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_apply_peer_addr_params(struct sctp_paddrparams *params,
				       struct sctp_transport   *trans,
				       struct sctp_association *asoc,
				       struct sctp_sock        *sp,
				       int                      hb_change,
				       int                      pmtud_change,
				       int                      sackdelay_change)
{
	int error;

	if (params->spp_flags & SPP_HB_DEMAND && trans) {
		struct net *net = sock_net(trans->asoc->base.sk);

		error = sctp_primitive_REQUESTHEARTBEAT(net, trans->asoc, trans);
		if (error)
			return error;
	}

	/* Note that unless the spp_flag is set to SPP_HB_ENABLE the value of
	 * this field is ignored.  Note also that a value of zero indicates
	 * the current setting should be left unchanged.
	 */
	if (params->spp_flags & SPP_HB_ENABLE) {

		/* Re-zero the interval if the SPP_HB_TIME_IS_ZERO is
		 * set.  This lets us use 0 value when this flag
		 * is set.
		 */
		if (params->spp_flags & SPP_HB_TIME_IS_ZERO)
			params->spp_hbinterval = 0;

		if (params->spp_hbinterval ||
		    (params->spp_flags & SPP_HB_TIME_IS_ZERO)) {
			if (trans) {
				trans->hbinterval =
				    msecs_to_jiffies(params->spp_hbinterval);
			} else if (asoc) {
				asoc->hbinterval =
				    msecs_to_jiffies(params->spp_hbinterval);
			} else {
				sp->hbinterval = params->spp_hbinterval;
			}
		}
	}

	if (hb_change) {
		if (trans) {
			trans->param_flags =
				(trans->param_flags & ~SPP_HB) | hb_change;
		} else if (asoc) {
			asoc->param_flags =
				(asoc->param_flags & ~SPP_HB) | hb_change;
		} else {
			sp->param_flags =
				(sp->param_flags & ~SPP_HB) | hb_change;
		}
	}

	/* When Path MTU discovery is disabled the value specified here will
	 * be the "fixed" path mtu (i.e. the value of the spp_flags field must
	 * include the flag SPP_PMTUD_DISABLE for this field to have any
	 * effect).
	 */
	if ((params->spp_flags & SPP_PMTUD_DISABLE) && params->spp_pathmtu) {
		if (trans) {
			trans->pathmtu = params->spp_pathmtu;
			sctp_assoc_sync_pmtu(sctp_opt2sk(sp), asoc);
		} else if (asoc) {
			asoc->pathmtu = params->spp_pathmtu;
		} else {
			sp->pathmtu = params->spp_pathmtu;
		}
	}

	if (pmtud_change) {
		if (trans) {
			int update = (trans->param_flags & SPP_PMTUD_DISABLE) &&
				(params->spp_flags & SPP_PMTUD_ENABLE);
			trans->param_flags =
				(trans->param_flags & ~SPP_PMTUD) | pmtud_change;
			if (update) {
				sctp_transport_pmtu(trans, sctp_opt2sk(sp));
				sctp_assoc_sync_pmtu(sctp_opt2sk(sp), asoc);
			}
		} else if (asoc) {
			asoc->param_flags =
				(asoc->param_flags & ~SPP_PMTUD) | pmtud_change;
		} else {
			sp->param_flags =
				(sp->param_flags & ~SPP_PMTUD) | pmtud_change;
		}
	}

	/* Note that unless the spp_flag is set to SPP_SACKDELAY_ENABLE the
	 * value of this field is ignored.  Note also that a value of zero
	 * indicates the current setting should be left unchanged.
	 */
	if ((params->spp_flags & SPP_SACKDELAY_ENABLE) && params->spp_sackdelay) {
		if (trans) {
			trans->sackdelay =
				msecs_to_jiffies(params->spp_sackdelay);
		} else if (asoc) {
			asoc->sackdelay =
				msecs_to_jiffies(params->spp_sackdelay);
		} else {
			sp->sackdelay = params->spp_sackdelay;
		}
	}

	if (sackdelay_change) {
		if (trans) {
			trans->param_flags =
				(trans->param_flags & ~SPP_SACKDELAY) |
				sackdelay_change;
		} else if (asoc) {
			asoc->param_flags =
				(asoc->param_flags & ~SPP_SACKDELAY) |
				sackdelay_change;
		} else {
			sp->param_flags =
				(sp->param_flags & ~SPP_SACKDELAY) |
				sackdelay_change;
		}
	}

