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Memory tagging is an awesome technology, and the scariest mitigation I have seen as an attacker, ever. As such I am excited to read the paper, but dismayed that the authors seem to be affected by Redmondian RIP/PC obsession. The true value of MT is not in control flow integrity.

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Matt Miller

@epakskape

Jul 19, 2019

Interested in memory safety exploits & mitigations? Here's a new research paper that explores an ISA extension which tries to make it more difficult to corrupt pointers. All feedback on the security efficacy and overall design is appreciated :) microsoft.com/en-us/research

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This. CFI is all snake oil aimed at giving attackers headaches at best. But MT admits mitigations that fully shut off lots of vulns.

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Unless I'm mistaken, even just 3 tag values suffice to ensure that no adjacent objects have same tag, which eliminates all sequential-store buffer overflows, or can be used to protect metadata between objects from *all* OOB stores.

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It can also be used for deterministic use-after-free mitigation for a given number of cycles. I've documented the approach that I'll be using for it in hardened_malloc: github.com/GrapheneOS/har There will be a value reserved for free memory, and then it increments the old value.

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This assumes freed chunks aren't splittable/mergeable, I think, which is another motivating factor for dropping dlmalloc design.

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A performance-oriented allocator would work well with a similar region-based slab allocation design. It could just use free lists within slabs instead of bitmaps, while still using the same approach to out-of-line metadata for tracking slabs. Main loss is double-free detection.

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There are other approaches to out-of-line metadata than the address space reservation. I'm doing that largely to get dedicated, isolated memory regions for metadata and each size class with the address space never being mixed / reused. Can also use an approach based on alignment.

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Memory tagging with a reserved free tag makes it so that the free list gets memory protected (free list accessed with pointers using the reserved free tag) and it's possible to detect double-free by checking for the free tag instead of checking if the slot is free in the bitmap.

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On that note, I'm really not a big fan of the ARMv8.3 pointer authentication feature. It's a very low entropy probabilistic mitigation without many compelling uses and I don't think it's a good use of scarce pointer bits. I'm already finding that address space is too scarce...

5:03 PM · Aug 8, 2019Twitter Web App

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And that's assuming free silicon as well