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diff --git a/Documentation/filesystems/ubifs-authentication.md b/Documentation/filesystems/ubifs-authentication.md new file mode 100644 index 000000000000..028b3e2e25f9 --- /dev/null +++ b/Documentation/filesystems/ubifs-authentication.md @@ -0,0 +1,426 @@ +% 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 |