1.. SPDX-License-Identifier: GPL-2.0
2
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