Re: [PATCH 0/3] Cryptomount detached headers

[Patrick Steinhardt <ps@pks.im>]

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On Fri, May 13, 2022 at 01:24:12PM +0200, Daniel Kiper wrote:
> On Tue, May 10, 2022 at 11:53:06PM -0500, Glenn Washburn wrote:
> > This patch series is, I believe, a better approach to supporting detached
> > headers for cryptomount and backends. This series will probably not apply
> > cleanly without the changes from the recent series entitled "[PATCH 0/4]
> > Cryptomount keyfile support". But its only because they touch code in the
> > same vicinity, not because there's any real dependency.
> >
> > Conceptually the approach is different than the previous detach header
> > series because this one uses the disk objects read hook to hook reads to
> > the disk in scan() and recover_key() such that if there is an associated
> > header file, the hook will cause the read to return data from the header
> > file instead of from the source disk.
> >
> > For this the read hook implementation needed to be upgaded because prior
> > it didn't get the read buffer sent from the read caller and so could not
> > modify its contents. Patch #1 updates the hook accordingly and all instances
> > of its use, but doesn't functionally change how GRUB operates.
> >
> > The second patch adds the header option to cryptomount and the read hook
> > to the source disk during the scan() and recover_key() stages. It takes
> > care of the case where there is already a previous read hook on the source
> > device and will call that read hook after modifying the read buffer. I don't
> > believe this is strictly necessary currently because I don't think there
> > ever is a read hook already set since the disk was just created with a
> > grub_disk_open(). I'm not opposed to getting rid of this code. The one
> > benefit I see if a bit of future proofing.
> 
> I would get rid of this code. The first question which comes to my mind
> is: which hook has to process the data first? If we do not know potential
> users of that "multi-hook" feature I would not introduce it to not
> create a feeling the hook interface is well defined. So, at this point
> I would suggest to stick with one hook only.
> 
> > The benefit of this approach is its simpler/less code and does not require
> > the modification of backends, except to potentially cause a failure in
> > scan() if the backend doesn't support the current implementation of detached
> > headers, as with the GELI backend. This implementation requires that the
> > crypto volume header reside at the beginning of the volume. GELI has its
> > header at the end, which is why it can not currently be supported. In
> > theory, GELI could be supported if extra assumptions about its source
> > access pattern during scan() and recovery_key() were made. I don't use GELI,
> > no one seems to be asking for GELI detached headers, and I don't think that
> > GELI even support detached headers with the official tools. So for me, not
> > supporting crypto volumes with headers at the end of the disk is not a big
> > deal.
> 
> I am OK with the idea though I would like to hear Patrick's opinion here.
> 
> Daniel

It's rather intimate with how the backends are working right now, but
has the big advantage that it's backend-agnostic except for GELI. I feel
like the code warrants some more comments to explain what the underlying
idea is, but overall I like it.

Patrick

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