pam_fscrypt: warn user if OLDAUTHTOK not given in chauthtok

If someone runs 'passwd USER' as root, the user is assigned a new login
passphrase without their fscrypt login protector being updated.  Detect
this case and show a warning message using pam_info().

Fixes #273

pam/pam.c

@@ -20,6 +20,7 @@
 #include "pam.h"
 
 #include <security/pam_appl.h>
+#include <security/pam_ext.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -107,3 +108,7 @@ void freeSecret(pam_handle_t* pamh, char* data, int error_status) {
   munlock(data, size);
   free(data);
 }
+
+void infoMessage(pam_handle_t* pamh, const char* message) {
+  pam_info(pamh, "%s", message);
+}

pam/pam.go

@@ -166,6 +166,13 @@ func (h *Handle) err() error {
 	return errors.New(s)
 }
 
+// InfoMessage sends a message to the application using pam_info().
+func (h *Handle) InfoMessage(message string) {
+	cMessage := C.CString(message)
+	defer C.free(unsafe.Pointer(cMessage))
+	C.infoMessage(h.handle, cMessage)
+}
+
 // Transaction represents a wrapped pam_handle_t type created with pam_start
 // form an application.
 type Transaction Handle

pam/pam.h

@@ -41,4 +41,7 @@ void *copyIntoSecret(void *data);
 // CleaupFunc that Zeros wipes a C string and unlocks and frees its memory.
 void freeSecret(pam_handle_t *pamh, char *data, int error_status);
 
+// Sends a message to the application using pam_info().
+void infoMessage(pam_handle_t *pamh, const char *message);
+
 #endif  // FSCRYPT_PAM_H

pam_fscrypt/pam_fscrypt.go

@@ -29,6 +29,7 @@ package main
 */
 import "C"
 import (
+	"fmt"
 	"log"
 	"unsafe"
 
@@ -300,6 +301,14 @@ func lockLoginPolicies(handle *pam.Handle) (bool, error) {
 	return needDropCaches, nil
 }
 
+var noOldAuthTokMessage string = `
+pam_fscrypt: cannot update login protector for '%s' because old passphrase
+was not given.  This is expected when changing a user's passphrase as root.
+You'll need to manually update the protector's passphrase using:
+
+   fscrypt metadata change-passphrase --protector=%s:%s
+`
+
 // Chauthtok rewraps the login protector when the passphrase changes.
 func Chauthtok(handle *pam.Handle, _ map[string]bool) error {
 	if err := handle.StartAsPamUser(); err != nil {
@@ -322,6 +331,9 @@ func Chauthtok(handle *pam.Handle, _ map[string]bool) error {
 		}
 		authtok, err := handle.GetItem(pam.Oldauthtok)
 		if err != nil {
+			handle.InfoMessage(fmt.Sprintf(noOldAuthTokMessage,
+				handle.PamUser.Username,
+				protector.Context.Mount.Path, protector.Descriptor()))
 			return nil, errors.Wrap(err, "could not get OLDAUTHTOK")
 		}
 		return crypto.NewKeyFromCString(authtok)