qcow2: extend specification to cover LUKS encryption
Update the qcow2 specification to describe how the LUKS header is placed inside a qcow2 file, when using LUKS encryption for the qcow2 payload instead of the legacy AES-CBC encryption Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Daniel P. Berrange <berrange@redhat.com> Message-id: 20170623162419.26068-13-berrange@redhat.com Signed-off-by: Max Reitz <mreitz@redhat.com>
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Daniel P. Berrange
Max Reitz
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@ -45,6 +45,7 @@ The first cluster of a qcow2 image contains the file header:
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32 - 35: crypt_method
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0 for no encryption
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1 for AES encryption
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2 for LUKS encryption
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36 - 39: l1_size
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Number of entries in the active L1 table
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@ -135,6 +136,7 @@ be stored. Each extension has a structure like the following:
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0xE2792ACA - Backing file format name
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0x6803f857 - Feature name table
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0x23852875 - Bitmaps extension
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0x0537be77 - Full disk encryption header pointer
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other - Unknown header extension, can be safely
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ignored
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@ -207,6 +209,107 @@ The fields of the bitmaps extension are:
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Offset into the image file at which the bitmap directory
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starts. Must be aligned to a cluster boundary.
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== Full disk encryption header pointer ==
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The full disk encryption header must be present if, and only if, the
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'crypt_method' header requires metadata. Currently this is only true
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of the 'LUKS' crypt method. The header extension must be absent for
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other methods.
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This header provides the offset at which the crypt method can store
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its additional data, as well as the length of such data.
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Byte 0 - 7: Offset into the image file at which the encryption
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header starts in bytes. Must be aligned to a cluster
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boundary.
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Byte 8 - 15: Length of the written encryption header in bytes.
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Note actual space allocated in the qcow2 file may
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be larger than this value, since it will be rounded
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to the nearest multiple of the cluster size. Any
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unused bytes in the allocated space will be initialized
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to 0.
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For the LUKS crypt method, the encryption header works as follows.
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The first 592 bytes of the header clusters will contain the LUKS
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partition header. This is then followed by the key material data areas.
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The size of the key material data areas is determined by the number of
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stripes in the key slot and key size. Refer to the LUKS format
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specification ('docs/on-disk-format.pdf' in the cryptsetup source
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package) for details of the LUKS partition header format.
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In the LUKS partition header, the "payload-offset" field will be
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calculated as normal for the LUKS spec. ie the size of the LUKS
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header, plus key material regions, plus padding, relative to the
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start of the LUKS header. This offset value is not required to be
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qcow2 cluster aligned. Its value is currently never used in the
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context of qcow2, since the qcow2 file format itself defines where
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the real payload offset is, but none the less a valid payload offset
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should always be present.
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In the LUKS key slots header, the "key-material-offset" is relative
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to the start of the LUKS header clusters in the qcow2 container,
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not the start of the qcow2 file.
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Logically the layout looks like
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+-----------------------------+
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| QCow2 header |
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| QCow2 header extension X |
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| QCow2 header extension FDE |
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| QCow2 header extension ... |
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| QCow2 header extension Z |
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+-----------------------------+
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| ....other QCow2 tables.... |
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. .
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. .
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+-----------------------------+
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| +-------------------------+ |
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| | LUKS partition header | |
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| +-------------------------+ |
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| | LUKS key material 1 | |
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| +-------------------------+ |
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| | LUKS key material 2 | |
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| +-------------------------+ |
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| | LUKS key material ... | |
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| +-------------------------+ |
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| | LUKS key material 8 | |
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| +-------------------------+ |
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+-----------------------------+
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| QCow2 cluster payload |
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. .
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. .
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. .
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| |
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+-----------------------------+
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== Data encryption ==
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When an encryption method is requested in the header, the image payload
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data must be encrypted/decrypted on every write/read. The image headers
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and metadata are never encrypted.
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The algorithms used for encryption vary depending on the method
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- AES:
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The AES cipher, in CBC mode, with 256 bit keys.
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Initialization vectors generated using plain64 method, with
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the virtual disk sector as the input tweak.
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This format is no longer supported in QEMU system emulators, due
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to a number of design flaws affecting its security. It is only
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supported in the command line tools for the sake of back compatibility
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and data liberation.
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- LUKS:
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The algorithms are specified in the LUKS header.
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Initialization vectors generated using the method specified
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in the LUKS header, with the physical disk sector as the
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input tweak.
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== Host cluster management ==
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