[seL4] Re: Confidentiality and realtime requirements

On 8/11/23 04:33, Gernot Heiser wrote:

This discussion is confusing some issues, among others, time protection (TP) and temporal isolation.

The definition of time protection (from Ge et al [2019]): "A collection of OS mechanism which jointly prevent interference between security domains that would make execution speed in one domain dependent on the activities of another.”

IOW, time protection is the mechanism that allows you to implement temporal isolation, which is a prevention of interference between domains that affects execution speed. It’s up to the OS to deploy TP to enforce its security policies.

Full temporal isolation, i.e. the absence of any temporal information flow, requires strict time partitioning, no if’s, no but’s. If the time of a context switch can be controlled by application code, you have a timing channel (and it’s one that is trivial to exploit).

That is correct.

Yes, strict temporal partitioning is extremely restrictive, way too restrictive for general-purpose computing. (But it happens to be what is used to ensure commercial aircraft don’t fall out of the sky.) Nothing new there.

Strict temporal partitioning also makes sense for cloud environments that do not oversubscribe CPU or memory.

If you want to relax this, you’ll need a very careful analysis of your security requirements, and deploy TP where needed and omit where it’s not needed. But you can't prevent µarch timing channels without TP. Everybody knows about D-cache channels these days and how easy they are to exploit, and similar with LLC channels. And if you want to prevent at least those, then full TP has no extra cost.

Does time protection require temporal isolation?

The main problem with using a more relaxed security policy (compared to full temporal isolation) is that I’m not aware of a theoretical framework that will allow you to make *any* guarantee, i.e. you’re back to ad-hoc security (which in the end tends to be not much different from no security).

Such a framework would be awesome!

Developing such a framework is on my agenda (it’s a core part of what I said I’m waiting for a good PhD student to work on). Basically I want to be able to make certain security guarantees where components have overt communication channels. An example is tab isolation in a browser: We want to have a fully functional web browser that allows processing of sensitive information in one tab, but guarantee that it cannot be leaked to a different tab.

Another good example is different apps on a mobile device. Android allows any two applications on the same profile to communicate with mutual consent, and yet also allows one application to keep information secret from another. One critical requirement is that such a framework must allow an efficient implementation, both in terms of performance and power consumption. If it isn’t efficient enough to be deployed, then it won’t do people any good.

You can easily think of ad-hoc approaches that seem to be able to deal with this. Until someone breaks them.

I hope this helps.

Gernot

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On 10 Aug 2023, at 07:48, Hugo V.C. wrote:

"For literally 100% of the cases I deal with, time partitioning is completely impractical. The inability of a time-partitioned system to adapt to workload changes means that it is not even worth considering."

I agree Demi. Maybe the problem is trying to solve everything wirh the same hardware and software. With current kind of hardware/CPUs is very difficult to solve both Desktop (human operated) devices general purpose software and embedded, task specific devices software challenges as they have, by nature, different problems to be solved. Human interaction with a computer is not deterministic, so forget about deterministic solutions... Instead try to solve just the most sensible parts of the full puzzle, so you can use specific software/hardware doing the job where errors are not an option and keep other pieces of the puzzle with "standard" software/hardware. Mixing all in a big soap of software is, nowadays, and with the horrible hardware support, an impossible mission.

El mié., 9 ago. 2023 19:38, Demi Marie Obenour escribió:

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On 8/9/23 04:47, Gernot Heiser wrote:

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On 9 Aug 2023, at 06:28, Demi Marie Obenour

wrote:

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> and full speculative taint tracking is required.

I don’t follow. If you clean all µarch state you don’t have to worry

about speculation traces, that’s (among others) the gist of Ge et al.

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Does it prevent Spectre v1? A bounds check will almost always predict

as in-bounds and that is potentially a problem.

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Taint tracking does prevent Spectre v1 because the speculatively read data is guaranteed to be unobservable. Strict temporal isolation also mitigates this, but IIUC it is also incompatible with load-balancing and therefore only practical in limited cases.

Spectre v1 uses speculation to put secrets into the cache, combined with a covert timing channel to move it across security domains. Without the covert channel it’s harmless. Time protection prevents the covert channel.

You cannot use time protection in a fully dynamic system, _especially_ not a desktop system. I should have made it clear that I was referring to dynamic systems.

