bridge: Only call /sbin/bridge-stp for the initial network namespace

commit ff621985 upstream.

[I stole this patch from Eric Biederman. He wrote:]

> There is no defined mechanism to pass network namespace information
> into /sbin/bridge-stp therefore don't even try to invoke it except
> for bridge devices in the initial network namespace.
>
> It is possible for unprivileged users to cause /sbin/bridge-stp to be
> invoked for any network device name which if /sbin/bridge-stp does not
> guard against unreasonable arguments or being invoked twice on the
> same network device could cause problems.

[Hannes: changed patch using netns_eq]

Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/bridge/br_stp_if.c

@@ -128,7 +128,10 @@ static void br_stp_start(struct net_bridge *br)
 	char *argv[] = { BR_STP_PROG, br->dev->name, "start", NULL };
 	char *envp[] = { NULL };
 
+	if (net_eq(dev_net(br->dev), &init_net))
 		r = call_usermodehelper(BR_STP_PROG, argv, envp, UMH_WAIT_PROC);
+	else
+		r = -ENOENT;
 
 	spin_lock_bh(&br->lock);