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/*
* login.go - Checks the validity of a login token key against PAM.
*
* Copyright 2017 Google Inc.
* Author: Joe Richey (joerichey@google.com)
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License. You may obtain a copy of
* the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations under
* the License.
*/
// Package pam contains all the functionality for interfacing with Linux
// Pluggable Authentication Modules (PAM). Currently, all this package does is
// check the validity of a user's login passphrase.
// See http://www.linux-pam.org/Linux-PAM-html/ for more information.
package pam
/*
#cgo LDFLAGS: -lpam
#include <stdlib.h>
#include "pam.h"
*/
import "C"
import (
"log"
"sync"
"unsafe"
"github.com/pkg/errors"
"fscrypt/crypto"
"fscrypt/util"
)
// Global state is needed for the PAM callback, so we guard this function with a
// lock. tokenToCheck is only ever non-nil when loginLock is held.
var (
ErrPamInternal = util.SystemError("internal pam error")
loginLock sync.Mutex
tokenToCheck *crypto.Key
)
// unexpectedMessage logs an error encountered in the PAM callback.
//export unexpectedMessage
func unexpectedMessage(msg *C.char) {
log.Printf("pam encountered unexpected %q", C.GoString(msg))
}
// pamInput is run when the PAM module needs some input from the user. The
// message parameter is the prompt that would be displayed to the user.
//export pamInput
func pamInput(msg *C.char) *C.char {
log.Printf("requesting secret data with %q", C.GoString(msg))
// Memory for the key must be moved into a C string allocated by C.
cLen := C.size_t(tokenToCheck.Len())
cData := C.malloc(cLen + 1)
// View the cData as a go slice
goData := (*[1 << 30]byte)(cData)
copy(goData[:cLen], tokenToCheck.UnsafeData())
goData[cLen] = 0 // Null terminator
return (*C.char)(cData)
}
// IsUserLoginToken returns true if the presented token is the user's login key,
// false if it is not their login key, and an error if this cannot be
// determined. Note that unless the currently running process is root, this
// check will only work for the user running this process.
func IsUserLoginToken(username string, token *crypto.Key) (_ bool, err error) {
log.Printf("Checking login token for %s", username)
// We require global state for the function. This function never takes
// ownership of the token, so it is not responsible for wiping it.
loginLock.Lock()
tokenToCheck = token
defer func() {
tokenToCheck = nil
loginLock.Unlock()
}()
cUsername := C.CString(username)
defer C.free(unsafe.Pointer(cUsername))
var conv C.struct_pam_conv
var handle *C.struct_pam_handle
C.pam_init(&conv)
// Start the pam transaction with the desired conversation and handle.
returnCode := C.pam_start(C.fscrypt_service, cUsername, &conv, &handle)
if returnCode != C.PAM_SUCCESS {
return false, errors.Wrapf(ErrPamInternal, "pam_start() = %d", returnCode)
}
defer func() {
// End the PAM transaction, setting the error if appropriate.
returnCode = C.pam_end(handle, returnCode)
if returnCode != C.PAM_SUCCESS && err == nil {
err = errors.Wrapf(ErrPamInternal, "pam_end() = %d", returnCode)
}
}()
// Ask PAM to authenticate the token. We either get an answer or an error
returnCode = C.pam_authenticate(handle, 0)
switch returnCode {
case C.PAM_SUCCESS:
return true, nil
case C.PAM_AUTH_ERR:
return false, nil
default:
// PAM didn't give us an answer to the authentication question
return false, errors.Wrapf(ErrPamInternal, "pam_authenticate() = %d", returnCode)
}
}
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