Change error handling to new package

This commit changes the error handing for the crypto, filesystem,
metadata, pam, and util packages to use the error handling library
github.com/pkg/errors. This means elimination of the FSError type, an
increased use of wrapping errors (as opposed to logging), switching
on the Cause() of an error (as opposed to its value), and improving our
integration tests involving TEST_FILESYSTEM_ROOT.

This commit also fixes a few bugs with the keyring code to ensure that
our {Find|Remove|Insert}PolicyKey functions are always operating on the
same keyring. The check for filesystem support has been moved from the
filesystem package to the metadata package. Finally, the API for the
filesystem package has been slightly modified:
	* filesystem.AllFilesystems() now returns all the filesystems in
	  sorted order
	* certain path methods are now public

O_SYNC is also removed for writing the metadata. We don't get that much
from syncing the metadata, as the actual file data could also be
corrupted by and IO error. The sync operation is also occasionally very
slow (~3 seconds) and can be unfriendly to battery life.

Change-Id: I392c2655141714b16dfdbc84ac09780072be2cf0

1 files changed, 22 insertions, 24 deletions

diff --git a/crypto/crypto.go b/crypto/crypto.go
index c6d6619..967243d 100644
--- a/crypto/crypto.go
+++ b/crypto/crypto.go
@@ -46,9 +46,9 @@ import (
 	"crypto/sha256"
 	"crypto/sha512"
 	"encoding/hex"
-	"errors"
 	"unsafe"
 
+	"github.com/pkg/errors"
 	"golang.org/x/crypto/hkdf"
 
 	"fscrypt/metadata"
@@ -57,34 +57,29 @@ import (
 
 // Crypto error values
 var (
-	ErrBadAuth          = errors.New("key authentication check failed")
-	ErrNegitiveLength   = errors.New("negative length requested for key")
-	ErrKeyAlloc         = util.SystemError("could not allocate memory for key")
-	ErrKeyFree          = util.SystemError("could not free memory of key")
-	ErrKeyringLocate    = util.SystemError("could not locate the session keyring")
-	ErrKeyringInsert    = util.SystemError("could not insert key into the session keyring")
-	ErrKeyringSearch    = util.SystemError("could not find key in the session keyring")
-	ErrKeyringDelete    = util.SystemError("could not delete key from the session keyring")
-	ErrRecoveryCode     = errors.New("provided recovery code had incorrect format")
-	ErrLowEntropy       = util.SystemError("insufficient entropy in pool to generate random bytes")
-	ErrRandNotSupported = util.SystemError("getrandom() not implemented; kernel must be v3.17 or later")
-	ErrRandFailed       = util.SystemError("cannot get random bytes")
+	ErrBadAuth        = errors.New("key authentication check failed")
+	ErrNegitiveLength = errors.New("keys cannot have negative lengths")
+	ErrRecoveryCode   = errors.New("invalid recovery code")
+	ErrGetrandomFail  = util.SystemError("getrandom() failed")
+	ErrKeyAlloc       = util.SystemError("could not allocate memory for key")
+	ErrKeyFree        = util.SystemError("could not free memory of key")
+	ErrKeyringLocate  = util.SystemError("could not locate the session keyring")
+	ErrKeyringInsert  = util.SystemError("could not insert key into the session keyring")
+	ErrKeyringSearch  = errors.New("could not find key with descriptor")
+	ErrKeyringDelete  = util.SystemError("could not delete key from the session keyring")
 )
 
 // panicInputLength panics if "name" has invalid length (expected != actual)
 func panicInputLength(name string, expected, actual int) {
-	if expected != actual {
-		util.NeverError(util.InvalidLengthError(name, expected, actual))
+	if err := util.CheckValidLength(expected, actual); err != nil {
+		panic(errors.Wrap(err, name))
 	}
 }
 
 // checkWrappingKey returns an error if the wrapping key has the wrong length
 func checkWrappingKey(wrappingKey *Key) error {
-	l := wrappingKey.Len()
-	if l != metadata.InternalKeyLen {
-		return util.InvalidLengthError("wrapping key", metadata.InternalKeyLen, l)
-	}
-	return nil
+	err := util.CheckValidLength(metadata.InternalKeyLen, wrappingKey.Len())
+	return errors.Wrap(err, "wrapping key")
 }
 
 // stretchKey stretches a key of length KeyLen using unsalted HKDF to make two
@@ -140,14 +135,14 @@ func getHMAC(key *Key, data ...[]byte) []byte {
 // and an HMAC to verify the wrapping key was correct. All of this is included
 // in the returned WrappedKeyData structure.
 func Wrap(wrappingKey, secretKey *Key) (*metadata.WrappedKeyData, error) {
-	err := checkWrappingKey(wrappingKey)
-	if err != nil {
+	if err := checkWrappingKey(wrappingKey); err != nil {
 		return nil, err
 	}
 
 	data := &metadata.WrappedKeyData{EncryptedKey: make([]byte, secretKey.Len())}
 
 	// Get random IV
+	var err error
 	if data.IV, err = NewRandomBuffer(metadata.IVLen); err != nil {
 		return nil, err
 	}
@@ -251,8 +246,11 @@ use it in "id" mode to provide extra protection against side-channel
 attacks. For more info see: https://github.com/P-H-C/phc-winner-argon2
 */
 func PassphraseHash(passphrase *Key, salt []byte, costs *metadata.HashingCosts) (*Key, error) {
-	if len(salt) != metadata.SaltLen {
-		return nil, util.InvalidLengthError("salt", metadata.SaltLen, len(salt))
+	if err := util.CheckValidLength(metadata.SaltLen, len(salt)); err != nil {
+		return nil, errors.Wrap(err, "passphrase hashing salt")
+	}
+	if err := costs.CheckValidity(); err != nil {
+		return nil, errors.Wrap(err, "passphrase hashing costs")
 	}
 
 	// This key will hold the hashing output