	/* Note that a value of zero indicates the current setting should be
	   left unchanged.
	 */
	if (params->spp_pathmaxrxt) {
		if (trans) {
			trans->pathmaxrxt = params->spp_pathmaxrxt;
		} else if (asoc) {
			asoc->pathmaxrxt = params->spp_pathmaxrxt;
		} else {
			sp->pathmaxrxt = params->spp_pathmaxrxt;
		}
	}

	return 0;
}

static int sctp_apply_peer_addr_params(struct sctp_paddrparams *params,
				       struct sctp_transport   *trans,
				       struct sctp_association *asoc,
				       struct sctp_sock        *sp,
				       int                      hb_change,
				       int                      pmtud_change,
				       int                      sackdelay_change)
{
	int error;

	if (params->spp_flags & SPP_HB_DEMAND && trans) {
		struct net *net = sock_net(trans->asoc->base.sk);

		error = sctp_primitive_REQUESTHEARTBEAT(net, trans->asoc, trans);
		if (error)
			return error;
	}

	/* Note that unless the spp_flag is set to SPP_HB_ENABLE the value of
	 * this field is ignored.  Note also that a value of zero indicates
	 * the current setting should be left unchanged.
	 */
	if (params->spp_flags & SPP_HB_ENABLE) {

		/* Re-zero the interval if the SPP_HB_TIME_IS_ZERO is
		 * set.  This lets us use 0 value when this flag
		 * is set.
		 */
		if (params->spp_flags & SPP_HB_TIME_IS_ZERO)
			params->spp_hbinterval = 0;

		if (params->spp_hbinterval ||
		    (params->spp_flags & SPP_HB_TIME_IS_ZERO)) {
			if (trans) {
				trans->hbinterval =
				    msecs_to_jiffies(params->spp_hbinterval);
			} else if (asoc) {
				asoc->hbinterval =
				    msecs_to_jiffies(params->spp_hbinterval);
			} else {
				sp->hbinterval = params->spp_hbinterval;
			}
		}
	}

	if (hb_change) {
		if (trans) {
			trans->param_flags =
				(trans->param_flags & ~SPP_HB) | hb_change;
		} else if (asoc) {
			asoc->param_flags =
				(asoc->param_flags & ~SPP_HB) | hb_change;
		} else {
			sp->param_flags =
				(sp->param_flags & ~SPP_HB) | hb_change;
		}
	}

	/* When Path MTU discovery is disabled the value specified here will
	 * be the "fixed" path mtu (i.e. the value of the spp_flags field must
	 * include the flag SPP_PMTUD_DISABLE for this field to have any
	 * effect).
	 */
	if ((params->spp_flags & SPP_PMTUD_DISABLE) && params->spp_pathmtu) {
		if (trans) {
			trans->pathmtu = params->spp_pathmtu;
			sctp_assoc_sync_pmtu(sctp_opt2sk(sp), asoc);
		} else if (asoc) {
			asoc->pathmtu = params->spp_pathmtu;
		} else {
			sp->pathmtu = params->spp_pathmtu;
		}
	}

	if (pmtud_change) {
		if (trans) {
			int update = (trans->param_flags & SPP_PMTUD_DISABLE) &&
				(params->spp_flags & SPP_PMTUD_ENABLE);
			trans->param_flags =
				(trans->param_flags & ~SPP_PMTUD) | pmtud_change;
			if (update) {
				sctp_transport_pmtu(trans, sctp_opt2sk(sp));
				sctp_assoc_sync_pmtu(sctp_opt2sk(sp), asoc);
			}
		} else if (asoc) {
			asoc->param_flags =
				(asoc->param_flags & ~SPP_PMTUD) | pmtud_change;
		} else {
			sp->param_flags =
				(sp->param_flags & ~SPP_PMTUD) | pmtud_change;
		}
	}

	/* Note that unless the spp_flag is set to SPP_SACKDELAY_ENABLE the
	 * value of this field is ignored.  Note also that a value of zero
	 * indicates the current setting should be left unchanged.
	 */
	if ((params->spp_flags & SPP_SACKDELAY_ENABLE) && params->spp_sackdelay) {
		if (trans) {
			trans->sackdelay =
				msecs_to_jiffies(params->spp_sackdelay);
		} else if (asoc) {
			asoc->sackdelay =
				msecs_to_jiffies(params->spp_sackdelay);
		} else {
			sp->sackdelay = params->spp_sackdelay;
		}
	}

	if (sackdelay_change) {
		if (trans) {
			trans->param_flags =
				(trans->param_flags & ~SPP_SACKDELAY) |
				sackdelay_change;
		} else if (asoc) {
			asoc->param_flags =
				(asoc->param_flags & ~SPP_SACKDELAY) |
				sackdelay_change;
		} else {
			sp->param_flags =
				(sp->param_flags & ~SPP_SACKDELAY) |
				sackdelay_change;
		}
	}

	/* Note that a value of zero indicates the current setting should be
	   left unchanged.
	 */
	if (params->spp_pathmaxrxt) {
		if (trans) {
			trans->pathmaxrxt = params->spp_pathmaxrxt;
		} else if (asoc) {
			asoc->pathmaxrxt = params->spp_pathmaxrxt;
		} else {
			sp->pathmaxrxt = params->spp_pathmaxrxt;
		}
	}

	return 0;
}
