coda: Restrict coda messages to the initial pid namespace

Remove the slight chance that pids in coda messages will be
interpreted in the wrong pid namespace.

- Explicitly send all pids in coda messages in the initial pid
  namespace.
- Only allow mounts from processes in the initial pid namespace.
- Only allow processes in the initial pid namespace to open the coda
  character device to communicate with coda.

Cc: Jan Harkes <jaharkes@cs.cmu.edu>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/coda/inode.c

@@ -20,6 +20,7 @@
 #include <linux/file.h>
 #include <linux/vfs.h>
 #include <linux/slab.h>
+#include <linux/pid_namespace.h>
 
 #include <asm/uaccess.h>
 
@@ -157,6 +158,9 @@ static int coda_fill_super(struct super_block *sb, void *data, int silent)
 	int error;
 	int idx;
 
+	if (task_active_pid_ns(current) != &init_pid_ns)
+		return -EINVAL;
+
 	idx = get_device_index((struct coda_mount_data *) data);
 
 	/* Ignore errors in data, for backward compatibility */