[dm-crypt] concurrency

Hugh Bragg hughbragg at dodo.com.au
Sun Mar 27 01:53:21 CET 2016

Hi Arno,

Thanks very much for taking time to respond Arno.
You're right. I'm sharing a virtual disk and trying to decrypt and mount
that in multiple locations.
The other thing I tried was mounting a disk image on a virtualbox shared

I don't want to need a dedicated server to deliver a decrypted
filesystem because I don't want the decrypted data to be exposed to the
network. I understand I could use secure communications, but this is all
way too much overhead compared to what I'm trying to achieve.

>From your response I gather that the answer is no, It doesn't support
sharing of the raw block device with concurrent mounting.
Is this just due to implementation or are there functional reason why
this is so?

I've been trying encfs and ecryptfs too, but they suffer from security
and functional deficiencies.
Is there some other solution that does support this setup?


On 27/03/2016 6:06 AM, Arno Wagner wrote:
> Hi,
> in order to have a shared filesystem work, you need, well,
> a shared filesystem! Do not under any circumstances share
> the block-device or the LUKS-remapped decrypted block
> device. I suspect you do soemthing like that, because
> otherwise the question would not even arise. 
> The rigth way to do this is 
>   raw-block-device -> LUKS decrypted block device -> Filesystem
>   -> export of that filesystem, e.g. via NFS.
> (last two steps possibly one with other network filesystyems)
> Of course, NFS (or the network filesystem of your choice)
> has some transactional assurances and is missing others.
> For example, on NFS, nothing is atomic, except locks 
> or rename operation (as far as I remember).
> But if you do follow the right layering, what you have is
> not a LUKS problem at all, but something stemming from the
> filesystem layer and possibly wrong assumptions about the 
> assurances it offers.
> Regards,
> Arno
> On Sat, Mar 26, 2016 at 15:50:10 CET, Hugh Bragg wrote:
>> Should I be able to use Luks concurrently on a shared filesystem from
>> different computers?
>> Attempts so far have failed with writes not being seen from the other
>> computer until both computers remount the filesystem or reboot.
>> Specifically, virtualbox shareable disks and shared folders, but
>> potentially, any nfs/cloud storage.
>> Am I missing something or is there another tool for this case?
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