[Trusty] netlink: Fix dump skb leak/double free

[Luis Henriques]

From: Herbert Xu <herbert at gondor.apana.org.au>

When we free cb->skb after a dump, we do it after releasing the
lock.  This means that a new dump could have started in the time
being and we'll end up freeing their skb instead of ours.

This patch saves the skb and module before we unlock so we free
the right memory.

Fixes: 16b304f3404f ("netlink: Eliminate kmalloc in netlink dump operation.")
Reported-by: Baozeng Ding <sploving1 at gmail.com>
Signed-off-by: Herbert Xu <herbert at gondor.apana.org.au>
Acked-by: Cong Wang <xiyou.wangcong at gmail.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 92964c79b357efd980812c4de5c1fd2ec8bb5520)
CVE-2016-9806
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/netlink/af_netlink.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index f4bc7ddc5840..0038f9bb8a09 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2663,6 +2663,7 @@ static int netlink_dump(struct sock *sk)
 	struct netlink_callback *cb;
 	struct sk_buff *skb = NULL;
 	struct nlmsghdr *nlh;
+	struct module *module;
 	int len, err = -ENOBUFS;
 	int alloc_size;
 
@@ -2732,9 +2733,11 @@ static int netlink_dump(struct sock *sk)
 		cb->done(cb);
 
 	nlk->cb_running = false;
+	module = cb->module;
+	skb = cb->skb;
 	mutex_unlock(nlk->cb_mutex);
-	module_put(cb->module);
-	consume_skb(cb->skb);
+	module_put(module);
+	consume_skb(skb);
 	return 0;
 
 errout_skb: