[PATCH 3.16 137/204] ipsec: Fix aborted xfrm policy dump crash

[Ben Hutchings]

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ben Hutchings <ben@xxxxxxxxxxxxxxx>

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.

Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -1570,7 +1570,8 @@ static int xfrm_dump_policy_done(struct
 	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);
+	if (cb->args[0])
+		xfrm_policy_walk_done(walk, net);
 	return 0;
 }