xfrm: fix crash in XFRM_MSG_GETSA netlink handler

If we hit any of the error conditions inside xfrm_dump_sa(), then
xfrm_state_walk_init() never gets called. However, we still call
xfrm_state_walk_done() from xfrm_dump_sa_done(), which will crash
because the state walk was never initialized properly.

We can fix this by setting cb->args[0] only after we've processed the
first element and checking this before calling xfrm_state_walk_done().

Fixes: d3623099 ("ipsec: add support of limited SA dump")
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_user.c

@@ -896,6 +896,7 @@ static int xfrm_dump_sa_done(struct netlink_callback *cb)
 	struct sock *sk = cb->skb->sk;
 	struct net *net = sock_net(sk);
 
+	if (cb->args[0])
 		xfrm_state_walk_done(walk, net);
 	return 0;
 }
@@ -921,8 +922,6 @@ static int xfrm_dump_sa(struct sk_buff *skb, struct netlink_callback *cb)
 		u8 proto = 0;
 		int err;
 
-		cb->args[0] = 1;
-
 		err = nlmsg_parse(cb->nlh, 0, attrs, XFRMA_MAX,
 				  xfrma_policy);
 		if (err < 0)
@@ -939,6 +938,7 @@ static int xfrm_dump_sa(struct sk_buff *skb, struct netlink_callback *cb)
 			proto = nla_get_u8(attrs[XFRMA_PROTO]);
 
 		xfrm_state_walk_init(walk, proto, filter);
+		cb->args[0] = 1;
 	}
 
 	(void) xfrm_state_walk(net, walk, dump_one_state, &info);