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Re: [PATCH v2 3/3] grub-core/kern/disk.c: handle LUKS2 devices

From: Josselin Poiret
Subject: Re: [PATCH v2 3/3] grub-core/kern/disk.c: handle LUKS2 devices
Date: Mon, 09 May 2022 22:27:30 +0200

Hello everyone,

Glenn Washburn <> writes:

> I don't really like this, but it gets the job done and is a work-around
> for a peculiarity of the LUKS2 backend. The cheat mount code for
> cryptodisk does only calls scan() and not recover_key(). For LUKS1 scan
> will return a grub_cryptodisk_t with log_sector_size set, but LUKS2
> will not. This is because for LUKS1 the log_sector_size is constant
> (LUKS1 also sets most of the other properties of the cryptodisk device,
> like crypto algorithm, because they are in the binary header). However,
> for LUKS2 the sector size (along with other properties) is in the json
> header, which isn't getting parsed in scan(). 
> For single segment LUKS2 containers, scan() could get the sector size
> from the json segment object. The LUKS2 spec says that normal LUKS2
> devices are single segment[1], so this should work in the the cases the
> care about (currently). scan() would not be able to fill in the other
> properties, like crypto algorithm, because that depends on the keyslot
> used, which needs key recovery to be determined. To avoid parsing the
> json data twice, once in scan() and once in recover_key(), which should
> be avoided, the parsed json object could be put in the grub_cryptodisk_t
> in scan(), and used and freed in recover_key(). We'd probably also want
> to add a way for grub_cryptodisk_t objects to get cleaned up by the
> backend using them, so that the json object could be freed even if
> recover_key() is never called.
> I think the above is the real fix, a moderate amount more work, and not
> something I'd expect Pierre-Louis to take up. So if we're not going to
> do this to get this functionality to work, we'll need a hack to get it
> working. However, I'd prefer a different one.
> I've not tested this, but it seems to me that we can set the
> log_sector_size field to GRUB_DISK_SECTOR_BITS _if_ it equals zero in
> grub_cryptodisk_cheat_insert(). This limits the hack to only GRUB
> host/user-space code.

Regarding these last lines, it's also possible to directly ask dm for
the actual sector size when cheatmounting, as well as the crypto
algorithm, bypassing the whole issue of parsing the json and finding the
right slot.  This is roughly what's done in patch 2 of [1], maybe this
workaround would be more to your liking?

I've distributed this patch to several people that were having issues on
GNU Guix and they've been happily using LUKS2 with GRUB with it.


Josselin Poiret

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