alt Hacker NewsTo expand a bit: 16-byte chunks of memory can be associated with a four bit tag. Then you steal four unused high bits from your pointers to store a tag value. When memory has a tag, a pointer used to access it must have the matching tag in its high bits. Your malloc implementation can then assign a different tag to adjacent allocations and any overflow into an adjacent allocation will have a mismatched tag and will trap. Likewise, change the tag on free and an attempt to use the pointer after the allocation has been freed will trap.
Of course, this doesn't come for free. Four bits per 16 bytes means a 3% increase in memory needed for tagged memory, hardware overhead for checking tags on memory accesses, and software overhead of setting/changing/clearing tags as necessary. This overhead is low (Apple shipped this in flagship hardware a year ago and nobody's complaining about performance there) but not zero, and an implementation with poor performance could be a real problem.
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What does the malloc or hardware do if for some reason your program needs to access memory addresses that overlap your tag bytes ?
I know that it's a really improbable scenario and the OS would also just refuse you allocations at some point, but what would the malloc implementation and the MTE do in such a case ? Fail the allocation ? trap when reading the pointer since it would point to the "wrong place" ?
4 bits per 16 bytes is ~ 1/4 the cost of ECC, which is possible to implement with just an extra cycle or two of latency in the memory controller. MTE seems similarly lightweight. Costs some transistors and a percent of a percent of power budget, but much like ECC it seems a fair bargain.
Implementing MTE is mostly another step in Apple’s efforts towards memory safety. There are bunch of things they’ve done to try to make this work a little better, but for obvious reasons, this is never going to be turned on by default for user apps and is only enabled for specific pathways.
> Because EMTE tag checking imposes a performance cost, we designed Memory Integrity Enforcement to take advantage of our secure allocators first and use EMTE to protect only smaller individual allocations within a type bucket, which software allocators can’t defend on their own.
Which seems to imply that outside of Apple’s targeted use in the kernel, the developer use of MTE on Apple platforms requires that you migrate your code to use their typed allocators…which I don’t know how many people are going to do that (and it is clear their answer is going to be to point people to Swift since it makes these types explicit for the compiler and even then, I am sure there are bugs in Swift in certain cases). Memory tagging is neat tech, but I don’t know if it’s going to gain all that much traction. It requires a burden that I suspect most app developers are not willing to take on and given that the user interaction is a crashy app. This is a path to nowhere except those of us who understand the trade-off.