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Speculation taint-tracking is a complex point-defence against one specific attack patterns, that needs to take a pessimistic approach to enforcing what the hardware thinks *might* be security boundaries, irrespective what the actual security policy is.

Time protection is a general, policy-free mechanism that prevents µarch timing channels under control of the OS, which can deploy it where needed.

The problem with time protection is that it is all-or-nothing. A general purpose system _cannot_ enforce time protection, because doing so requires statically allocating CPU time to different security domains. This is obviously impossible in any desktop system, because it is the human at the console who decides what needs to run and when.

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As I keep saying, the seL4 mechanism that is (unfortunately, somewhat misleadingly and for purely historic reasons) called “IPC” shouldn’t be considered a general-purpose communication mechanism, but a protected procedure call – the microkernel equivalent to a Linux system call. As such, the trust relationship is not symmetric: you use it to invoke some more privileged operation (and definitely need to trust the callee to a degree).

I should have said “not-mutually-trusting”, then.

Makes a big difference: Enforcement then comes down to enforcing the security policy, which may or may not require temporal isolation. If it does, there’s an (unavoidable) cost. If not then not.

Or maybe the security policy requires something between “nothing” and “full temporal isolation”.

Consider a server that performs cryptographic operations. The security policy is that clients cannot access or alter data belonging to other clients and that secret keys cannot be extracted by any client. Since the cryptographic operations are constant-time there is no need for temporal isolation, _provided that speculative execution does not cause problems_. Enforcing temporal isolation would likely cause such a large performance penalty that the whole concept is not viable.

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fence.t (or something similar) is the mechanism you need to let the OS do it’s job, and it is simple and cheap to implement, and costs you no more than the L1-D flush you need anyway, as Wistoff et al demonstrated.

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I missed the “costs you no more than the L1-D flush you need anyway” part. On x86, instructions like IBPB can easily take thousands of cycles IIUC.

Invalidating the L1-D cache takes 1000s of cycles, and is unavoidable for temporal isolation – D-cache timing attacks are the easiest ones to do. The point is that compared to the inevitable D-cache flush, everything else is cheap, and can be done in parallel to the D-cache flush, so doesn’t affect overall latency.

I’m not sure why IBPB should take 1000s of cycles (unless Intel executes complex microcode to do it). Resetting flip-flops is cheap. What makes the D-cache expensive is the need to write back dirty data. Other µarch state can be reset without any write-back as it caches R/O information.

There is a separate issue of indirect costs, which can be significant, but not in a time-partitioned system. If the security policy requires isolating a server from it’s client, then these costs would become significant, but that’s inherent in the problem.

For literally 100% of the cases I deal with, time partitioning is completely impractical. The inability of a time-partitioned system to adapt to workload changes means that it is not even worth considering. Any feasible solution needs to be able to allocate 90+% of system CPU time to security domain X, and then allocate 90+% of system CPU time to security domain Y, _without knowing in advance that these changes will happen_. Time partitioning is awesome for your static embedded systems that will only ever run workloads known in advance, but for the systems I work on, it is a complete non-starter both now and in the foreseeable future.

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Would fence.t have equally catastrophic overhead on an out-of-order RISC-V processor? https://riscv-europe.org/media/proceedings/posters/2023-06-08-Nils-WISTOFF-a... seems simple to implement in hardware, but does not seem efficient. https://carrv.github.io/2020/papers/CARRV2020_paper_10_Wistoff.pdf claims to be decently efficient, but is for an in-order CPU.

It is highly efficient, and completely hidden behind the D-cache flush.

Implementation on an OoO processor isn’t published yet, but confirms the results obtained on the IO CV6.

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Also, in the future, would you mind including the full URL of any articles? I don’t know what “Wistoff et al” and “Ge et al” refer to, and my mail client is configured to only display plain text (not HTML) because the attack surface of HTML rendering is absurdly high.

All papers are listed on the time-protection project page.

[Ge et al, EuroSys’19]: https://trustworthy.systems/publications/abstracts/Ge_YCH_19.abstract [Wistoff et al, DATE’21]: https://trustworthy.systems/publications/abstracts/Wistoff_SGBH_21.abstract

Both won Best-Paper awards, btw

I’m not surprised! For the systems that _can_ use it, it is awesome. -- Sincerely, Demi Marie Obenour (she/her/hers)

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-- Sincerely, Demi Marie Obenour (she/her/hers)