[pizlonator]

MTE and CHERI are so different that it’s hard and maybe not even possible to do both at the same time (you might not have enough spare bits in a CHERI 128 bit ptr for the MTE tag)

They also imply a very different system architecture.

[jrtc27]

We actually have ideas for how to combine the two; see section C.5 of https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-987.pdf

[leoc]

Sure, I'm not suggesting that Apple might actually do both at the same time. They could however implement the less burdensome one now while intending to replace it with the the all-singing-all-dancing alternative down the line.

[pizlonator]

Gotcha. My point about different systems architectures makes me think it’s unlikely that you’d want to do that

[quotemstr]

> MTE and CHERI are so different that it’s hard and maybe not even possible to do both at the same time (you might not have enough spare bits in a CHERI 128 bit ptr for the MTE tag)

Why would you need MTE if you have CHERI?

[bri3d]

Why would you need CHERI if you have working mitigations that don't demand a second bus?

I think it's two halves of the same coin and Apple chose the second half of the coin.

The two systems are largely orthogonal; I think if Apple chose to go from one to the other it will be a generational change rather than an incremental one. The advantage of MTE/MIE is you can do it incrementally by just changing the high bits the allocator supplies; CHERI requires a fundamental paradigm shift. Apple love paradigm shifts but there's no indication they're going to do one here; if they do, it will be a separate effort.

[pizlonator]

CHERI is deterministic.

That’s strictly better, in theory.

(Not sure it’s practically better. You could make an argument that it’s not.)

[bri3d]

FWIW (I am a nobody compared to you; I didn't make FIL-C :) ) - I think that MIE/MTE are practically superior to CHERI.

I also think this argument is compelling because one exists in millions of consumer drives, to-be-more (MTE -> MIE) and one does not.

[VogonPoetry]

This is on the verge of pedantry - CHERI determinism isn't strictly true, garbage collecting abandoned descriptors is currently done asynchronously. Malicious code could attempt to reuse an abandoned descriptor before it is "disappeared". I think it might be possible to construct a synthetic situation where two threads operating with perhaps different privilege in the same address space (something CHERI can support!) have an IPC channel might be affected by the timing.

There is a section in the technical reports that talks about garbage collection.

I don't think CHERI is currently being used with different privileged threads in the same address space.

[Findecanor]

I suspect that the parent poster was referring to MTE's memory protection being probabilistic. There are only 16 tag values for an attacker to guess. You can combine MTE and PAC, but PAC is also only probabilistic.

With CHERI, there is nothing to guess. You either have a capability or you don't.

[labcomputer]

Right, but the problem with CHERI is that you may (probabilistically) continue to have that capability even after you shouldn't. That's the problem.

That's because the capability (tagged pointer) itself is what gives you the right to access memory. So you have to find all the capabilities pointing to a segment of memory and invalidate them. Remember, capabilities are meant to be copied.

Early work on CHERI (CHERIvoke) proposed a stop-the-world barrier to revoke capabilities by doing a full scan of the program's memory (ouch!) to find and invalidate any stale capabilities. Because that is so expensive, the scan is only performed after a certain threshold amount of memory has been freed. That threshold introduces a security / battery life trade-off.

That was followed by "Cornucopia", which proposed a concurrent in-kernel scan (with some per-page flags to reduce the number of pages scanned) followed by a shorter stop-the-world. In 2024 (just last year), "Reloaded" was proposed, which add still more MMU hardware to nearly eliminate pauses, at the cost of 10% more memory traffic.

Unfortunately, the time between free and revocation introduces a short-but-not-zero window for UAF bugs/attacks. This time gap is even explicitly acknowledged in the Reloaded paper! Moreover, the Reloaded revocation algo requires blocking all threads of an application to ensure no dead capabilities are hidden in registers.

In contrast, with MTE, you just change the memory's tag on free, which immediately causes all formerly-valid pointers to the memory granule to become invalid. That's why you would want both: They're complementary.

* MTE gives truly instantaneous invalidation with zero battery impact, but only probabilistic spatial protections from attackers.

* CHERI gives deterministic spatial protection with eventually-consistent temporal invalidation semantics.

[als0]

Second bus?

[bri3d]

CHERI fundamentally relies on capabilities living in memory that is architecturally separate from program memory. You could do so using a bus firewall, but then you're at the same place as MIE with the SPTM.

