Code:  static int xfrm_dump_policy_done(struct netlink_callback *cb)
 {
	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *) &cb->args[1];
 	struct net *net = sock_net(cb->skb->sk);
 
 	xfrm_policy_walk_done(walk, net);
 	return 0;
 }

Vulnerability Type: DoS +Priv 
CWE ID: CWE-416
Summary: The XFRM dump policy implementation in net/xfrm/xfrm_user.c in the Linux kernel before 4.13.11 allows local users to gain privileges or cause a denial of service (use-after-free) via a crafted SO_RCVBUF setsockopt system call in conjunction with XFRM_MSG_GETPOLICY Netlink messages.
Commit Message: 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>