1 files changed, 221 insertions, 0 deletions
diff --git a/keyring/user_keyring.go b/keyring/user_keyring.go
new file mode 100644
index 0000000..adac71a
--- /dev/null
+++ b/keyring/user_keyring.go
@@ -0,0 +1,221 @@
+/*
+ * user_keyring.go - Add/remove encryption policy keys to/from user keyrings
+ *
+ * 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 keyring
+
+import (
+	"os/user"
+	"runtime"
+	"unsafe"
+
+	"github.com/pkg/errors"
+	"golang.org/x/sys/unix"
+
+	"fmt"
+	"log"
+
+	"github.com/google/fscrypt/crypto"
+	"github.com/google/fscrypt/security"
+	"github.com/google/fscrypt/util"
+)
+
+// KeyType is always logon as required by filesystem encryption.
+const KeyType = "logon"
+
+// userAddKey puts the provided policy key into the user keyring for the
+// specified user with the provided description, and type logon.
+func userAddKey(key *crypto.Key, description string, targetUser *user.User) error {
+	runtime.LockOSThread() // ensure target user keyring remains possessed in thread keyring
+	defer runtime.UnlockOSThread()
+
+	// Create our payload (containing an FscryptKey)
+	payload, err := crypto.NewBlankKey(int(unsafe.Sizeof(unix.FscryptKey{})))
+	if err != nil {
+		return err
+	}
+	defer payload.Wipe()
+
+	// Cast the payload to an FscryptKey so we can initialize the fields.
+	fscryptKey := (*unix.FscryptKey)(payload.UnsafePtr())
+	// Mode is ignored by the kernel
+	fscryptKey.Mode = 0
+	fscryptKey.Size = uint32(key.Len())
+	copy(fscryptKey.Raw[:], key.Data())
+
+	keyringID, err := UserKeyringID(targetUser, true)
+	if err != nil {
+		return err
+	}
+	keyID, err := unix.AddKey(KeyType, description, payload.Data(), keyringID)
+	log.Printf("KeyctlAddKey(%s, %s, <data>, %d) = %d, %v",
+		KeyType, description, keyringID, keyID, err)
+	if err != nil {
+		return errors.Wrap(ErrKeyAdd, err.Error())
+	}
+	return nil
+}
+
+// userRemoveKey tries to remove a policy key from the user keyring with the
+// provided description. An error is returned if the key does not exist.
+func userRemoveKey(description string, targetUser *user.User) error {
+	runtime.LockOSThread() // ensure target user keyring remains possessed in thread keyring
+	defer runtime.UnlockOSThread()
+
+	keyID, err := userFindKey(description, targetUser)
+	if err != nil {
+		return ErrKeyNotPresent
+	}
+
+	// We use KEYCTL_INVALIDATE instead of KEYCTL_REVOKE because
+	// invalidating a key immediately removes it.
+	_, err = unix.KeyctlInt(unix.KEYCTL_INVALIDATE, keyID, 0, 0, 0)
+	log.Printf("KeyctlInvalidate(%d) = %v", keyID, err)
+	if err != nil {
+		return errors.Wrap(ErrKeyRemove, err.Error())
+	}
+	return nil
+}
+
+// userFindKey tries to locate a key in the user keyring with the provided
+// description. The key ID is returned if we can find the key. An error is
+// returned if the key does not exist.
+func userFindKey(description string, targetUser *user.User) (int, error) {
+	runtime.LockOSThread() // ensure target user keyring remains possessed in thread keyring
+	defer runtime.UnlockOSThread()
+
+	keyringID, err := UserKeyringID(targetUser, false)
+	if err != nil {
+		return 0, err
+	}
+
+	keyID, err := unix.KeyctlSearch(keyringID, KeyType, description, 0)
+	log.Printf("KeyctlSearch(%d, %s, %s) = %d, %v", keyringID, KeyType, description, keyID, err)
+	if err != nil {
+		return 0, errors.Wrap(ErrKeySearch, err.Error())
+	}
+	return keyID, err
+}
+
+// UserKeyringID returns the key id of the target user's user keyring. We also
+// ensure that the keyring will be accessible by linking it into the thread
+// keyring and linking it into the root user keyring (permissions allowing). If
+// checkSession is true, an error is returned if a normal user requests their
+// user keyring, but it is not in the current session keyring.