[jrtc27]

That's not true. Capabilities are in main memory as much as any other data. The tags are in separate memory (whether a wider SRAM, DRAM ECC bits, or a separate table off on the side in a fraction of memory that's managed by the memory controller; all three schemes have been implemented and have trade-offs). But this is also true of MTE; you do not want those tags in normal software-visible main memory either, they need to be protected.

[Findecanor]

A CHERI capability is stored in main memory but with the tag bit for that location set. The tags are stored in separate memory pages, also in main memory in current designs.

Maybe you've been confused by a description of how it works inside a processor. In early CHERI designs, capabilities were in different architectural processor registers from integers.

In recent CHERI designs, the same register numbers are used for capabilities and other registers. A micro-architecture could be designed to have either all registers be capability registers with the tag bit, or use register renaming to separate integer and capability registers.

I suppose a CHERI MCU for embedded systems with small memory could theoretically have tag pages in separate SRAM instead of caching main memory, but I have not seen that.

[MBCook]

So something like having built in RAM for the pagetables that aren’t part of the normal pool? That way no matter what kind of attack you come up with user space cannot pass a pointer to it?

[pizlonator]

Not saying you’d want both. Just answering why MTE isn’t a path to CHERI

But here’s a reason to do both: CHERI’s UAF story isn’t great. Adding MTE means you get a probabilistic story at least

[bri3d]

True! On the flip side, MTE sucks at intra-object corruption: if I get access to a heap object with pointers, MTE doesn't affect me, I can go ahead and write to that object because I own the tag.

Overall my _personal_ opinion is that CHERI is a huge win at a huge cost, while MTE is a huge win at a low cost. But, there are definitely vulnerability classes that each system excels at.

[pizlonator]

I think the intra object issue might be niche enough to not matter.

And CHERI fixes it only optionally, if you accept having to change a lot more code

[jrtc27]

Where studies suggest "a lot" is sub-0.1%. For example, https://www.capabilitieslimited.co.uk/_files/ugd/f4d681_e0f2... was a study into porting 6 million lines of C and C++ to run a KDE+X11 desktop stack on CHERI, and saw 0.026% LoC change, or ~1.5k LoC out of ~6 million LoC, all done in just 3 months by one person. That's even an overestimate, because it includes many changes to build systems just to be able to cross-compile the projects. It's not nothing, but it's the kind of thing where a single engineer can feasibly port large bodies of code. Yes, certain systems code will be worse (like JITs), but the vast majority of cases are not that, and even those are still feasible (e.g. we have people working with Chromium and V8).

[pizlonator]

Does that study include enabling intra object overflow protection, or not?

When I say that this optional feature would force you to change a lot more code I’m comparing CHERI without intra object overflow protection to CHERI with intra object object overflow protection.

Finally, 6 million lines of code is not that impressive. Real OSes are measured in billions

[bri3d]

I think I broadly agree with you. IMO tagging is practically much, much more valuable than capabilities systems modeled like CHERI.

[quotemstr]

Yes, but CHERI opens whole new system design possibilities, including things like ultra-cheap intra-address-space security boundaries. See https://www.cl.cam.ac.uk/research/security/ctsrd/pdfs/201607...

> We have used CHERI’s ISA facilities as a foundation to build a software object-capability model supporting orders of magnitude greater compartmentalization performance, and hence granularity, than current designs. We use capabilities to build a hardware-software domain-transition mechanism and programming model suitable for safe communication between mutually distrusting software

and https://github.com/CTSRD-CHERI/cheripedia/wiki/Colocation-Tu...

> Processes are Unix' natural compartments, and a lot of existing software makes use of that model. The problem is, they are heavy-weight; communication and context switching overhead make using them for fine-grained compartmentalisation impractical. Cocalls, being fast (order of magnitude slower than a function call, order of magnitude faster than a cheapest syscall), aim to fix that problem.

This functionality revolves around two functions: cocall(2) for the caller (client) side, and coaccept(2) for the callee (service) side. Underneath they are implemented using CHERI magic in the form of CInvoke / LDPBR CPU instruction to switch protection domains without the need to enter the kernel, but from the API user point of view they mostly look like ordinary system calls and follow the same conventions, errno et al.

There's a decent chance that we get back whatever performance we pay for CHERI with interest as new systems architecture possibilities open up.

MTE helps us secure existing architectures. CHERI makes new architectures possible.

[quotemstr]

Some progress on UAF though! https://dl.acm.org/doi/10.1145/3703595.3705878