind() or sctp_bindx() is not called prior to
			 * an sctp_connectx() call, the system picks an
			 * ephemeral port and will choose an address set
			 * equivalent to binding with a wildcard address.
			 */
			if (!ep->base.bind_addr.port) {
				if (sctp_autobind(sk)) {
					err = -EAGAIN;
					goto out_free;
				}
			} else {
				/*
				 * If an unprivileged user inherits a 1-many
				 * style socket with open associations on a
				 * privileged port, it MAY be permitted to
				 * accept new associations, but it SHOULD NOT
				 * be permitted to open new associations.
				 */
				if (ep->base.bind_addr.port <
				    inet_prot_sock(net) &&
				    !ns_capable(net->user_ns,
				    CAP_NET_BIND_SERVICE)) {
					err = -EACCES;
					goto out_free;
				}
			}

			scope = sctp_scope(&to);
			asoc = sctp_association_new(ep, sk, scope, GFP_KERNEL);
			if (!asoc) {
				err = -ENOMEM;
				goto out_free;
			}

			err = sctp_assoc_set_bind_addr_from_ep(asoc, scope,
							      GFP_KERNEL);
			if (err < 0) {
				goto out_free;
			}

		}

		/* Prime the peer's transport structures.  */
		transport = sctp_assoc_add_peer(asoc, &to, GFP_KERNEL,
						SCTP_UNKNOWN);
		if (!transport) {
			err = -ENOMEM;
			goto out_free;
		}

		addrcnt++;
		addr_buf += af->sockaddr_len;
		walk_size += af->sockaddr_len;
	}

	/* In case the user of sctp_connectx() wants an association
	 * id back, assign one now.
	 */
	if (assoc_id) {
		err = sctp_assoc_set_id(asoc, GFP_KERNEL);
		if (err < 0)
			goto out_free;
	}

	err = sctp_primitive_ASSOCIATE(net, asoc, NULL);
	if (err < 0) {
		goto out_free;
	}

	/* Initialize sk's dport and daddr for getpeername() */
	inet_sk(sk)->inet_dport = htons(asoc->peer.port);
	sp->pf->to_sk_daddr(sa_addr, sk);
	sk->sk_err = 0;

	/* in-kernel sockets don't generally have a file allocated to them
	 * if all they do is call sock_create_kern().
	 */
	if (sk->sk_socket->file)
		f_flags = sk->sk_socket->file->f_flags;

	timeo = sock_sndtimeo(sk, f_flags & O_NONBLOCK);

	if (assoc_id)
		*assoc_id = asoc->assoc_id;
	err = sctp_wait_for_connect(asoc, &timeo);
	/* Note: the asoc may be freed after the return of
	 * sctp_wait_for_connect.
	 */

	/* Don't free association on exit. */
	asoc = NULL;

out_free:
	pr_debug("%s: took out_free path with asoc:%p kaddrs:%p err:%d\n",
		 __func__, asoc, kaddrs, err);

	if (asoc) {
		/* sctp_primitive_ASSOCIATE may have added this association
		 * To the hash table, try to unhash it, just in case, its a noop
		 * if it wasn't hashed so we're safe
		 */
		sctp_association_free(asoc);
	}
	return err;
}

static int __sctp_connect(struct sock *sk,
			  struct sockaddr *kaddrs,
			  int addrs_size,
			  sctp_assoc_t *assoc_id)
{
	struct net *net = sock_net(sk);
	struct sctp_sock *sp;
	struct sctp_endpoint *ep;
	struct sctp_association *asoc = NULL;
	struct sctp_association *asoc2;
	struct sctp_transport *transport;
	union sctp_addr to;
	sctp_scope_t scope;
	long timeo;
	int err = 0;
	int addrcnt = 0;
	int walk_size = 0;
	union sctp_addr *sa_addr = NULL;
	void *addr_buf;
	unsigned short port;
	unsigned int f_flags = 0;

	sp = sctp_sk(sk);
	ep = sp->ep;

	/* connect() cannot be done on a socket that is already in ESTABLISHED
	 * state - UDP-style peeled off socket or a TCP-style socket that
	 * is already connected.
	 * It cannot be done even on a TCP-style listening socket.
	 */
	if (sctp_sstate(sk, ESTABLISHED) || sctp_sstate(sk, CLOSING) ||
	    (sctp_style(sk, TCP) && sctp_sstate(sk, LISTENING))) {
		err = -EISCONN;
		goto out_free;
	}

	/* Walk through the addrs buffer and count the number of addresses. */
	addr_buf = kaddrs;
	while (walk_size < addrs_size) {
		struct sctp_af *af;

		if (walk_size + sizeof(sa_family_t) > addrs_size) {
			err = -EINVAL;
			goto out_free;
		}

		sa_addr = addr_buf;
		af = sctp_get_af_specific(sa_addr->sa.sa_family);

