nfs: don't share mounts between network namespaces

There's no guarantee that an IP address in a different network namespace
actually represents the same endpoint.

Also, if we allow unprivileged nfs mounts some day then this might allow
an unprivileged user in another network namespace to misdirect somebody
else's nfs mounts.

If sharing between containers is really what's wanted then that could
still be arranged explicitly, for example with bind mounts.

Reported-by: "Eric W. Biederman" <ebiederm@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/super.c

@@ -2408,6 +2408,11 @@ static int nfs_compare_super_address(struct nfs_server *server1,
 				     struct nfs_server *server2)
 {
 	struct sockaddr *sap1, *sap2;
+	struct rpc_xprt *xprt1 = server1->client->cl_xprt;
+	struct rpc_xprt *xprt2 = server2->client->cl_xprt;
+
+	if (!net_eq(xprt1->xprt_net, xprt2->xprt_net))
+		return 0;
 
 	sap1 = (struct sockaddr *)&server1->nfs_client->cl_addr;
 	sap2 = (struct sockaddr *)&server2->nfs_client->cl_addr;