:orphan:

  .. UBIFS Authentication
  .. sigma star gmbh
  .. 2018
  
  Introduction
  ============

  
  UBIFS utilizes the fscrypt framework to provide confidentiality for file
  contents and file names. This prevents attacks where an attacker is able to
  read contents of the filesystem on a single point in time. A classic example
  is a lost smartphone where the attacker is unable to read personal data stored
  on the device without the filesystem decryption key.
  
  At the current state, UBIFS encryption however does not prevent attacks where
  the attacker is able to modify the filesystem contents and the user uses the
  device afterwards. In such a scenario an attacker can modify filesystem
  contents arbitrarily without the user noticing. One example is to modify a
  binary to perform a malicious action when executed [DMC-CBC-ATTACK]. Since
  most of the filesystem metadata of UBIFS is stored in plain, this makes it
  fairly easy to swap files and replace their contents.
  
  Other full disk encryption systems like dm-crypt cover all filesystem metadata,
  which makes such kinds of attacks more complicated, but not impossible.
  Especially, if the attacker is given access to the device multiple points in
  time. For dm-crypt and other filesystems that build upon the Linux block IO
  layer, the dm-integrity or dm-verity subsystems [DM-INTEGRITY, DM-VERITY]
  can be used to get full data authentication at the block layer.
  These can also be combined with dm-crypt [CRYPTSETUP2].
  
  This document describes an approach to get file contents _and_ full metadata
  authentication for UBIFS. Since UBIFS uses fscrypt for file contents and file
  name encryption, the authentication system could be tied into fscrypt such that
  existing features like key derivation can be utilized. It should however also
  be possible to use UBIFS authentication without using encryption.

  MTD, UBI & UBIFS
  ----------------

  
  On Linux, the MTD (Memory Technology Devices) subsystem provides a uniform
  interface to access raw flash devices. One of the more prominent subsystems that
  work on top of MTD is UBI (Unsorted Block Images). It provides volume management
  for flash devices and is thus somewhat similar to LVM for block devices. In
  addition, it deals with flash-specific wear-leveling and transparent I/O error
  handling. UBI offers logical erase blocks (LEBs) to the layers on top of it
  and maps them transparently to physical erase blocks (PEBs) on the flash.
  
  UBIFS is a filesystem for raw flash which operates on top of UBI. Thus, wear
  leveling and some flash specifics are left to UBI, while UBIFS focuses on
  scalability, performance and recoverability.

  ::

  
  	+------------+ +*******+ +-----------+ +-----+
  	|            | * UBIFS * | UBI-BLOCK | | ... |
  	| JFFS/JFFS2 | +*******+ +-----------+ +-----+
  	|            | +-----------------------------+ +-----------+ +-----+
  	|            | |              UBI            | | MTD-BLOCK | | ... |
  	+------------+ +-----------------------------+ +-----------+ +-----+
  	+------------------------------------------------------------------+
  	|                  MEMORY TECHNOLOGY DEVICES (MTD)                 |
  	+------------------------------------------------------------------+
  	+-----------------------------+ +--------------------------+ +-----+
  	|         NAND DRIVERS        | |        NOR DRIVERS       | | ... |
  	+-----------------------------+ +--------------------------+ +-----+
  
              Figure 1: Linux kernel subsystems for dealing with raw flash
  
  
  
  Internally, UBIFS maintains multiple data structures which are persisted on
  the flash:
  
  - *Index*: an on-flash B+ tree where the leaf nodes contain filesystem data
  - *Journal*: an additional data structure to collect FS changes before updating
    the on-flash index and reduce flash wear.
  - *Tree Node Cache (TNC)*: an in-memory B+ tree that reflects the current FS
    state to avoid frequent flash reads. It is basically the in-memory
    representation of the index, but contains additional attributes.
  - *LEB property tree (LPT)*: an on-flash B+ tree for free space accounting per
    UBI LEB.
  
