445 строки
22 KiB
ReStructuredText
445 строки
22 KiB
ReStructuredText
:orphan:
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.. UBIFS Authentication
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.. sigma star gmbh
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.. 2018
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Introduction
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============
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UBIFS utilizes the fscrypt framework to provide confidentiality for file
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contents and file names. This prevents attacks where an attacker is able to
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read contents of the filesystem on a single point in time. A classic example
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is a lost smartphone where the attacker is unable to read personal data stored
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on the device without the filesystem decryption key.
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At the current state, UBIFS encryption however does not prevent attacks where
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the attacker is able to modify the filesystem contents and the user uses the
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device afterwards. In such a scenario an attacker can modify filesystem
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contents arbitrarily without the user noticing. One example is to modify a
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binary to perform a malicious action when executed [DMC-CBC-ATTACK]. Since
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most of the filesystem metadata of UBIFS is stored in plain, this makes it
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fairly easy to swap files and replace their contents.
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Other full disk encryption systems like dm-crypt cover all filesystem metadata,
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which makes such kinds of attacks more complicated, but not impossible.
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Especially, if the attacker is given access to the device multiple points in
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time. For dm-crypt and other filesystems that build upon the Linux block IO
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layer, the dm-integrity or dm-verity subsystems [DM-INTEGRITY, DM-VERITY]
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can be used to get full data authentication at the block layer.
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These can also be combined with dm-crypt [CRYPTSETUP2].
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This document describes an approach to get file contents _and_ full metadata
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authentication for UBIFS. Since UBIFS uses fscrypt for file contents and file
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name encryption, the authentication system could be tied into fscrypt such that
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existing features like key derivation can be utilized. It should however also
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be possible to use UBIFS authentication without using encryption.
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MTD, UBI & UBIFS
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----------------
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On Linux, the MTD (Memory Technology Devices) subsystem provides a uniform
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interface to access raw flash devices. One of the more prominent subsystems that
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work on top of MTD is UBI (Unsorted Block Images). It provides volume management
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for flash devices and is thus somewhat similar to LVM for block devices. In
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addition, it deals with flash-specific wear-leveling and transparent I/O error
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handling. UBI offers logical erase blocks (LEBs) to the layers on top of it
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and maps them transparently to physical erase blocks (PEBs) on the flash.
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UBIFS is a filesystem for raw flash which operates on top of UBI. Thus, wear
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leveling and some flash specifics are left to UBI, while UBIFS focuses on
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scalability, performance and recoverability.
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::
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+------------+ +*******+ +-----------+ +-----+
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| | * UBIFS * | UBI-BLOCK | | ... |
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| JFFS/JFFS2 | +*******+ +-----------+ +-----+
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| | +-----------------------------+ +-----------+ +-----+
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| | | UBI | | MTD-BLOCK | | ... |
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+------------+ +-----------------------------+ +-----------+ +-----+
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+------------------------------------------------------------------+
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| MEMORY TECHNOLOGY DEVICES (MTD) |
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+------------------------------------------------------------------+
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+-----------------------------+ +--------------------------+ +-----+
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| NAND DRIVERS | | NOR DRIVERS | | ... |
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+-----------------------------+ +--------------------------+ +-----+
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Figure 1: Linux kernel subsystems for dealing with raw flash
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Internally, UBIFS maintains multiple data structures which are persisted on
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the flash:
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- *Index*: an on-flash B+ tree where the leaf nodes contain filesystem data
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- *Journal*: an additional data structure to collect FS changes before updating
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the on-flash index and reduce flash wear.
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- *Tree Node Cache (TNC)*: an in-memory B+ tree that reflects the current FS
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state to avoid frequent flash reads. It is basically the in-memory
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representation of the index, but contains additional attributes.
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- *LEB property tree (LPT)*: an on-flash B+ tree for free space accounting per
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UBI LEB.
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In the remainder of this section we will cover the on-flash UBIFS data
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structures in more detail. The TNC is of less importance here since it is never
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persisted onto the flash directly. More details on UBIFS can also be found in
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[UBIFS-WP].
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UBIFS Index & Tree Node Cache
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Basic on-flash UBIFS entities are called *nodes*. UBIFS knows different types
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of nodes. Eg. data nodes (`struct ubifs_data_node`) which store chunks of file
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contents or inode nodes (`struct ubifs_ino_node`) which represent VFS inodes.
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Almost all types of nodes share a common header (`ubifs_ch`) containing basic
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information like node type, node length, a sequence number, etc. (see
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`fs/ubifs/ubifs-media.h`in kernel source). Exceptions are entries of the LPT
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and some less important node types like padding nodes which are used to pad
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unusable content at the end of LEBs.
