ipsec: Fix aborted xfrm policy dump crash

An independent security researcher, Mohamed Ghannam, has reported
this vulnerability to Beyond Security's SecuriTeam Secure Disclosure
program.

The xfrm_dump_policy_done function expects xfrm_dump_policy to
have been called at least once or it will crash.  This can be
triggered if a dump fails because the target socket's receive
buffer is full.

This patch fixes it by using the cb->start mechanism to ensure that
the initialisation is always done regardless of the buffer situation.

Fixes: 12a169e7d8f4 ("ipsec: Put dumpers on the dump list")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
static inline int verify_replay(struct xfrm_usersa_info *p,
				struct nlattr **attrs)
{
	struct nlattr *rt = attrs[XFRMA_REPLAY_ESN_VAL];
	struct xfrm_replay_state_esn *rs;

	if (p->flags & XFRM_STATE_ESN) {
		if (!rt)
			return -EINVAL;

		rs = nla_data(rt);

		if (rs->bmp_len > XFRMA_REPLAY_ESN_MAX / sizeof(rs->bmp[0]) / 8)
			return -EINVAL;

		if (nla_len(rt) < xfrm_replay_state_esn_len(rs) &&
		    nla_len(rt) != sizeof(*rs))
			return -EINVAL;
	}

	if (!rt)
		return 0;

	/* As only ESP and AH support ESN feature. */
	if ((p->id.proto != IPPROTO_ESP) && (p->id.proto != IPPROTO_AH))
		return -EINVAL;

	if (p->replay_window != 0)
		return -EINVAL;

	return 0;
}

static inline int verify_replay(struct xfrm_usersa_info *p,
				struct nlattr **attrs)
{
	struct nlattr *rt = attrs[XFRMA_REPLAY_ESN_VAL];
	struct xfrm_replay_state_esn *rs;

	if (p->flags & XFRM_STATE_ESN) {
		if (!rt)
			return -EINVAL;

		rs = nla_data(rt);

		if (rs->bmp_len > XFRMA_REPLAY_ESN_MAX / sizeof(rs->bmp[0]) / 8)
			return -EINVAL;

		if (nla_len(rt) < xfrm_replay_state_esn_len(rs) &&
		    nla_len(rt) != sizeof(*rs))
			return -EINVAL;
	}

	if (!rt)
		return 0;

	/* As only ESP and AH support ESN feature. */
	if ((p->id.proto != IPPROTO_ESP) && (p->id.proto != IPPROTO_AH))
		return -EINVAL;

	if (p->replay_window != 0)
		return -EINVAL;

	return 0;
}