  In the remainder of this section we will cover the on-flash UBIFS data
  structures in more detail. The TNC is of less importance here since it is never
  persisted onto the flash directly. More details on UBIFS can also be found in
  [UBIFS-WP].

  UBIFS Index & Tree Node Cache
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  
  Basic on-flash UBIFS entities are called *nodes*. UBIFS knows different types
  of nodes. Eg. data nodes (`struct ubifs_data_node`) which store chunks of file
  contents or inode nodes (`struct ubifs_ino_node`) which represent VFS inodes.
  Almost all types of nodes share a common header (`ubifs_ch`) containing basic
  information like node type, node length, a sequence number, etc. (see
  `fs/ubifs/ubifs-media.h`in kernel source). Exceptions are entries of the LPT
  and some less important node types like padding nodes which are used to pad
  unusable content at the end of LEBs.
  
  To avoid re-writing the whole B+ tree on every single change, it is implemented
  as *wandering tree*, where only the changed nodes are re-written and previous
  versions of them are obsoleted without erasing them right away. As a result,
  the index is not stored in a single place on the flash, but *wanders* around
  and there are obsolete parts on the flash as long as the LEB containing them is
  not reused by UBIFS. To find the most recent version of the index, UBIFS stores
  a special node called *master node* into UBI LEB 1 which always points to the
  most recent root node of the UBIFS index. For recoverability, the master node
  is additionally duplicated to LEB 2. Mounting UBIFS is thus a simple read of
  LEB 1 and 2 to get the current master node and from there get the location of
  the most recent on-flash index.
  
  The TNC is the in-memory representation of the on-flash index. It contains some
  additional runtime attributes per node which are not persisted. One of these is
  a dirty-flag which marks nodes that have to be persisted the next time the
  index is written onto the flash. The TNC acts as a write-back cache and all
  modifications of the on-flash index are done through the TNC. Like other caches,
  the TNC does not have to mirror the full index into memory, but reads parts of
  it from flash whenever needed. A *commit* is the UBIFS operation of updating the
  on-flash filesystem structures like the index. On every commit, the TNC nodes
  marked as dirty are written to the flash to update the persisted index.

  Journal
  ~~~~~~~

  
  To avoid wearing out the flash, the index is only persisted (*commited*) when

  certain conditions are met (eg. ``fsync(2)``). The journal is used to record

  any changes (in form of inode nodes, data nodes etc.) between commits
  of the index. During mount, the journal is read from the flash and replayed
  onto the TNC (which will be created on-demand from the on-flash index).
  
  UBIFS reserves a bunch of LEBs just for the journal called *log area*. The
  amount of log area LEBs is configured on filesystem creation (using

  ``mkfs.ubifs``) and stored in the superblock node. The log area contains only

  two types of nodes: *reference nodes* and *commit start nodes*. A commit start
  node is written whenever an index commit is performed. Reference nodes are
  written on every journal update. Each reference node points to the position of
  other nodes (inode nodes, data nodes etc.) on the flash that are part of this
  journal entry. These nodes are called *buds* and describe the actual filesystem
  changes including their data.
  
  The log area is maintained as a ring. Whenever the journal is almost full,
  a commit is initiated. This also writes a commit start node so that during
  mount, UBIFS will seek for the most recent commit start node and just replay
  every reference node after that. Every reference node before the commit start
  node will be ignored as they are already part of the on-flash index.
  
  When writing a journal entry, UBIFS first ensures that enough space is
  available to write the reference node and buds part of this entry. Then, the
  reference node is written and afterwards the buds describing the file changes.
  On replay, UBIFS will record every reference node and inspect the location of
  the referenced LEBs to discover the buds. If these are corrupt or missing,
  UBIFS will attempt to recover them by re-reading the LEB. This is however only
  done for the last referenced LEB of the journal. Only this can become corrupt
  because of a power cut. If the recovery fails, UBIFS will not mount. An error
  for every other LEB will directly cause UBIFS to fail the mount operation.

  ::

  
         | ----    LOG AREA     ---- | ----------    MAIN AREA    ------------ |
  
          -----+------+-----+--------+----   ------+-----+-----+---------------
          \    |      |     |        |   /  /      |     |     |               \
          / CS |  REF | REF |        |   \  \ DENT | INO | INO |               /
          \    |      |     |        |   /  /      |     |     |               \
           ----+------+-----+--------+---   -------+-----+-----+----------------
                   |     |                  ^            ^
                   |     |                  |            |
                   +------------------------+            |
                         |                               |
                         +-------------------------------+
  
  
                  Figure 2: UBIFS flash layout of log area with commit start nodes
                            (CS) and reference nodes (REF) pointing to main area
                            containing their buds

  LEB Property Tree/Table
  ~~~~~~~~~~~~~~~~~~~~~~~

