linux/Documentation/filesystems/ubifs-authentication.rst
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   1.. SPDX-License-Identifier: GPL-2.0
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   3.. UBIFS Authentication
   4.. sigma star gmbh
   5.. 2018
   6
   7============================
   8UBIFS Authentication Support
   9============================
  10
  11Introduction
  12============
  13
  14UBIFS utilizes the fscrypt framework to provide confidentiality for file
  15contents and file names. This prevents attacks where an attacker is able to
  16read contents of the filesystem on a single point in time. A classic example
  17is a lost smartphone where the attacker is unable to read personal data stored
  18on the device without the filesystem decryption key.
  19
  20At the current state, UBIFS encryption however does not prevent attacks where
  21the attacker is able to modify the filesystem contents and the user uses the
  22device afterwards. In such a scenario an attacker can modify filesystem
  23contents arbitrarily without the user noticing. One example is to modify a
  24binary to perform a malicious action when executed [DMC-CBC-ATTACK]. Since
  25most of the filesystem metadata of UBIFS is stored in plain, this makes it
  26fairly easy to swap files and replace their contents.
  27
  28Other full disk encryption systems like dm-crypt cover all filesystem metadata,
  29which makes such kinds of attacks more complicated, but not impossible.
  30Especially, if the attacker is given access to the device multiple points in
  31time. For dm-crypt and other filesystems that build upon the Linux block IO
  32layer, the dm-integrity or dm-verity subsystems [DM-INTEGRITY, DM-VERITY]
  33can be used to get full data authentication at the block layer.
  34These can also be combined with dm-crypt [CRYPTSETUP2].
  35
  36This document describes an approach to get file contents _and_ full metadata
  37authentication for UBIFS. Since UBIFS uses fscrypt for file contents and file
  38name encryption, the authentication system could be tied into fscrypt such that
  39existing features like key derivation can be utilized. It should however also
  40be possible to use UBIFS authentication without using encryption.
  41
  42
  43MTD, UBI & UBIFS
  44----------------
  45
  46On Linux, the MTD (Memory Technology Devices) subsystem provides a uniform
  47interface to access raw flash devices. One of the more prominent subsystems that
  48work on top of MTD is UBI (Unsorted Block Images). It provides volume management
  49for flash devices and is thus somewhat similar to LVM for block devices. In
  50addition, it deals with flash-specific wear-leveling and transparent I/O error
  51handling. UBI offers logical erase blocks (LEBs) to the layers on top of it
  52and maps them transparently to physical erase blocks (PEBs) on the flash.
  53
  54UBIFS is a filesystem for raw flash which operates on top of UBI. Thus, wear
  55leveling and some flash specifics are left to UBI, while UBIFS focuses on
  56scalability, performance and recoverability.
  57
  58::
  59
  60        +------------+ +*******+ +-----------+ +-----+
  61        |            | * UBIFS * | UBI-BLOCK | | ... |
  62        | JFFS/JFFS2 | +*******+ +-----------+ +-----+
  63        |            | +-----------------------------+ +-----------+ +-----+
  64        |            | |              UBI            | | MTD-BLOCK | | ... |
  65        +------------+ +-----------------------------+ +-----------+ +-----+
  66        +------------------------------------------------------------------+
  67        |                  MEMORY TECHNOLOGY DEVICES (MTD)                 |
  68        +------------------------------------------------------------------+
  69        +-----------------------------+ +--------------------------+ +-----+
  70        |         NAND DRIVERS        | |        NOR DRIVERS       | | ... |
  71        +-----------------------------+ +--------------------------+ +-----+
  72
  73            Figure 1: Linux kernel subsystems for dealing with raw flash
  74
  75
  76
  77Internally, UBIFS maintains multiple data structures which are persisted on
  78the flash:
  79
  80- *Index*: an on-flash B+ tree where the leaf nodes contain filesystem data
  81- *Journal*: an additional data structure to collect FS changes before updating
  82  the on-flash index and reduce flash wear.
  83- *Tree Node Cache (TNC)*: an in-memory B+ tree that reflects the current FS
  84  state to avoid frequent flash reads. It is basically the in-memory
  85  representation of the index, but contains additional attributes.
  86- *LEB property tree (LPT)*: an on-flash B+ tree for free space accounting per
  87  UBI LEB.
  88
  89In the remainder of this section we will cover the on-flash UBIFS data
  90structures in more detail. The TNC is of less importance here since it is never
  91persisted onto the flash directly. More details on UBIFS can also be found in
  92[UBIFS-WP].
  93
  94
  95UBIFS Index & Tree Node Cache
  96~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  97
  98Basic on-flash UBIFS entities are called *nodes*. UBIFS knows different types
  99of nodes. Eg. data nodes (``struct ubifs_data_node``) which store chunks of file
 100contents or inode nodes (``struct ubifs_ino_node``) which represent VFS inodes.
 101Almost all types of nodes share a common header (``ubifs_ch``) containing basic
 102information like node type, node length, a sequence number, etc. (see
 103``fs/ubifs/ubifs-media.h`` in kernel source). Exceptions are entries of the LPT
 104and some less important node types like padding nodes which are used to pad
 105unusable content at the end of LEBs.
 106
 107To avoid re-writing the whole B+ tree on every single change, it is implemented
 108as *wandering tree*, where only the changed nodes are re-written and previous
 109versions of them are obsoleted without erasing them right away. As a result,
 110the index is not stored in a single place on the flash, but *wanders* around
 111and there are obsolete parts on the flash as long as the LEB containing them is
 112not reused by UBIFS. To find the most recent version of the index, UBIFS stores
 113a special node called *master node* into UBI LEB 1 which always points to the
 114most recent root node of the UBIFS index. For recoverability, the master node
 115is additionally duplicated to LEB 2. Mounting UBIFS is thus a simple read of
 116LEB 1 and 2 to get the current master node and from there get the location of
 117the most recent on-flash index.
 118
 119The TNC is the in-memory representation of the on-flash index. It contains some
 120additional runtime attributes per node which are not persisted. One of these is
 121a dirty-flag which marks nodes that have to be persisted the next time the
 122index is written onto the flash. The TNC acts as a write-back cache and all
 123modifications of the on-flash index are done through the TNC. Like other caches,
 124the TNC does not have to mirror the full index into memory, but reads parts of
 125it from flash whenever needed. A *commit* is the UBIFS operation of updating the
 126on-flash filesystem structures like the index. On every commit, the TNC nodes
 127marked as dirty are written to the flash to update the persisted index.
 128
 129
 130Journal
 131~~~~~~~
 132
 133To avoid wearing out the flash, the index is only persisted (*commited*) when
 134certain conditions are met (eg. ``fsync(2)``). The journal is used to record
 135any changes (in form of inode nodes, data nodes etc.) between commits
 136of the index. During mount, the journal is read from the flash and replayed
 137onto the TNC (which will be created on-demand from the on-flash index).
 138
 139UBIFS reserves a bunch of LEBs just for the journal called *log area*. The
 140amount of log area LEBs is configured on filesystem creation (using
 141``mkfs.ubifs``) and stored in the superblock node. The log area contains only
 142two types of nodes: *reference nodes* and *commit start nodes*. A commit start
 143node is written whenever an index commit is performed. Reference nodes are
 144written on every journal update. Each reference node points to the position of
 145other nodes (inode nodes, data nodes etc.) on the flash that are part of this
 146journal entry. These nodes are called *buds* and describe the actual filesystem
 147changes including their data.
 148
 149The log area is maintained as a ring. Whenever the journal is almost full,
 150a commit is initiated. This also writes a commit start node so that during
 151mount, UBIFS will seek for the most recent commit start node and just replay
 152every reference node after that. Every reference node before the commit start
 153node will be ignored as they are already part of the on-flash index.
 154
 155When writing a journal entry, UBIFS first ensures that enough space is
 156available to write the reference node and buds part of this entry. Then, the
 157reference node is written and afterwards the buds describing the file changes.
 158On replay, UBIFS will record every reference node and inspect the location of
 159the referenced LEBs to discover the buds. If these are corrupt or missing,
 160UBIFS will attempt to recover them by re-reading the LEB. This is however only
 161done for the last referenced LEB of the journal. Only this can become corrupt
 162because of a power cut. If the recovery fails, UBIFS will not mount. An error
 163for every other LEB will directly cause UBIFS to fail the mount operation.
 164
 165::
 166
 167       | ----    LOG AREA     ---- | ----------    MAIN AREA    ------------ |
 168
 169        -----+------+-----+--------+----   ------+-----+-----+---------------
 170        \    |      |     |        |   /  /      |     |     |               \
 171        / CS |  REF | REF |        |   \  \ DENT | INO | INO |               /
 172        \    |      |     |        |   /  /      |     |     |               \
 173         ----+------+-----+--------+---   -------+-----+-----+----------------
 174                 |     |                  ^            ^
 175                 |     |                  |            |
 176                 +------------------------+            |
 177                       |                               |
 178                       +-------------------------------+
 179
 180
 181                Figure 2: UBIFS flash layout of log area with commit start nodes
 182                          (CS) and reference nodes (REF) pointing to main area
 183                          containing their buds
 184
 185
 186LEB Property Tree/Table
 187~~~~~~~~~~~~~~~~~~~~~~~
 188
 189The LEB property tree is used to store per-LEB information. This includes the
 190LEB type and amount of free and *dirty* (old, obsolete content) space [1]_ on
 191the LEB. The type is important, because UBIFS never mixes index nodes with data
 192nodes on a single LEB and thus each LEB has a specific purpose. This again is
 193useful for free space calculations. See [UBIFS-WP] for more details.
 194
 195The LEB property tree again is a B+ tree, but it is much smaller than the
 196index. Due to its smaller size it is always written as one chunk on every
 197commit. Thus, saving the LPT is an atomic operation.
 198
 199
 200.. [1] Since LEBs can only be appended and never overwritten, there is a
 201   difference between free space ie. the remaining space left on the LEB to be
 202   written to without erasing it and previously written content that is obsolete
 203   but can't be overwritten without erasing the full LEB.
 204
 205
 206UBIFS Authentication
 207====================
 208
 209This chapter introduces UBIFS authentication which enables UBIFS to verify
 210the authenticity and integrity of metadata and file contents stored on flash.
 211
 212
 213Threat Model
 214------------
 215
 216UBIFS authentication enables detection of offline data modification. While it
 217does not prevent it, it enables (trusted) code to check the integrity and
 218authenticity of on-flash file contents and filesystem metadata. This covers
 219attacks where file contents are swapped.
 220
 221UBIFS authentication will not protect against rollback of full flash contents.
 222Ie. an attacker can still dump the flash and restore it at a later time without
 223detection. It will also not protect against partial rollback of individual
 224index commits. That means that an attacker is able to partially undo changes.
 225This is possible because UBIFS does not immediately overwrites obsolete
 226versions of the index tree or the journal, but instead marks them as obsolete
 227and garbage collection erases them at a later time. An attacker can use this by
 228erasing parts of the current tree and restoring old versions that are still on
 229the flash and have not yet been erased. This is possible, because every commit
 230will always write a new version of the index root node and the master node
 231without overwriting the previous version. This is further helped by the
 232wear-leveling operations of UBI which copies contents from one physical
 233eraseblock to another and does not atomically erase the first eraseblock.
 234
 235UBIFS authentication does not cover attacks where an attacker is able to
 236execute code on the device after the authentication key was provided.
 237Additional measures like secure boot and trusted boot have to be taken to
 238ensure that only trusted code is executed on a device.
 239
 240
 241Authentication
 242--------------
 243
 244To be able to fully trust data read from flash, all UBIFS data structures
 245stored on flash are authenticated. That is:
 246
 247- The index which includes file contents, file metadata like extended
 248  attributes, file length etc.
 249- The journal which also contains file contents and metadata by recording changes
 250  to the filesystem
 251- The LPT which stores UBI LEB metadata which UBIFS uses for free space accounting
 252
 253
 254Index Authentication
 255~~~~~~~~~~~~~~~~~~~~
 256
 257Through UBIFS' concept of a wandering tree, it already takes care of only
 258updating and persisting changed parts from leaf node up to the root node
 259of the full B+ tree. This enables us to augment the index nodes of the tree
 260with a hash over each node's child nodes. As a result, the index basically also
 261a Merkle tree. Since the leaf nodes of the index contain the actual filesystem
 262data, the hashes of their parent index nodes thus cover all the file contents
 263and file metadata. When a file changes, the UBIFS index is updated accordingly
 264from the leaf nodes up to the root node including the master node. This process
 265can be hooked to recompute the hash only for each changed node at the same time.
 266Whenever a file is read, UBIFS can verify the hashes from each leaf node up to
 267the root node to ensure the node's integrity.
 268
 269To ensure the authenticity of the whole index, the UBIFS master node stores a
 270keyed hash (HMAC) over its own contents and a hash of the root node of the index
 271tree. As mentioned above, the master node is always written to the flash whenever
 272the index is persisted (ie. on index commit).
 273
 274Using this approach only UBIFS index nodes and the master node are changed to
 275include a hash. All other types of nodes will remain unchanged. This reduces
 276the storage overhead which is precious for users of UBIFS (ie. embedded
 277devices).
 278
 279::
 280
 281                             +---------------+
 282                             |  Master Node  |
 283                             |    (hash)     |
 284                             +---------------+
 285                                     |
 286                                     v
 287                            +-------------------+
 288                            |  Index Node #1    |
 289                            |                   |
 290                            | branch0   branchn |
 291                            | (hash)    (hash)  |
 292                            +-------------------+
 293                               |    ...   |  (fanout: 8)
 294                               |          |
 295                       +-------+          +------+
 296                       |                         |
 297                       v                         v
 298            +-------------------+       +-------------------+
 299            |  Index Node #2    |       |  Index Node #3    |
 300            |                   |       |                   |
 301            | branch0   branchn |       | branch0   branchn |
 302            | (hash)    (hash)  |       | (hash)    (hash)  |
 303            +-------------------+       +-------------------+
 304                 |   ...                     |   ...   |
 305                 v                           v         v
 306               +-----------+         +----------+  +-----------+
 307               | Data Node |         | INO Node |  | DENT Node |
 308               +-----------+         +----------+  +-----------+
 309
 310
 311           Figure 3: Coverage areas of index node hash and master node HMAC
 312
 313
 314
 315The most important part for robustness and power-cut safety is to atomically
 316persist the hash and file contents. Here the existing UBIFS logic for how
 317changed nodes are persisted is already designed for this purpose such that
 318UBIFS can safely recover if a power-cut occurs while persisting. Adding
 319hashes to index nodes does not change this since each hash will be persisted
 320atomically together with its respective node.
 321
 322
 323Journal Authentication
 324~~~~~~~~~~~~~~~~~~~~~~
 325
 326The journal is authenticated too. Since the journal is continuously written
 327it is necessary to also add authentication information frequently to the
 328journal so that in case of a powercut not too much data can't be authenticated.
 329This is done by creating a continuous hash beginning from the commit start node
 330over the previous reference nodes, the current reference node, and the bud
 331nodes. From time to time whenever it is suitable authentication nodes are added
 332between the bud nodes. This new node type contains a HMAC over the current state
 333of the hash chain. That way a journal can be authenticated up to the last
 334authentication node. The tail of the journal which may not have a authentication
 335node cannot be authenticated and is skipped during journal replay.
 336
 337We get this picture for journal authentication::
 338
 339    ,,,,,,,,
 340    ,......,...........................................
 341    ,. CS  ,               hash1.----.           hash2.----.
 342    ,.  |  ,                    .    |hmac            .    |hmac
 343    ,.  v  ,                    .    v                .    v
 344    ,.REF#0,-> bud -> bud -> bud.-> auth -> bud -> bud.-> auth ...
 345    ,..|...,...........................................
 346    ,  |   ,
 347    ,  |   ,,,,,,,,,,,,,,,
 348    .  |            hash3,----.
 349    ,  |                 ,    |hmac
 350    ,  v                 ,    v
 351    , REF#1 -> bud -> bud,-> auth ...
 352    ,,,|,,,,,,,,,,,,,,,,,,
 353       v
 354      REF#2 -> ...
 355       |
 356       V
 357      ...
 358
 359Since the hash also includes the reference nodes an attacker cannot reorder or
 360skip any journal heads for replay. An attacker can only remove bud nodes or
 361reference nodes from the end of the journal, effectively rewinding the
 362filesystem at maximum back to the last commit.
