diff options
Diffstat (limited to 'docs/interop/qcow2.txt')
| -rw-r--r-- | docs/interop/qcow2.txt | 111 |
1 files changed, 106 insertions, 5 deletions
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index 80cdfd0e91..d7fdb1fee3 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -45,6 +45,7 @@ The first cluster of a qcow2 image contains the file header: 32 - 35: crypt_method 0 for no encryption 1 for AES encryption + 2 for LUKS encryption 36 - 39: l1_size Number of entries in the active L1 table @@ -135,6 +136,7 @@ be stored. Each extension has a structure like the following: 0xE2792ACA - Backing file format name 0x6803f857 - Feature name table 0x23852875 - Bitmaps extension + 0x0537be77 - Full disk encryption header pointer other - Unknown header extension, can be safely ignored @@ -201,12 +203,113 @@ The fields of the bitmaps extension are: 8 - 15: bitmap_directory_size Size of the bitmap directory in bytes. It is the cumulative - size of all (nb_bitmaps) bitmap headers. + size of all (nb_bitmaps) bitmap directory entries. 16 - 23: bitmap_directory_offset Offset into the image file at which the bitmap directory starts. Must be aligned to a cluster boundary. +== Full disk encryption header pointer == + +The full disk encryption header must be present if, and only if, the +'crypt_method' header requires metadata. Currently this is only true +of the 'LUKS' crypt method. The header extension must be absent for +other methods. + +This header provides the offset at which the crypt method can store +its additional data, as well as the length of such data. + + Byte 0 - 7: Offset into the image file at which the encryption + header starts in bytes. Must be aligned to a cluster + boundary. + Byte 8 - 15: Length of the written encryption header in bytes. + Note actual space allocated in the qcow2 file may + be larger than this value, since it will be rounded + to the nearest multiple of the cluster size. Any + unused bytes in the allocated space will be initialized + to 0. + +For the LUKS crypt method, the encryption header works as follows. + +The first 592 bytes of the header clusters will contain the LUKS +partition header. This is then followed by the key material data areas. +The size of the key material data areas is determined by the number of +stripes in the key slot and key size. Refer to the LUKS format +specification ('docs/on-disk-format.pdf' in the cryptsetup source +package) for details of the LUKS partition header format. + +In the LUKS partition header, the "payload-offset" field will be +calculated as normal for the LUKS spec. ie the size of the LUKS +header, plus key material regions, plus padding, relative to the +start of the LUKS header. This offset value is not required to be +qcow2 cluster aligned. Its value is currently never used in the +context of qcow2, since the qcow2 file format itself defines where +the real payload offset is, but none the less a valid payload offset +should always be present. + +In the LUKS key slots header, the "key-material-offset" is relative +to the start of the LUKS header clusters in the qcow2 container, +not the start of the qcow2 file. + +Logically the layout looks like + + +-----------------------------+ + | QCow2 header | + | QCow2 header extension X | + | QCow2 header extension FDE | + | QCow2 header extension ... | + | QCow2 header extension Z | + +-----------------------------+ + | ....other QCow2 tables.... | + . . + . . + +-----------------------------+ + | +-------------------------+ | + | | LUKS partition header | | + | +-------------------------+ | + | | LUKS key material 1 | | + | +-------------------------+ | + | | LUKS key material 2 | | + | +-------------------------+ | + | | LUKS key material ... | | + | +-------------------------+ | + | | LUKS key material 8 | | + | +-------------------------+ | + +-----------------------------+ + | QCow2 cluster payload | + . . + . . + . . + | | + +-----------------------------+ + +== Data encryption == + +When an encryption method is requested in the header, the image payload +data must be encrypted/decrypted on every write/read. The image headers +and metadata are never encrypted. + +The algorithms used for encryption vary depending on the method + + - AES: + + The AES cipher, in CBC mode, with 256 bit keys. + + Initialization vectors generated using plain64 method, with + the virtual disk sector as the input tweak. + + This format is no longer supported in QEMU system emulators, due + to a number of design flaws affecting its security. It is only + supported in the command line tools for the sake of back compatibility + and data liberation. + + - LUKS: + + The algorithms are specified in the LUKS header. + + Initialization vectors generated using the method specified + in the LUKS header, with the physical disk sector as the + input tweak. == Host cluster management == @@ -426,8 +529,7 @@ Each bitmap saved in the image is described in a bitmap directory entry. The bitmap directory is a contiguous area in the image file, whose starting offset and length are given by the header extension fields bitmap_directory_offset and bitmap_directory_size. The entries of the bitmap directory have variable -length, depending on the lengths of the bitmap name and extra data. These -entries are also called bitmap headers. +length, depending on the lengths of the bitmap name and extra data. Structure of a bitmap directory entry: @@ -472,8 +574,7 @@ Structure of a bitmap directory entry: 17: granularity_bits Granularity bits. Valid values: 0 - 63. - Note: Qemu currently doesn't support granularity_bits - greater than 31. + Note: Qemu currently supports only values 9 - 31. Granularity is calculated as granularity = 1 << granularity_bits |