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docs: ubifs-authentication.md: convert to ReST
The documentation standard is ReST and not markdown. Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org> Acked-by: Rob Herring <robh@kernel.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mauro Carvalho Chehab
Jonathan Corbet
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% UBIFS Authentication
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% sigma star gmbh
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% 2018
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:orphan:
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# Introduction
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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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@ -33,7 +36,8 @@ 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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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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@ -47,7 +51,7 @@ 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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@ -84,7 +88,8 @@ 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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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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@ -118,17 +123,18 @@ 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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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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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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``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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@ -152,6 +158,7 @@ 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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@ -172,10 +179,11 @@ for every other LEB will directly cause UBIFS to fail the mount operation.
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containing their buds
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### LEB Property Tree/Table
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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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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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@ -185,19 +193,21 @@ 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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.. [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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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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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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@ -224,7 +234,8 @@ 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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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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@ -236,7 +247,8 @@ stored on flash are authenticated. That is:
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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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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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@ -260,6 +272,7 @@ 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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@ -303,7 +316,8 @@ 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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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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@ -316,7 +330,7 @@ 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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We get this picture for journal authentication::
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,,,,,,,,
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,......,...........................................
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@ -352,7 +366,8 @@ 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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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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@ -363,7 +378,8 @@ 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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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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@ -399,7 +415,8 @@ 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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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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@ -411,7 +428,8 @@ 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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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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