 363
 364The location of the log area is stored in the master node. Since the master
 365node is authenticated with a HMAC as described above, it is not possible to
 366tamper with that without detection. The size of the log area is specified when
 367the filesystem is created using `mkfs.ubifs` and stored in the superblock node.
 368To avoid tampering with this and other values stored there, a HMAC is added to
 369the superblock struct. The superblock node is stored in LEB 0 and is only
 370modified on feature flag or similar changes, but never on file changes.
 371
 372
 373LPT Authentication
 374~~~~~~~~~~~~~~~~~~
 375
 376The location of the LPT root node on the flash is stored in the UBIFS master
 377node. Since the LPT is written and read atomically on every commit, there is
 378no need to authenticate individual nodes of the tree. It suffices to
 379protect the integrity of the full LPT by a simple hash stored in the master
 380node. Since the master node itself is authenticated, the LPTs authenticity can
 381be verified by verifying the authenticity of the master node and comparing the
 382LTP hash stored there with the hash computed from the read on-flash LPT.
 383
 384
 385Key Management
 386--------------
 387
 388For simplicity, UBIFS authentication uses a single key to compute the HMACs
 389of superblock, master, commit start and reference nodes. This key has to be
 390available on creation of the filesystem (`mkfs.ubifs`) to authenticate the
 391superblock node. Further, it has to be available on mount of the filesystem
 392to verify authenticated nodes and generate new HMACs for changes.
 393
 394UBIFS authentication is intended to operate side-by-side with UBIFS encryption
 395(fscrypt) to provide confidentiality and authenticity. Since UBIFS encryption
 396has a different approach of encryption policies per directory, there can be
 397multiple fscrypt master keys and there might be folders without encryption.
 398UBIFS authentication on the other hand has an all-or-nothing approach in the
 399sense that it either authenticates everything of the filesystem or nothing.
 400Because of this and because UBIFS authentication should also be usable without
 401encryption, it does not share the same master key with fscrypt, but manages
 402a dedicated authentication key.
 403
 404The API for providing the authentication key has yet to be defined, but the
 405key can eg. be provided by userspace through a keyring similar to the way it
 406is currently done in fscrypt. It should however be noted that the current
 407fscrypt approach has shown its flaws and the userspace API will eventually
 408change [FSCRYPT-POLICY2].
 409
 410Nevertheless, it will be possible for a user to provide a single passphrase
 411or key in userspace that covers UBIFS authentication and encryption. This can
 412be solved by the corresponding userspace tools which derive a second key for
 413authentication in addition to the derived fscrypt master key used for
 414encryption.
 415
 416To be able to check if the proper key is available on mount, the UBIFS
 417superblock node will additionally store a hash of the authentication key. This
 418approach is similar to the approach proposed for fscrypt encryption policy v2
 419[FSCRYPT-POLICY2].
 420
 421
 422Future Extensions
 423=================
 424
 425In certain cases where a vendor wants to provide an authenticated filesystem
 426image to customers, it should be possible to do so without sharing the secret
 427UBIFS authentication key. Instead, in addition the each HMAC a digital
 428signature could be stored where the vendor shares the public key alongside the
 429filesystem image. In case this filesystem has to be modified afterwards,
 430UBIFS can exchange all digital signatures with HMACs on first mount similar
 431to the way the IMA/EVM subsystem deals with such situations. The HMAC key
 432will then have to be provided beforehand in the normal way.
 433
 434
 435References
 436==========
 437
 438[CRYPTSETUP2]        https://www.saout.de/pipermail/dm-crypt/2017-November/005745.html
 439
 440[DMC-CBC-ATTACK]     https://www.jakoblell.com/blog/2013/12/22/practical-malleability-attack-against-cbc-encrypted-luks-partitions/
 441
 442[DM-INTEGRITY]       https://www.kernel.org/doc/Documentation/device-mapper/dm-integrity.rst
 443
 444[DM-VERITY]          https://www.kernel.org/doc/Documentation/device-mapper/verity.rst
 445
 446[FSCRYPT-POLICY2]    https://www.spinics.net/lists/linux-ext4/msg58710.html
 447
 448[UBIFS-WP]           http://www.linux-mtd.infradead.org/doc/ubifs_whitepaper.pdf
 449