		/* If the address family is not supported or if this address
		 * causes the address buffer to overflow return EINVAL.
		 */
		if (!af || (walk_size + af->sockaddr_len) > addrs_size) {
			err = -EINVAL;
			goto out_free;
		}

		port = ntohs(sa_addr->v4.sin_port);

		/* Save current address so we can work with it */
		memcpy(&to, sa_addr, af->sockaddr_len);

		err = sctp_verify_addr(sk, &to, af->sockaddr_len);
		if (err)
			goto out_free;

		/* Make sure the destination port is correctly set
		 * in all addresses.
		 */
		if (asoc && asoc->peer.port && asoc->peer.port != port) {
			err = -EINVAL;
			goto out_free;
		}

		/* Check if there already is a matching association on the
		 * endpoint (other than the one created here).
		 */
		asoc2 = sctp_endpoint_lookup_assoc(ep, &to, &transport);
		if (asoc2 && asoc2 != asoc) {
			if (asoc2->state >= SCTP_STATE_ESTABLISHED)
				err = -EISCONN;
			else
				err = -EALREADY;
			goto out_free;
		}

		/* If we could not find a matching association on the endpoint,
		 * make sure that there is no peeled-off association matching
		 * the peer address even on another socket.
		 */
		if (sctp_endpoint_is_peeled_off(ep, &to)) {
			err = -EADDRNOTAVAIL;
			goto out_free;
		}

		if (!asoc) {
			/* If a bind() or sctp_bindx() is not called prior to
			 * an sctp_connectx() call, the system picks an
			 * ephemeral port and will choose an address set
			 * equivalent to binding with a wildcard address.
			 */
			if (!ep->base.bind_addr.port) {
				if (sctp_autobind(sk)) {
					err = -EAGAIN;
					goto out_free;
				}
			} else {
				/*
				 * If an unprivileged user inherits a 1-many
				 * style socket with open associations on a
				 * privileged port, it MAY be permitted to
				 * accept new associations, but it SHOULD NOT
				 * be permitted to open new associations.
				 */
				if (ep->base.bind_addr.port <
				    inet_prot_sock(net) &&
				    !ns_capable(net->user_ns,
				    CAP_NET_BIND_SERVICE)) {
					err = -EACCES;
					goto out_free;
				}
			}

			scope = sctp_scope(&to);
			asoc = sctp_association_new(ep, sk, scope, GFP_KERNEL);
			if (!asoc) {
				err = -ENOMEM;
				goto out_free;
			}

			err = sctp_assoc_set_bind_addr_from_ep(asoc, scope,
							      GFP_KERNEL);
			if (err < 0) {
				goto out_free;
			}

		}

		/* Prime the peer's transport structures.  */
		transport = sctp_assoc_add_peer(asoc, &to, GFP_KERNEL,
						SCTP_UNKNOWN);
		if (!transport) {
			err = -ENOMEM;
			goto out_free;
		}

		addrcnt++;
		addr_buf += af->sockaddr_len;
		walk_size += af->sockaddr_len;
	}

	/* In case the user of sctp_connectx() wants an association
	 * id back, assign one now.
	 */
	if (assoc_id) {
		err = sctp_assoc_set_id(asoc, GFP_KERNEL);
		if (err < 0)
			goto out_free;
	}

	err = sctp_primitive_ASSOCIATE(net, asoc, NULL);
	if (err < 0) {
		goto out_free;
	}

	/* Initialize sk's dport and daddr for getpeername() */
	inet_sk(sk)->inet_dport = htons(asoc->peer.port);
	sp->pf->to_sk_daddr(sa_addr, sk);
	sk->sk_err = 0;

	/* in-kernel sockets don't generally have a file allocated to them
	 * if all they do is call sock_create_kern().
	 */
	if (sk->sk_socket->file)
		f_flags = sk->sk_socket->file->f_flags;

	timeo = sock_sndtimeo(sk, f_flags & O_NONBLOCK);

	if (assoc_id)
		*assoc_id = asoc->assoc_id;
	err = sctp_wait_for_connect(asoc, &timeo);
	/* Note: the asoc may be freed after the return of
	 * sctp_wait_for_connect.
	 */

	/* Don't free association on exit. */
	asoc = NULL;

out_free:
	pr_debug("%s: took out_free path with asoc:%p kaddrs:%p err:%d\n",
		 __func__, asoc, kaddrs, err);

	if (asoc) {
		/* sctp_primitive_ASSOCIATE may have added this association
		 * To the hash table, try to unhash it, just in case, its a noop
		 * if it wasn't hashed so we're safe
		 */
		sctp_association_free(asoc);
	}
	return err;
}