  
  The LEB property tree is used to store per-LEB information. This includes the

  LEB type and amount of free and *dirty* (old, obsolete content) space [1]_ on

  the LEB. The type is important, because UBIFS never mixes index nodes with data
  nodes on a single LEB and thus each LEB has a specific purpose. This again is
  useful for free space calculations. See [UBIFS-WP] for more details.
  
  The LEB property tree again is a B+ tree, but it is much smaller than the
  index. Due to its smaller size it is always written as one chunk on every
  commit. Thus, saving the LPT is an atomic operation.

  .. [1] Since LEBs can only be appended and never overwritten, there is a
     difference between free space ie. the remaining space left on the LEB to be
     written to without erasing it and previously written content that is obsolete
     but can't be overwritten without erasing the full LEB.

  UBIFS Authentication
  ====================

  
  This chapter introduces UBIFS authentication which enables UBIFS to verify
  the authenticity and integrity of metadata and file contents stored on flash.

  Threat Model
  ------------

  
  UBIFS authentication enables detection of offline data modification. While it
  does not prevent it, it enables (trusted) code to check the integrity and
  authenticity of on-flash file contents and filesystem metadata. This covers
  attacks where file contents are swapped.
  
  UBIFS authentication will not protect against rollback of full flash contents.
  Ie. an attacker can still dump the flash and restore it at a later time without
  detection. It will also not protect against partial rollback of individual
  index commits. That means that an attacker is able to partially undo changes.
  This is possible because UBIFS does not immediately overwrites obsolete
  versions of the index tree or the journal, but instead marks them as obsolete
  and garbage collection erases them at a later time. An attacker can use this by
  erasing parts of the current tree and restoring old versions that are still on
  the flash and have not yet been erased. This is possible, because every commit
  will always write a new version of the index root node and the master node
  without overwriting the previous version. This is further helped by the
  wear-leveling operations of UBI which copies contents from one physical
  eraseblock to another and does not atomically erase the first eraseblock.
  
  UBIFS authentication does not cover attacks where an attacker is able to
  execute code on the device after the authentication key was provided.
  Additional measures like secure boot and trusted boot have to be taken to
  ensure that only trusted code is executed on a device.

  Authentication
  --------------

  
  To be able to fully trust data read from flash, all UBIFS data structures
  stored on flash are authenticated. That is:
  
  - The index which includes file contents, file metadata like extended
    attributes, file length etc.
  - The journal which also contains file contents and metadata by recording changes
    to the filesystem
  - The LPT which stores UBI LEB metadata which UBIFS uses for free space accounting

