Re: [discussion] proposal to bypass zero data for dm-crypt

[Milan Broz <gmazyland@gmail.com>]

On 1/6/25 2:43 AM, Yu Kuai wrote:
>> On 1/3/25 5:25 PM, Mikulas Patocka wrote:
>>> Milan, what do you think about this from a cryptographic point of view?
>>> Does it make sense to add an option that would detect zero data and skip
>>> decryption in this case?
>>
>> It is a very dangerous thing.
>>
>> Disk encryption is a length-preserving encryption, so it cannot prevent
>> decryption of modified ciphertext. However, such ciphertext modification
>> (without key knowledge) will cause a pseudorandom plaintext output
>> (IOW attacker cannot easily flip bits or whole sectors by ciphertext
>> modification).
>>
>> If you allow the zeroed sector to transform to valid plaintext directly,
>> the attacker can wipe arbitrary plaintext sector. It can lead to fatal
>> issues (for example, wiping filesystem metadata bitmaps on some known
>> location).
> 
> Will there be difference if the attacher wipe the data to zero data or
> random data? And AFAIK, for this case, should user consider dm-integrity
> to prevent such attack?

I think I just explained this - you can directly set specific data with
zeroed plaintext. With pseudorandom decrypted data, you can only destroy
it and hope it will do something useful.

(I did not mention side channels as "decryption" will be much faster.)

With such a feature, it will not be full disk encryption but a weakened variant.
Even if it is ok for your threat model, someone can later use it improperly.

Just use encryption that suits your intended use case (perhaps filesystem
encryption orproperly configure your storage stack, dunno).
>> Stack FDE (dm-crypt) below the filesystem or other storage layer
>> (like thin provision) that supports sparse data, and you will get
>> the expected behavior without such tricks.
> 
> All we want to do is to offer an additional option for user, to enable
> dm-crypt or not. And if we stack dm-crypt below our storage layer, then
> all users will have to use dm-crypt. In order to prevent that, the
> storage layer will have to be much complex, and it will be impossible
> to perform a hot upgrade without affecting existing use cases. :(

This is not a reason for weakening encryption in dm-crypt.
You can always have a virtual volume per user that is optionally encrypted.

Milan

> 
> Thanks,
> Kuai
> 
>>
>> Milan
>>
>>
>>>
>>> Mikulas
>>>
>>> On Sat, 21 Dec 2024, Yu Kuai wrote:
>>>
>>>> Background
>>>>
>>>> We provide virtual machines for customers to use, which include an
>>>> important
>>>> feature: in the initial state, the disks in the virtual machine do
>>>> not occupy
>>>> actual storage space, and the data read by users is all zeros until
>>>> the user
>>>> writes data for the first time. This can save a large amount of storage.
>>>>
>>>> Problem
>>>>
>>>> However, after introducing dm-crypt, this feature has failed. Because we
>>>> expect the data read by users in the initial state to be zero, we
>>>> have to
>>>> write all zeros from dm-crypt.
>>>>
>>>> Hence we'd like to propose to bypass zero data for dm-crypt, for
>>>> example:
>>>>
>>>> before:
>>>> zero data -> encrypted zero data
>>>> decrypted zero data -> zero data
>>>> others
>>>>
>>>> after:
>>>> zero data -> zero data
>>>> decrypted zero data -> encrypted zero data
>>>> others(doesn't change)
>>>>
>>>> We'd like to hear from the community for suggestions first, before we
>>>> start. :)
>>>>
>>>> Thanks,
>>>> Kuai
>>>>
>>>
>>
>> .
>>
>