Re: [PATCH v3 0/7] dma-buf: heaps: Add secure heap

[Pekka Paalanen]

On Wed, 13 Dec 2023 14:22:29 +0100
Joakim Bech <joakim.bech@xxxxxxxxxx> wrote:

> On Wed, Dec 13, 2023 at 01:38:25PM +0200, Pekka Paalanen wrote:
> > On Wed, 13 Dec 2023 11:15:49 +0100
> > Joakim Bech <joakim.bech@xxxxxxxxxx> wrote:
> >   
> > > On Wed, Dec 13, 2023 at 11:05:17AM +0200, Pekka Paalanen wrote:  
> > > > On Tue, 12 Dec 2023 16:36:35 +0000
> > > > Simon Ser <contact@xxxxxxxxxxx> wrote:
> > > >     
> > > > > Is there a chance to pick a better name than "secure" here?
> > > > > 
> > > > > "Secure" is super overloaded, it's not clear at all what it means from
> > > > > just the name. Something like "restricted" would be an improvement.
> > > > >     
> > > > 
> > > > My thoughts exactly. Every time I see "secure" used for something that
> > > > either gives you garbage, refuses to work, or crashes your whole machine
> > > > *intentionally* when you try to do normal usual things to it in
> > > > userspace (like use it for GL texturing, or try to use KMS writeback), I
> > > > get an unscratchable itch.
> > > > 
> > > > There is nothing "secure" from security perspective there for end users
> > > > and developers. It's just inaccessible buffers.
> > > > 
> > > > I've been biting my lip until now, thinking it's too late.
> > > >     
> > > The characteristics we're looking for here is a buffer where the content
> > > is inaccessible to the normal OS and user space, i.e., Non-secure EL0 to
> > > EL2. I.e, the content of the buffer is meant to be used and accessible
> > > primarily by the secure side and other devices that has been granted  
> > 
> > s/secure side/proprietary side/
> >   
> I'm using the nomenclature as written by Arm (other architectures have
> other names for their secure execution states).
> 
> > I presume nothing of the other side can ever be in any way open?
> >   
> I'm sure there are lots of examples of things running on the secure side
> that are open. The OP-TEE project where I'm a maintainer has been fully
> open source since 2014, to give one example that I'm familiar with
> myself.

Oh, I didn't realise there were FOSS implementations of the other side
that tie in with this secure/restricted heap here. Sorry.

I think the patch series cover letter should point to those to give a
view of the other side to the reviewers, just like DRM requires
userspace to be open for new UAPI.

> > Maybe the other side is even less secure than the FOSS side...
> >   
> > > access to it (for example decoders, display controllers if we're talking
> > > about video use cases). However, since the use cases for this exercises
> > > the whole stack, from non-secure user space (EL0) all the way to secure
> > > user space (S-EL0), with various devices needing access to the buffer at
> > > various times, it makes sense to let Linux manage the buffers, although
> > > it still cannot access the content. That's the overall context.  
> > 
> > Yes, we know all this (except for the exact meaning of EL0 etc.).
> >   
> Great!
> 
> > > As for the name, it's always difficult to find a name suitable precisely
> > > describing what it is. "Secure" is perhaps vague, but it might still a
> > > good choice, if you carefully describe what secure means for this
> > > particular heap (in the source code and the documentation for it). For  
> > 
> > Carefully describe, as in, re-define.
> >   
> > > example, the definition of "secure" for a secure heap as here could mean
> > > that buffer content is inaccessible to the host OS and user space
> > > running in normal world (using Arm nomenclature). I wouldn't have any
> > > problems with calling it secure if, as said it's defined what we mean by
> > > saying so. But I'm all ears for other suggestions as well.
> > > 
> > > Safe, protected, shielded, unreachable, isolated, inaccessible,
> > > unaccessible, fortified, ... would any of these make more sense?  
> > 
> > "Restricted" sounds like a good compromise to me. The buffers' usage is
> > severely restricted.
> >   
> Yes, restricted isn't a bad choice. We would still need to describe what
> we mean by saying it's restricted, i.e., what is it restricted from,
> since I'd guess that "restricted" by itself also could be a bit open
> ended for a lot of people.

Yes, but "restricted" also does not give out an immediate wrong
impression. Label something as "secure", and it immediately raises the
questions of what kind of attacks it prevents and how can I make use of
it.

Is there any use of restricted buffers outside of Digital Rights
Management though? Could a private person somehow make use of it to
protect their own contents? Like a photographer sending drafts to a
customer while not wanting give out any digital copies?

If not, then restricted buffers are something people would generally
like to avoid, not embrace. "Secure" gives a positive impression,
"restricted" a negative impression.

> > It is the opposite of "safe", because accessing the contents the wrong
> > way can return garbage or intentionally crash the whole system,
> > depending on the hardware implementation. One example is attempting to
> > put such a buffer on a KMS plane while the connector HDCP state is not
> > high enough, or a writeback connector is connected to the CRTC. It is
> > really fragile. (Do KMS drivers fail an atomic commit that would
> > violate the heap rules? Somehow I doubt that, who'd even know what the
> > rules are.)
> >   
> I believe one of the goals with reviewing the patches is to highlight
> issues like this and try to figure out how to avoid ending up in
> situations like what you described by suggesting alternative solutions
> and ideas.
> 
> > It is protected/shielded/fortified from all the kernel and userspace,
> > but a more familiar word to describe that is inaccessible.
> > "Inaccessible buffer" per se OTOH sounds like a useless concept.
> > 
> > It is not secure, because it does not involve security in any way. In
> > fact, given it's so fragile, I'd classify it as mildly opposite of
> > secure, as e.g. clients of a Wayland compositor can potentially DoS the
> > compositor with it by simply sending such a dmabuf. Or DoS the whole
> > system.
> >   
> I hear what you are saying and DoS is a known problem and attack vector,
> but regardless, we have use cases where we don't want to expose
> information in the clear and where we also would like to have some
> guarantees about correctness. That is where various secure elements and
> more generally security is needed.
> 
> So, it sounds like we have two things here, the first is the naming and
> the meaning behind it. I'm pretty sure the people following and
> contributing to this thread can agree on a name that makes sense. Would
> you personally be OK with "restricted" as the name? It sounds like that.

I would. I'm also just a by-stander, not a maintainer of kernel
anything. I have no power to accept nor reject anything here.

> The other thing is the feature and functionality itself offered by this
> patch series. My impression from reading your replies is that you think
> this is the wrong approach. If my impression is correct, what would you
> suggest as an alternative approach?

I just generally dislike locking people out of what their systems hold,
but I also understand why extremely big companies want this Digital
Rights Management technology for their business model. If Linux does
not support that business model, they and the whole broadcast industry
might use something else. At least it pays for kernel developers who
can hopefully do some genuinely useful work on the side as well,
benefiting the community.

Let's just be honest about what is what.


Thanks,
pq

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