  Index Authentication
  ~~~~~~~~~~~~~~~~~~~~

  
  Through UBIFS' concept of a wandering tree, it already takes care of only
  updating and persisting changed parts from leaf node up to the root node
  of the full B+ tree. This enables us to augment the index nodes of the tree
  with a hash over each node's child nodes. As a result, the index basically also
  a Merkle tree. Since the leaf nodes of the index contain the actual filesystem
  data, the hashes of their parent index nodes thus cover all the file contents
  and file metadata. When a file changes, the UBIFS index is updated accordingly
  from the leaf nodes up to the root node including the master node. This process
  can be hooked to recompute the hash only for each changed node at the same time.
  Whenever a file is read, UBIFS can verify the hashes from each leaf node up to
  the root node to ensure the node's integrity.
  
  To ensure the authenticity of the whole index, the UBIFS master node stores a
  keyed hash (HMAC) over its own contents and a hash of the root node of the index
  tree. As mentioned above, the master node is always written to the flash whenever
  the index is persisted (ie. on index commit).
  
  Using this approach only UBIFS index nodes and the master node are changed to
  include a hash. All other types of nodes will remain unchanged. This reduces
  the storage overhead which is precious for users of UBIFS (ie. embedded
  devices).

  ::

  
                               +---------------+
                               |  Master Node  |
                               |    (hash)     |
                               +---------------+
                                       |
                                       v
                              +-------------------+
                              |  Index Node #1    |
                              |                   |
                              | branch0   branchn |
                              | (hash)    (hash)  |
                              +-------------------+
                                 |    ...   |  (fanout: 8)
                                 |          |
                         +-------+          +------+
                         |                         |
                         v                         v
              +-------------------+       +-------------------+
              |  Index Node #2    |       |  Index Node #3    |
              |                   |       |                   |
              | branch0   branchn |       | branch0   branchn |
              | (hash)    (hash)  |       | (hash)    (hash)  |
              +-------------------+       +-------------------+
                   |   ...                     |   ...   |
                   v                           v         v
                 +-----------+         +----------+  +-----------+
                 | Data Node |         | INO Node |  | DENT Node |
                 +-----------+         +----------+  +-----------+
  
  
             Figure 3: Coverage areas of index node hash and master node HMAC
  
  
  
  The most important part for robustness and power-cut safety is to atomically
  persist the hash and file contents. Here the existing UBIFS logic for how
  changed nodes are persisted is already designed for this purpose such that
  UBIFS can safely recover if a power-cut occurs while persisting. Adding
  hashes to index nodes does not change this since each hash will be persisted
  atomically together with its respective node.

  Journal Authentication
  ~~~~~~~~~~~~~~~~~~~~~~

  
  The journal is authenticated too. Since the journal is continuously written
  it is necessary to also add authentication information frequently to the
  journal so that in case of a powercut not too much data can't be authenticated.
  This is done by creating a continuous hash beginning from the commit start node
  over the previous reference nodes, the current reference node, and the bud
  nodes. From time to time whenever it is suitable authentication nodes are added
  between the bud nodes. This new node type contains a HMAC over the current state
  of the hash chain. That way a journal can be authenticated up to the last
  authentication node. The tail of the journal which may not have a authentication
  node cannot be authenticated and is skipped during journal replay.

  We get this picture for journal authentication::

  
      ,,,,,,,,
      ,......,...........................................
      ,. CS  ,               hash1.----.           hash2.----.
      ,.  |  ,                    .    |hmac            .    |hmac
      ,.  v  ,                    .    v                .    v
      ,.REF#0,-> bud -> bud -> bud.-> auth -> bud -> bud.-> auth ...
      ,..|...,...........................................
      ,  |   ,
      ,  |   ,,,,,,,,,,,,,,,
      .  |            hash3,----.
      ,  |                 ,    |hmac
      ,  v                 ,    v
      , REF#1 -> bud -> bud,-> auth ...
      ,,,|,,,,,,,,,,,,,,,,,,
         v
        REF#2 -> ...
         |
         V
        ...
  
  Since the hash also includes the reference nodes an attacker cannot reorder or
  skip any journal heads for replay. An attacker can only remove bud nodes or
  reference nodes from the end of the journal, effectively rewinding the
  filesystem at maximum back to the last commit.
  
