ipsec: Fix aborted xfrm policy dump crash

commit 1137b5e2529a8f5ca8ee709288ecba3e68044df2 upstream.

This is a fix for CVE-2017-16939 suitable for older stable branches.
The upstream fix is commit 1137b5e2529a8f5ca8ee709288ecba3e68044df2,
from which the following explanation is taken:

    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.

It was not possible to define a 'start' callback for netlink dumps
until Linux 4.5, so instead add a check for the initialisation flag in
the 'done' callback.

Change-Id: I5726d3a089f362e8cc2d1d62bacc86b843703489
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/xfrm/xfrm_user.c

@@ -1558,7 +1558,8 @@ static int xfrm_dump_policy_done(struct netlink_callback *cb)
 {
 	struct xfrm_policy_walk *walk = (struct xfrm_policy_walk *) &cb->args[1];
 
-	xfrm_policy_walk_done(walk);
+	if (cb->args[0])
+		xfrm_policy_walk_done(walk);
 	return 0;
 }