Re: [PATCH v6 06/12] cryptodisk: Properly handle non-512 byte sized sectors

[Daniel Kiper]

On Fri, Nov 27, 2020 at 03:03:38AM -0600, Glenn Washburn wrote:
> By default, dm-crypt internally uses an IV that corresponds to 512-byte
> sectors, even when a larger sector size is specified. What this means is
> that when using a larger sector size, the IV is incremented every sector.
> However, the amount the IV is incremented is the number of 512 byte blocks
> in a sector (ie 8 for 4K sectors). Confusingly the IV does not corespond to
> the number of, for example, 4K sectors. So each 512 byte cipher block in a
> sector will be encrypted with the same IV and the IV will be incremented
> afterwards by the number of 512 byte cipher blocks in the sector.
>
> There are some encryption utilities which do it the intuitive way and have
> the IV equal to the sector number regardless of sector size (ie. the fifth
> sector would have an IV of 4 for each cipher block). And this is supported
> by dm-crypt with the iv_large_sectors option and also cryptsetup as of 2.3.3
> with the --iv-large-sectors, though not with LUKS headers (only with --type
> plain). However, support for this has not been included as grub does not
> support plain devices right now.
>
> One gotcha here is that the encrypted split keys are encrypted with a hard-
> coded 512-byte sector size. So even if your data is encrypted with 4K sector
> sizes, the split key encrypted area must be decrypted with a block size of
> 512 (ie the IV increments every 512 bytes). This made these changes less
> aestetically pleasing than desired.
>
> Signed-off-by: Glenn Washburn <development@efficientek.com>

Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Daniel