[CVE-2016-4951][vivid, wily, xenial, yakkety] tipc: check nl sock before parsing nested attributes

[Luis Henriques]

From: Richard Alpe <richard.alpe at ericsson.com>

Make sure the socket for which the user is listing publication exists
before parsing the socket netlink attributes.

Prior to this patch a call without any socket caused a NULL pointer
dereference in tipc_nl_publ_dump().

Tested-and-reported-by: Baozeng Ding <sploving1 at gmail.com>
Signed-off-by: Richard Alpe <richard.alpe at ericsson.com>
Acked-by: Jon Maloy <jon.maloy at ericsson.cm>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 45e093ae2830cd1264677d47ff9a95a71f5d9f9c)
CVE-2016-4951
BugLink: https://bugs.launchpad.net/bugs/1585365
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/tipc/socket.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index 84cf2f805ee9..0bc60e8fc8e2 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -2976,6 +2976,9 @@ int tipc_nl_publ_dump(struct sk_buff *skb, struct netlink_callback *cb)
 		if (err)
 			return err;
 
+		if (!attrs[TIPC_NLA_SOCK])
+			return -EINVAL;
+
 		err = nla_parse_nested(sock, TIPC_NLA_SOCK_MAX,
 				       attrs[TIPC_NLA_SOCK],
 				       tipc_nl_sock_policy);