+func UserKeyringID(targetUser *user.User, checkSession bool) (int, error) {
+	runtime.LockOSThread() // ensure target user keyring remains possessed in thread keyring
+	defer runtime.UnlockOSThread()
+
+	uid := util.AtoiOrPanic(targetUser.Uid)
+	targetKeyring, err := userKeyringIDLookup(uid)
+	if err != nil {
+		return 0, errors.Wrap(ErrAccessUserKeyring, err.Error())
+	}
+
+	if !util.IsUserRoot() {
+		// Make sure the returned keyring will be accessible by checking
+		// that it is in the session keyring.
+		if checkSession && !isUserKeyringInSession(uid) {
+			return 0, ErrSessionUserKeying
+		}
+		return targetKeyring, nil
+	}
+
+	// Make sure the returned keyring will be accessible by linking it into
+	// the root user's user keyring (which will not be garbage collected).
+	rootKeyring, err := userKeyringIDLookup(0)
+	if err != nil {
+		return 0, errors.Wrap(ErrLinkUserKeyring, err.Error())
+	}
+
+	if rootKeyring != targetKeyring {
+		if err = keyringLink(targetKeyring, rootKeyring); err != nil {
+			return 0, errors.Wrap(ErrLinkUserKeyring, err.Error())
+		}
+	}
+	return targetKeyring, nil
+}
+
+func userKeyringIDLookup(uid int) (keyringID int, err error) {
+
+	// Our goals here are to:
+	//    - Find the user keyring (for the provided uid)
+	//    - Link it into the current thread keyring (so we can use it)
+	//    - Make no permanent changes to the process privileges
+	// Complicating this are the facts that:
+	//    - The value of KEY_SPEC_USER_KEYRING is determined by the ruid
+	//    - Keyring linking permissions use the euid
+	// So we have to change both the ruid and euid to make this work,
+	// setting the suid to 0 so that we can later switch back.
+	ruid, euid, suid := security.GetUids()
+	if ruid != uid || euid != uid {
+		if err = security.SetUids(uid, uid, 0); err != nil {
+			return
+		}
+		defer func() {
+			resetErr := security.SetUids(ruid, euid, suid)
+			if resetErr != nil {
+				err = resetErr
+			}
+		}()
+	}
+
+	// We get the value of KEY_SPEC_USER_KEYRING. Note that this will also
+	// trigger the creation of the uid keyring if it does not yet exist.
+	keyringID, err = unix.KeyctlGetKeyringID(unix.KEY_SPEC_USER_KEYRING, true)
+	log.Printf("keyringID(_uid.%d) = %d, %v", uid, keyringID, err)
+	if err != nil {
+		return 0, err
+	}
+
+	// We still want to use this keyring after our privileges are reset. So
+	// we link it into the thread keyring, preventing a loss of access.
+	//
+	// We must be under LockOSThread() for this to work reliably.  Note that
+	// we can't just use the process keyring, since it doesn't work reliably
+	// in Go programs, due to the Go runtime creating threads before the
+	// program starts and has a chance to create the process keyring.
+	if err = keyringLink(keyringID, unix.KEY_SPEC_THREAD_KEYRING); err != nil {
+		return 0, err
+	}
+
+	return keyringID, nil
+}
+
+// isUserKeyringInSession tells us if the user's uid keyring is in the current
+// session keyring.
+func isUserKeyringInSession(uid int) bool {
+	// We cannot use unix.KEY_SPEC_SESSION_KEYRING directly as that might
+	// create a session keyring if one does not exist.
+	sessionKeyring, err := unix.KeyctlGetKeyringID(unix.KEY_SPEC_SESSION_KEYRING, false)
+	log.Printf("keyringID(session) = %d, %v", sessionKeyring, err)
+	if err != nil {
+		return false
+	}
+
+	description := fmt.Sprintf("_uid.%d", uid)
+	id, err := unix.KeyctlSearch(sessionKeyring, "keyring", description, 0)
+	log.Printf("KeyctlSearch(%d, keyring, %s) = %d, %v", sessionKeyring, description, id, err)
+	return err == nil
+}
+
+func keyringLink(keyID int, keyringID int) error {
+	_, err := unix.KeyctlInt(unix.KEYCTL_LINK, keyID, keyringID, 0, 0)
+	log.Printf("KeyctlLink(%d, %d) = %v", keyID, keyringID, err)
+	return err
+}