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/psdev.c

@@ -37,6 +37,7 @@
 #include <linux/list.h>
 #include <linux/mutex.h>
 #include <linux/device.h>
+#include <linux/pid_namespace.h>
 #include <asm/io.h>
 #include <asm/poll.h>
 #include <asm/uaccess.h>
@@ -266,6 +267,9 @@ static int coda_psdev_open(struct inode * inode, struct file * file)
 	struct venus_comm *vcp;
 	int idx, err;
 
+	if (task_active_pid_ns(current) != &init_pid_ns)
+		return -EINVAL;
+
 	idx = iminor(inode);
 	if (idx < 0 || idx >= MAX_CODADEVS)
 		return -ENODEV;