[PATCH 25/25] Documentation: ubifs: Add authentication whitepaper

[Sascha Hauer]

Signed-off-by: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
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+% 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.txt
+
+[DM-VERITY]          https://www.kernel.org/doc/Documentation/device-mapper/verity.txt
+
+[FSCRYPT-POLICY2]    https://www.spinics.net/lists/linux-ext4/msg58710.html
+
+[UBIFS-WP]           http://www.linux-mtd.infradead.org/doc/ubifs_whitepaper.pdf
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