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To avoid re-writing the whole B+ tree on every single change, it is implemented
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as *wandering tree*, where only the changed nodes are re-written and previous
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versions of them are obsoleted without erasing them right away. As a result,
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the index is not stored in a single place on the flash, but *wanders* around
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and there are obsolete parts on the flash as long as the LEB containing them is
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not reused by UBIFS. To find the most recent version of the index, UBIFS stores
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a special node called *master node* into UBI LEB 1 which always points to the
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most recent root node of the UBIFS index. For recoverability, the master node
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is additionally duplicated to LEB 2. Mounting UBIFS is thus a simple read of
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LEB 1 and 2 to get the current master node and from there get the location of
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the most recent on-flash index.
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The TNC is the in-memory representation of the on-flash index. It contains some
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additional runtime attributes per node which are not persisted. One of these is
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a dirty-flag which marks nodes that have to be persisted the next time the
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index is written onto the flash. The TNC acts as a write-back cache and all
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modifications of the on-flash index are done through the TNC. Like other caches,
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the TNC does not have to mirror the full index into memory, but reads parts of
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it from flash whenever needed. A *commit* is the UBIFS operation of updating the
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on-flash filesystem structures like the index. On every commit, the TNC nodes
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marked as dirty are written to the flash to update the persisted index.
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Journal
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~~~~~~~
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To avoid wearing out the flash, the index is only persisted (*commited*) when
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certain conditions are met (eg. ``fsync(2)``). The journal is used to record
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any changes (in form of inode nodes, data nodes etc.) between commits
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of the index. During mount, the journal is read from the flash and replayed
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onto the TNC (which will be created on-demand from the on-flash index).
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UBIFS reserves a bunch of LEBs just for the journal called *log area*. The
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amount of log area LEBs is configured on filesystem creation (using
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``mkfs.ubifs``) and stored in the superblock node. The log area contains only
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two types of nodes: *reference nodes* and *commit start nodes*. A commit start
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node is written whenever an index commit is performed. Reference nodes are
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written on every journal update. Each reference node points to the position of
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other nodes (inode nodes, data nodes etc.) on the flash that are part of this
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journal entry. These nodes are called *buds* and describe the actual filesystem
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changes including their data.
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The log area is maintained as a ring. Whenever the journal is almost full,
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a commit is initiated. This also writes a commit start node so that during
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mount, UBIFS will seek for the most recent commit start node and just replay
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every reference node after that. Every reference node before the commit start
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node will be ignored as they are already part of the on-flash index.
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When writing a journal entry, UBIFS first ensures that enough space is
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available to write the reference node and buds part of this entry. Then, the
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reference node is written and afterwards the buds describing the file changes.
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On replay, UBIFS will record every reference node and inspect the location of
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the referenced LEBs to discover the buds. If these are corrupt or missing,
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UBIFS will attempt to recover them by re-reading the LEB. This is however only
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done for the last referenced LEB of the journal. Only this can become corrupt
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because of a power cut. If the recovery fails, UBIFS will not mount. An error
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for every other LEB will directly cause UBIFS to fail the mount operation.
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::
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| ---- LOG AREA ---- | ---------- MAIN AREA ------------ |
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-----+------+-----+--------+---- ------+-----+-----+---------------
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\ | | | | / / | | | \
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/ CS | REF | REF | | \ \ DENT | INO | INO | /
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\ | | | | / / | | | \
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----+------+-----+--------+--- -------+-----+-----+----------------
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| | ^ ^
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+------------------------+ |
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+-------------------------------+
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Figure 2: UBIFS flash layout of log area with commit start nodes
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(CS) and reference nodes (REF) pointing to main area
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containing their buds
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LEB Property Tree/Table
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~~~~~~~~~~~~~~~~~~~~~~~
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The LEB property tree is used to store per-LEB information. This includes the
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LEB type and amount of free and *dirty* (old, obsolete content) space [1]_ on
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the LEB. The type is important, because UBIFS never mixes index nodes with data
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nodes on a single LEB and thus each LEB has a specific purpose. This again is
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useful for free space calculations. See [UBIFS-WP] for more details.
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The LEB property tree again is a B+ tree, but it is much smaller than the
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index. Due to its smaller size it is always written as one chunk on every
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commit. Thus, saving the LPT is an atomic operation.
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.. [1] Since LEBs can only be appended and never overwritten, there is a
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difference between free space ie. the remaining space left on the LEB to be
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written to without erasing it and previously written content that is obsolete
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but can't be overwritten without erasing the full LEB.
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UBIFS Authentication
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====================
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This chapter introduces UBIFS authentication which enables UBIFS to verify
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the authenticity and integrity of metadata and file contents stored on flash.
