Re: [RFC PATCH v7 12/16] fsverity|security: add security hooks to fsverity digest and signature

Deven Bowers
2021-10-28T18:11:21Z

On 10/27/2021 8:48 PM, Eric Biggers wrote:
> On Tue, Oct 26, 2021 at 12:03:53PM -0700, Deven Bowers wrote:
>>>> The proposed LSM (IPE) of this series will be the only one to need
>>>> this information at the  moment. IPE’s goal is to have provide
>>>> trust-based access control. Trust and Integrity are tied together,
>>>> as you cannot prove trust without proving integrity.
>>> I think you mean authenticity, not integrity?
>> I’ve heard a lot of people use these terms in overloaded ways.
>>
>> If we’re working with the definition of authenticity being
>> “the property that a resource was _actually_ sent/created by a
>> party”, and integrity being “the property that a resource was not
>> modified from a point of time”, then yes. Though the statement isn’t
>> false, though, because you’d need to prove integrity in the process of
>> proving authenticity.
>>
>> If not, could you clarify what you mean by authenticity and integrity,
>> so that we can use consistent definitions?
> In cryptography, integrity normally means knowing whether data has been
> non-maliciously changed, while authenticity means knowing whether data is from a
> particular source, which implies knowing whether it has been changed at all
> (whether maliciously or not). Consider that there are "Message Authentication
> Codes" (MACs) and "Authenticated Encryption", not "Message Integrity Codes" and
> "Intact Encryption".
>
> Unfortunately lots of people do overload "integrity" to mean authenticity, so
> you're not alone. But it's confusing, so if you're going to do that then please
> make sure to clearly explain what you mean.
>
>>> Also how does this differ from IMA? I know that IMA doesn't support fs-verity
>>> file hashes, but that could be changed. Why not extend IMA to cover your use
>>> case(s)?
>> We looked at extending IMA to cover our requirements extensively the past
>> year
>> based on feedback the last time I posted these patches. We implemented a
>> prototype that had half of our requirements, but found it resulted in a
>> large change list that would result in a large amount of pain in respect
>> to maintenance, in addition to other more architectural concerns about the
>> implementation. We weren’t convinced it was the correct direction, for our
>> needs.
>>
>> There was a presentation done at LSS 2021 around this prototype done by my
>> colleague, Fan, who authored this patch and implemented the aforementioned
>> prototype.
>>
>> In general, IMA provides a whole suite of amazing functionality when it
>> comes to everything integrity, as the fs-verity documentation states
>> itself:
>>
>> IMA specifies a system-wide policy that specifies which
>> files are hashed and what to do with those hashes, such
>> as log them, authenticate them, or add them to a
>> measurement list.
>>
>> Instead, IPE provides a fine-tuned way to _only_ enforce an access control
>> policy to these files based on the defined trust requirements in the policy,
>> under various contexts, (you might have different requirements for what
>> executes in a general purpose, versus loadable kernel modules, for example).
>> It will never provide bother to log, measure, or revalidate these hashes
>> because
>> that’s not its purpose. This is why it belongs at the LSM layer instead of
>> the
>> integrity subsystem layer, as it is providing access control based on a
>> policy,
>> versus providing deep integrations with the actual integrity claim.
>>
>> IPE is trying to be agnostic to how precisely “trust” is provided, as
>> opposed to be deeply integrated into the mechanism that provides
>> “trust”.
> IMA doesn't require logging or "measuring" hashes, though. Those are just some
> of its supported features. And I thought the IMA developers were planning to
> add support for fs-verity hashes, and that it wouldn't require an entirely new
> architecture to do so.
>
> Anyway, while it does sound to me like you're duplicating IMA, I don't really
> have a horse in this race, and I defer to the IMA developers on this. I trust
> that you've been engaging with them? This patchset isn't even Cc'ed to
> linux-integrity, so it's unclear that's been happening.
That was entirely my mistake. Mimi and the linux-integrity list was CC'd
on previous
versions (Roberto actually added the list to his responses) - when I was
reconstructing
the To: line with get-maintainers.pl, the list didn't pop up and I did
not remember to
add it manually. I've corrected my mailing script to re-add them again.