Daniel Micay on Twitter: "@lattera On a system without MREMAP_MAYMOVE, that portion would just be disabled so it falls through to the standard handling at the bottom: https://t.co/01iVITooDi There's another use of mremap without MREMAP_MAYMOVE for in-place growth. MAP_FIXED_NOREPLACE would also work for that." / Twitter

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I really like #FreeBSD's MAP_GUARD implementation for guard pages. It's used in #bhyve to protect the VM's address space from the host's perspective. MAP_GUARD is essentially PROT_NONE on steroids. It prevents growable segments (ie, MAP_STACK) from expanding into the region.

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On Linux, MAP_STACK is a no-op and secondary stack mappings are just a contiguous, static mapping like other mmap mappings so it doesn't really matter. The main thread stack is a special case and has a properly enforced gap that's not a PROT_NONE region so it can't grow into one.

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The enforced gap is a special reserved area rather than an actual normal mapping. Secondary stacks are just a mapping made by the libc pthread_create implementation (or the caller, if they provide their own stack) with a guard region. I improved that in my past libc hardening.

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In my previous Bionic hardening, I added randomly sized guard regions on both ends similar to how I handle large allocations in the hardened allocator along with randomizing the stack pointer within the initial page to a certain extent up to a limit like 2% of available space.

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That's something I need to look into for HardenedBSD again: random-sized MAP_GUARD stack guard pages for per-thread stacks.

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You can see how it's implemented: github.com/AndroidHardeni It's only tricky for aligned allocations (not applicable to stacks) and isn't that complex. Here's where it chooses the random guard size: github.com/AndroidHardeni It's stored alongside size: github.com/AndroidHardeni

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I meant to link to pages.c rather than memory.c: github.com/AndroidHardeni The whole purpose of pages.c / pages.h is to provide a wrapper around memory.c / memory.h implementing support for guard pages in a mostly transparent way. It's not entirely transparent due to realloc.

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Since realloc actually has to be aware of how the guard pages work for the efficient approaches to large allocation shrinking, growth and moving pages to a new location with the mapping already reserved via MREMAP_MAYMOVE|MREMAP_FIXED. Those are just optimized fast paths though.

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So this realloc section is essentially part of the implementation of guard pages too and is by far the most complex part of it: github.com/AndroidHardeni It's quite tricky to implement some of this with proper handling for out-of-memory errors. It can barely use MREMAP_MAYMOVE.

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It has to actually allocate a new mapping in the usual way to provide it with guard pages, and then move the non-guard pages to the new inner portion via MREMAP_MAYMOVE|MREMAP_FIXED. It essentially uses it as an optimized memcpy and falls back to memcpy if that system call fails.

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On a system without MREMAP_MAYMOVE, that portion would just be disabled so it falls through to the standard handling at the bottom: github.com/AndroidHardeni There's another use of mremap without MREMAP_MAYMOVE for in-place growth. MAP_FIXED_NOREPLACE would also work for that.

5:57 PM · Feb 4, 2019Twitter Web Client

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Those are both just optimizations. There's no harm in disabling both. It's rarely ever possible to do in-place growth anyway, at least on Linux where the mmap heap starts at a high address and grows downwards. MREMAP_MAYMOVE helps a huge amount but it's only for massive realloc.

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Also worth noting that one of the few forms of misuse this allocator implementation isn't hardened against is a buggy program with realloc races on the same allocation between different threads. I've thought about how to address that but it's quite difficult and would be costly.

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