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Threat Model
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------------
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UBIFS authentication enables detection of offline data modification. While it
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does not prevent it, it enables (trusted) code to check the integrity and
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authenticity of on-flash file contents and filesystem metadata. This covers
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attacks where file contents are swapped.
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UBIFS authentication will not protect against rollback of full flash contents.
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Ie. an attacker can still dump the flash and restore it at a later time without
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detection. It will also not protect against partial rollback of individual
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index commits. That means that an attacker is able to partially undo changes.
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This is possible because UBIFS does not immediately overwrites obsolete
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versions of the index tree or the journal, but instead marks them as obsolete
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and garbage collection erases them at a later time. An attacker can use this by
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erasing parts of the current tree and restoring old versions that are still on
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the flash and have not yet been erased. This is possible, because every commit
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will always write a new version of the index root node and the master node
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without overwriting the previous version. This is further helped by the
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wear-leveling operations of UBI which copies contents from one physical
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eraseblock to another and does not atomically erase the first eraseblock.
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UBIFS authentication does not cover attacks where an attacker is able to
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execute code on the device after the authentication key was provided.
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Additional measures like secure boot and trusted boot have to be taken to
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ensure that only trusted code is executed on a device.
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Authentication
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--------------
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To be able to fully trust data read from flash, all UBIFS data structures
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stored on flash are authenticated. That is:
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- The index which includes file contents, file metadata like extended
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attributes, file length etc.
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- The journal which also contains file contents and metadata by recording changes
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to the filesystem
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- The LPT which stores UBI LEB metadata which UBIFS uses for free space accounting
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Index Authentication
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~~~~~~~~~~~~~~~~~~~~
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Through UBIFS' concept of a wandering tree, it already takes care of only
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updating and persisting changed parts from leaf node up to the root node
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of the full B+ tree. This enables us to augment the index nodes of the tree
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with a hash over each node's child nodes. As a result, the index basically also
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a Merkle tree. Since the leaf nodes of the index contain the actual filesystem
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data, the hashes of their parent index nodes thus cover all the file contents
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and file metadata. When a file changes, the UBIFS index is updated accordingly
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from the leaf nodes up to the root node including the master node. This process
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can be hooked to recompute the hash only for each changed node at the same time.
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Whenever a file is read, UBIFS can verify the hashes from each leaf node up to
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the root node to ensure the node's integrity.
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To ensure the authenticity of the whole index, the UBIFS master node stores a
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keyed hash (HMAC) over its own contents and a hash of the root node of the index
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tree. As mentioned above, the master node is always written to the flash whenever
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the index is persisted (ie. on index commit).
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Using this approach only UBIFS index nodes and the master node are changed to
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include a hash. All other types of nodes will remain unchanged. This reduces
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the storage overhead which is precious for users of UBIFS (ie. embedded
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devices).
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::
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+---------------+
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| Master Node |
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| (hash) |
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+---------------+
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v
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+-------------------+
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| Index Node #1 |
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| |
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| branch0 branchn |
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| (hash) (hash) |
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+-------------------+
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| ... | (fanout: 8)
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+-------+ +------+
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v v
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+-------------------+ +-------------------+
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| Index Node #2 | | Index Node #3 |
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| | | |
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| branch0 branchn | | branch0 branchn |
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| (hash) (hash) | | (hash) (hash) |
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+-------------------+ +-------------------+
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| ... | ... |
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v v v
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+-----------+ +----------+ +-----------+
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| Data Node | | INO Node | | DENT Node |
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+-----------+ +----------+ +-----------+
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Figure 3: Coverage areas of index node hash and master node HMAC
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The most important part for robustness and power-cut safety is to atomically
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persist the hash and file contents. Here the existing UBIFS logic for how
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changed nodes are persisted is already designed for this purpose such that
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UBIFS can safely recover if a power-cut occurs while persisting. Adding
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hashes to index nodes does not change this since each hash will be persisted
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atomically together with its respective node.
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Journal Authentication
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~~~~~~~~~~~~~~~~~~~~~~
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The journal is authenticated too. Since the journal is continuously written
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it is necessary to also add authentication information frequently to the
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journal so that in case of a powercut not too much data can't be authenticated.
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This is done by creating a continuous hash beginning from the commit start node
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over the previous reference nodes, the current reference node, and the bud
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nodes. From time to time whenever it is suitable authentication nodes are added
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between the bud nodes. This new node type contains a HMAC over the current state
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of the hash chain. That way a journal can be authenticated up to the last
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authentication node. The tail of the journal which may not have a authentication
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node cannot be authenticated and is skipped during journal replay.
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We get this picture for journal authentication::
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,,,,,,,,
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,......,...........................................
