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LUKS2-locking.txt
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LUKS2 device locking overview ============================= Why ~~~ LUKS2 format keeps two identical copies of metadata stored consecutively at the head of metadata device (file or bdev). The metadata area (both copies) must be updated in a single atomic operation to avoid header corruption during concurrent write. While with LUKS1 users may have clear knowledge of when a LUKS header is being updated (written to) or when it's being read solely the need for locking with legacy format was not so obvious as it is with the LUKSv2 format. With LUKS2 the boundary between read-only and read-write is blurry and what used to be the exclusively read-only operation (i.e., cryptsetup open command) may easily become read-update operation silently without user's knowledge. Major feature of LUKS2 format is resilience against accidental corruption of metadata (i.e., partial header overwrite by parted or cfdisk while creating partition on mistaken block device). Such header corruption is detected early on header read and auto-recovery procedure takes place (the corrupted header with checksum mismatch is being replaced by the secondary one if that one is intact). On current Linux systems header load operation may be triggered without user direct intervention for example by udev rule or from systemd service. Such clash of header read and auto-recovery procedure could have severe consequences with the worst case of having LUKS2 device unaccessible or being broken beyond repair. The whole locking of LUKSv2 device headers split into two categories depending what backend the header is stored on: I) block device ~~~~~~~~~~~~~~~ We perform flock() on file descriptors of files stored in a private directory (by default /run/lock/cryptsetup). The file name is derived from major:minor couple of affected block device. Note we recommend that access to private locking directory is supposed to be limited to superuser only. For this method to work the distribution needs to install the locking directory with appropriate access rights. II) regular files ~~~~~~~~~~~~~~~~~ First notable difference between headers stored in a file vs. headers stored in a block device is that headers in a file may be manipulated by the regular user unlike headers on block devices. Therefore we perform flock() protection on file with the luks2 header directly. Limitations ~~~~~~~~~~~ a) In general, the locking model provides serialization of I/Os targeting the header only. It means the header is always written or read at once while locking is enabled. We do not suppress any other negative effect that two or more concurrent writers of the same header may cause. b) The locking is not cluster aware in any way.