  The location of the log area is stored in the master node. Since the master
  node is authenticated with a HMAC as described above, it is not possible to
  tamper with that without detection. The size of the log area is specified when
  the filesystem is created using `mkfs.ubifs` and stored in the superblock node.
  To avoid tampering with this and other values stored there, a HMAC is added to
  the superblock struct. The superblock node is stored in LEB 0 and is only
  modified on feature flag or similar changes, but never on file changes.

  LPT Authentication
  ~~~~~~~~~~~~~~~~~~

  
  The location of the LPT root node on the flash is stored in the UBIFS master
  node. Since the LPT is written and read atomically on every commit, there is
  no need to authenticate individual nodes of the tree. It suffices to
  protect the integrity of the full LPT by a simple hash stored in the master
  node. Since the master node itself is authenticated, the LPTs authenticity can
  be verified by verifying the authenticity of the master node and comparing the
  LTP hash stored there with the hash computed from the read on-flash LPT.

  Key Management
  --------------

  
  For simplicity, UBIFS authentication uses a single key to compute the HMACs
  of superblock, master, commit start and reference nodes. This key has to be
  available on creation of the filesystem (`mkfs.ubifs`) to authenticate the
  superblock node. Further, it has to be available on mount of the filesystem
  to verify authenticated nodes and generate new HMACs for changes.
  
  UBIFS authentication is intended to operate side-by-side with UBIFS encryption
  (fscrypt) to provide confidentiality and authenticity. Since UBIFS encryption
  has a different approach of encryption policies per directory, there can be
  multiple fscrypt master keys and there might be folders without encryption.
  UBIFS authentication on the other hand has an all-or-nothing approach in the
  sense that it either authenticates everything of the filesystem or nothing.
  Because of this and because UBIFS authentication should also be usable without
  encryption, it does not share the same master key with fscrypt, but manages
  a dedicated authentication key.
  
  The API for providing the authentication key has yet to be defined, but the
  key can eg. be provided by userspace through a keyring similar to the way it
  is currently done in fscrypt. It should however be noted that the current
  fscrypt approach has shown its flaws and the userspace API will eventually
  change [FSCRYPT-POLICY2].
  
  Nevertheless, it will be possible for a user to provide a single passphrase
  or key in userspace that covers UBIFS authentication and encryption. This can
  be solved by the corresponding userspace tools which derive a second key for
  authentication in addition to the derived fscrypt master key used for
  encryption.
  
  To be able to check if the proper key is available on mount, the UBIFS
  superblock node will additionally store a hash of the authentication key. This
  approach is similar to the approach proposed for fscrypt encryption policy v2
  [FSCRYPT-POLICY2].

  Future Extensions
  =================

  
  In certain cases where a vendor wants to provide an authenticated filesystem
  image to customers, it should be possible to do so without sharing the secret
  UBIFS authentication key. Instead, in addition the each HMAC a digital
  signature could be stored where the vendor shares the public key alongside the
  filesystem image. In case this filesystem has to be modified afterwards,
  UBIFS can exchange all digital signatures with HMACs on first mount similar
  to the way the IMA/EVM subsystem deals with such situations. The HMAC key
  will then have to be provided beforehand in the normal way.

  References
  ==========

  
  [CRYPTSETUP2]        http://www.saout.de/pipermail/dm-crypt/2017-November/005745.html
  
  [DMC-CBC-ATTACK]     http://www.jakoblell.com/blog/2013/12/22/practical-malleability-attack-against-cbc-encrypted-luks-partitions/

  [DM-INTEGRITY]       https://www.kernel.org/doc/Documentation/device-mapper/dm-integrity.rst

  [DM-VERITY]          https://www.kernel.org/doc/Documentation/device-mapper/verity.rst

  
  [FSCRYPT-POLICY2]    https://www.spinics.net/lists/linux-ext4/msg58710.html
  
  [UBIFS-WP]           http://www.linux-mtd.infradead.org/doc/ubifs_whitepaper.pdf