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,. CS , hash1.----. hash2.----.
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,. | , . |hmac . |hmac
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,. v , . v . v
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,.REF#0,-> bud -> bud -> bud.-> auth -> bud -> bud.-> auth ...
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,..|...,...........................................
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, | ,
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, | ,,,,,,,,,,,,,,,
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. | hash3,----.
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, | , |hmac
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, v , v
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, REF#1 -> bud -> bud,-> auth ...
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,,,|,,,,,,,,,,,,,,,,,,
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v
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REF#2 -> ...
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V
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...
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Since the hash also includes the reference nodes an attacker cannot reorder or
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skip any journal heads for replay. An attacker can only remove bud nodes or
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reference nodes from the end of the journal, effectively rewinding the
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filesystem at maximum back to the last commit.
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The location of the log area is stored in the master node. Since the master
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node is authenticated with a HMAC as described above, it is not possible to
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tamper with that without detection. The size of the log area is specified when
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the filesystem is created using `mkfs.ubifs` and stored in the superblock node.
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To avoid tampering with this and other values stored there, a HMAC is added to
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the superblock struct. The superblock node is stored in LEB 0 and is only
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modified on feature flag or similar changes, but never on file changes.
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LPT Authentication
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~~~~~~~~~~~~~~~~~~
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The location of the LPT root node on the flash is stored in the UBIFS master
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node. Since the LPT is written and read atomically on every commit, there is
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no need to authenticate individual nodes of the tree. It suffices to
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protect the integrity of the full LPT by a simple hash stored in the master
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node. Since the master node itself is authenticated, the LPTs authenticity can
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be verified by verifying the authenticity of the master node and comparing the
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LTP hash stored there with the hash computed from the read on-flash LPT.
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Key Management
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--------------
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For simplicity, UBIFS authentication uses a single key to compute the HMACs
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of superblock, master, commit start and reference nodes. This key has to be
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available on creation of the filesystem (`mkfs.ubifs`) to authenticate the
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superblock node. Further, it has to be available on mount of the filesystem
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to verify authenticated nodes and generate new HMACs for changes.
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UBIFS authentication is intended to operate side-by-side with UBIFS encryption
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(fscrypt) to provide confidentiality and authenticity. Since UBIFS encryption
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has a different approach of encryption policies per directory, there can be
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multiple fscrypt master keys and there might be folders without encryption.
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UBIFS authentication on the other hand has an all-or-nothing approach in the
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sense that it either authenticates everything of the filesystem or nothing.
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Because of this and because UBIFS authentication should also be usable without
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encryption, it does not share the same master key with fscrypt, but manages
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a dedicated authentication key.
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The API for providing the authentication key has yet to be defined, but the
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key can eg. be provided by userspace through a keyring similar to the way it
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is currently done in fscrypt. It should however be noted that the current
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fscrypt approach has shown its flaws and the userspace API will eventually
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change [FSCRYPT-POLICY2].
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Nevertheless, it will be possible for a user to provide a single passphrase
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or key in userspace that covers UBIFS authentication and encryption. This can
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be solved by the corresponding userspace tools which derive a second key for
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authentication in addition to the derived fscrypt master key used for
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encryption.
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To be able to check if the proper key is available on mount, the UBIFS
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superblock node will additionally store a hash of the authentication key. This
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approach is similar to the approach proposed for fscrypt encryption policy v2
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[FSCRYPT-POLICY2].
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Future Extensions
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=================
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In certain cases where a vendor wants to provide an authenticated filesystem
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image to customers, it should be possible to do so without sharing the secret
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UBIFS authentication key. Instead, in addition the each HMAC a digital
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signature could be stored where the vendor shares the public key alongside the
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filesystem image. In case this filesystem has to be modified afterwards,
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UBIFS can exchange all digital signatures with HMACs on first mount similar
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to the way the IMA/EVM subsystem deals with such situations. The HMAC key
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will then have to be provided beforehand in the normal way.
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References
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==========
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[CRYPTSETUP2] http://www.saout.de/pipermail/dm-crypt/2017-November/005745.html
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[DMC-CBC-ATTACK] http://www.jakoblell.com/blog/2013/12/22/practical-malleability-attack-against-cbc-encrypted-luks-partitions/
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[DM-INTEGRITY] https://www.kernel.org/doc/Documentation/device-mapper/dm-integrity.rst
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[DM-VERITY] https://www.kernel.org/doc/Documentation/device-mapper/verity.rst
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[FSCRYPT-POLICY2] https://www.spinics.net/lists/linux-ext4/msg58710.html
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[UBIFS-WP] http://www.linux-mtd.infradead.org/doc/ubifs_whitepaper.pdf
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