| .. SPDX-License-Identifier: GPL-2.0 | |
| .. UBIFS Authentication | |
| .. sigma star gmbh | |
| .. 2018 | |
| ============================ | |
| UBIFS Authentication Support | |
| ============================ | |
| 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 (*committed*) 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] https://www.saout.de/pipermail/dm-crypt/2017-November/005745.html | |
| [DMC-CBC-ATTACK] https://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://lore.kernel.org/r/20171023214058.128121-1-ebiggers3@gmail.com/ | |
| [UBIFS-WP] http://www.linux-mtd.infradead.org/doc/ubifs_whitepaper.pdf |