Re: [PATCH 0/5] Support kdump with LUKS encryption by reusing LUKS volume key

[Milan Broz]

On 6/5/23 04:31, Coiby Xu wrote:
> Hi Eric and Milan,
>
> On Sat, Jun 03, 2023 at 11:22:52AM +0200, Milan Broz wrote:
> > On 6/2/23 23:34, Eric Biggers wrote:
> > > On Thu, Jun 01, 2023 at 03:24:39PM +0800, Coiby Xu wrote:
> > > > [PATCH 0/5] Support kdump with LUKS encryption by reusing LUKS volume key
> > >
> > > The kernel has no concept of LUKS at all.  It provides dm-crypt, which LUKS
> > > happens to use.  But LUKS is a userspace concept.
> > >
> > > This is a kernel patchset, so why does it make sense for it to be talking about
> > > LUKS at all?  Perhaps you mean dm-crypt?
> >
> > Exactly.
>
> Thanks for raising the above concern! The use cases like CoreOS and
> Confidential VMs explicitly want kdump to work for LUKS. And correct me
> if I'm wrong, I think the two problems addressed by this patch set only
> apply to LUKS so the kdump part of the kernel only cares about the LUKS
> case. If there are use cases where similar approach is needed, I'll be
> happy to make the solution more generic.
>
> > I had the same comment almost a year ago... and it still applies:
> > https://lore.kernel.org/all/c857dcf8-024e-ab8a-fd26-295ce2e0ae41@xxxxxxxxx/
> >
> > Anyway, please fix the naming before this patchset can be read or reviewed!
> >
> > LUKS is user-space key management only (on-disk metadata); the kernel has
> > no idea how the key is derived or what LUKS is - dm-crypt only knows the key
> > (either through keyring or directly in the mapping table).
> >
> > Polluting kernel namespace with "luks" names variables is wrong - dm-crypt
> > is used in many other mappings (plain, bitlocker, veracrypt, ...)
> > Just use the dm-crypt key, do not reference LUKS at all.
>
> Thanks for the reminding! That comment was on the first RFC version. But
> starting with "RFC v2", there is no longer any interaction with dm-crypt
> (to save a copy of the LUKS volume key for the kdump kernel) and now I
> make cryptsetup talks to the kdump part of the kernel via the sysfs to
> reuse the volume key. So only the kdump part of the kernel needs to know
> LUKS which is what it cares. Thus I don't think there is any kernel
> namespace pollution now.

Hi,

I am sorry if I did understand correctly, but I thought that kdump is part
of the kernel.

I am trying to say that kernel generally has no concept of LUKS;
this is a userspace abstraction for key management.

Even the cryptsetup dm-crypt configuration mapping table generated from LUKS
has nothing LUKS special in it (only in DM-UUID as a name prefix).

So I do not understand why you need to mention LUKS even in kdump part.
Perhaps it is still only a naming problem, nothing more.

All you need is to preserve key and configuration parameters (for dm-crypt).
If it is set by cryptsetup, dmsetup, or any other way is not important - on this
kernel layer, it has nothing to do with LUKS key management metadata.

No problem if you support only LUKS in userspace, but really, all this machinery
should work for any dm-crypt devices. Perhaps your patch even works for it already.

Milan