Memory Integrity Enforcement

This is really impressive.

It’s my understanding that this won’t protect you in the case where the attacker has a chance to try multiple times.

The approach would be something like: go out of bounds far enough to skip the directly adjacent object, or do a use after free with a lot of grooming, so that you get a a chance of getting a matching tag. The probability of getting a matching tag is 1/16.

But this post doesn’t provide enough details for me to be super confident about what I’m saying. Time will tell! If this is successful then the remaining exploit chains will have to rely on logic bugs, which would be super painful for the bad guys

Even with Android MTE, one of the workarounds was probabilistic attacks on the small tag size, which imply multiple tries. One of the big distinctions here is uniform synchronous enforcement, so writes trap immediately and not on the next context switch.

It's typically used in synchronous or asymmetric mode on Android. The asymmetric mode preserves nearly the same performance as asymmetric while only having writes remain asynchronous. It's enforced once there's a read or system call. Synchronous is more important in the kernel due to how many holes there are for bypassing it, which is why GrapheneOS is using it as synchronous in the kernel and asymmetric in userspace. io_uring is a major example of how there could be a major bypass of asymmetric mode, although Android doesn't allow it for more than a few core processes. Deploying asynchronous is still useful since it's a widely distributed bug finding tool with near zero cost. The main cost is that it finds so many bugs which need to be addressed which is a barrier for deploying it for third party apps.

The main weakness is that MTE is only 4 bits... and it's not even 1/16 but typically 1/15 chance of bypassing it since a tag is usually reserved for metadata, free data, etc. The Linux kernel's standard implementation for in-kernel usage unnecessarily reserves more than 1 to make debugging easier. MTE clears the way for a more serious security focused memory tagging implementation with far more bits and other features. It provides a clear path to providing very strong protection against the main classes of vulnerabilities used in exploits, especially remote/proximity ones. It's a great feature but it's more what it leads to that's very impressive than the current 4 bit MTE. Getting rid of some known side channels doesn't make it into a memory safety implementation.

You'd know better than I would; I'm a bystander on this whole area of development. I was really just responding to the notion that these countermeasures fall to attackers who get multiple bites at the apple --- those attackers are explicitly part of the threat model. I think I have realistic expectations about what this revision of MIE is going to do (raise costs, maybe over time wash out a lower tier of exploit developers on the platform).

I think they've likely done a great job implementing it and think it will significantly improve iPhone security. I dislike the over the top marketing resembling a technical blog post. It's as if they've deployed CHERI in production with near 0 overhead rather than an incremental improvement over what standard ARM Cortex cores shipped years ago which people have been using in production.

Others are aware of where MTE needs improvement and are working on it for years. Cortex shipped MTE with a side channel issue which is better than not shipping it and it will get addressed. Apple has plenty of their own side channel vulnerabilities for their CPUs. Deterministic protections provided via MTE aren't negatively impacted by the side channel and also avoid depending on only 4 bits of entropy. The obvious way to use MTE is not the only way to use it.

GrapheneOS began using MTE in production right after the Pixel 8 provided a production quality implementation, which was significantly later than it could have been made available since Pixels aren't early adopters of new Cortex cores. On those cores, asynchronous MTE is near free and asymmetric is comparable to something like -fstack-protector-strong. Synchronous is relatively expensive, so making that perform better than the early Cortex cores providing MTE seems to be where Apple made a significant improvement. Apple has higher end, larger cores than the current line of Cortex cores. Qualcomm's MTE implementation will be available soon and will be an interesting comparison. We expect Android to heavily adopt it and therefore it will be made faster out of necessity. The security advantage of synchronous over asymmetric for userspace is questionable. It's clearer within the kernel, where little CPU time is spent on an end user device. We use synchronous in the kernel and asymmetric in userspace. We haven't offered full synchronous as an option mainly because we don't have any example of it making a difference. System calls act as a synchronization point in addition to reads. io_uring isn't available beyond a few core processes, etc.