This project currently aims to support Android, musl and glibc. It may support other non-Linux operating systems in the future. For Android and musl, there will be custom integration and other hardening features. The glibc support will be limited to replacing the malloc implementation because musl is a much more robust and cleaner base to build on and can cover the same use cases.

Debian stable determines the most ancient set of supported dependencies:

• glibc 2.24
• Linux 4.9
• Clang 3.8 or GCC 6.3

However, using more recent releases is highly recommended. Older versions of the dependencies may be compatible at the moment but are not tested and will explicitly not be supported.

For external malloc replacement with musl, musl 1.1.20 is required. However, there will be custom integration offering better performance in the future along with other hardening for the C standard library implementation.

Major releases of Android will be supported until tags stop being pushed to the Android Open Source Project (AOSP). Google supports each major release with security patches for 3 years, but tagged releases of the Android Open Source Project are more than just security patches and are no longer pushed once no officially supported devices are using them anymore. For example, at the time of writing (September 2018), AOSP only has tagged releases for 8.1 (Nexus 5X, Nexus 5X, Pixel C) and 9.0 (Pixel, Pixel XL, Pixel 2, Pixel 2 XL). There are ongoing security patches for 6.0, 6.0.1, 7.0, 7.1.1, 7.1.2, 8.0, 8.1 and 9.0 but only the active AOSP branches (8.1 and 9.0) are supported by this project and it doesn't make much sense to use much older releases with far less privacy and security hardening.

Testing

The preload.sh script can be used for testing with dynamically linked executables using glibc or musl:

./preload.sh krita --new-image RGBA,U8,500,500

It can be necessary to substantially increase the vm.max_map_count sysctl to accomodate the large number of mappings caused by guard slabs and large allocation guard regions. There will be a configuration option in config.h for tuning the proportion of slabs to guard slabs too, since the default 1:1 proportion makes the address space quite sparse.

It can offer slightly better performance when integrated into the C standard library and there are other opportunities for similar hardening within C standard library and dynamic linker implementations. For example, a library region can be implemented to offer similar isolation for dynamic libraries as this allocator offers across different size classes. The intention is that this will be offered as part of hardened variants of the Bionic and musl C standard libraries.

Configuration

Compile-time configuration is available in the config.h file for controlling the balance between security and performance / memory usage. By default, all the optional security features are enabled. Options are only provided for the features with a significant performance or memory usage cost.

#define GUARD_SLABS true
#define WRITE_AFTER_FREE_CHECK true
#define SLOT_RANDOMIZE true
#define ZERO_ON_FREE true
#define SLAB_CANARY true

There will be more control over enabled features in the future along with control over fairly arbitrarily chosen values like the size of empty slab caches (making them smaller improves security), the maximum size of guard regions for large allocations and the proportion of slabs to guard slabs.

Basic design

The current design is very simple and will become a bit more sophisticated as the basic features are completed and the implementation is hardened and optimized. The allocator is exclusive to 64-bit platforms in order to take full advantage of the abundant address space without being constrained by needing to keep the design compatible with 32-bit.

Small allocations are always located in a large memory region reserved for slab allocations. It can be determined that an allocation is one of the small size classes from the address range. Each small size class has a separate reserved region within the larger region, and the size of a small allocation can simply be determined from the range. Each small size class has a separate out-of-line metadata array outside of the overall allocation region, with the index of the metadata struct within the array mapping to the index of the slab within the dedicated size class region. Slabs are a multiple of the page size and are page aligned. The entire small size class region starts out memory protected and becomes readable / writable as it gets allocated, with idle slabs beyond the cache limit having their pages dropped and the memory protected again.

Large allocations are tracked via a global hash table mapping their address to their size and guard size. They're simply memory mappings and get mapped on allocation and then unmapped on free.

Security properties

• Fully out-of-line metadata
• Deterministic detection of any invalid free (unallocated, unaligned, etc.)
• Isolated memory region for slab allocations
  • Divided up into isolated inner regions for each size class
    • High entropy random base for each size class region
    • No deterministic / low entropy offsets between allocations with different size classes
  • Metadata is completely outside the slab allocation region
    • No references to metadata within the slab allocation region
    • No deterministic / low entropy offsets to metadata
  • Entire slab region starts out non-readable and non-writable
  • Slabs beyond the cache limit are purged and become non-readable and non-writable memory again
• Fine-grained randomization within memory regions
  • Randomly sized guard regions for large allocations
  • Random slot selection within slabs
  • [in-progress] Randomized delayed free for slab allocations
  • [in-progress] Randomized allocation of slabs
  • [more randomization coming as the implementation is matured]
• Slab allocations are zeroed on free and large allocations are unmapped
• Detection of write-after-free by verifying zero filling is intact
• Memory in fresh allocations is consistently zeroed due to it either being fresh pages or zeroed on free after previous usage
• [in-progress] Delayed free via a combination of FIFO and randomization for slab allocations
• Random canaries placed after each slab allocation to absorb and then later detect overflows/underflows
  • High entropy per-slab random values
  • Leading byte is zeroed to contain C string overflows
  • [in-progress] Mangled into a unique value per slab slot (although not with a strong keyed hash due to performance limitations)
• Possible slab locations are skipped and remain memory protected, leaving slab size class regions interspersed with guard pages
• Zero size allocations are memory protected
• Protected allocator metadata
  • Address space for metadata is never used for allocations and vice versa
  • [implementing stronger protection is in-progress]
• Extension for retrieving the size of allocations with fallback to a sentinel for pointers not managed by the allocator
  • Can also return accurate values for pointers within small allocations
  • The same applies to pointers within the first page of large allocations, otherwise it currently has to return a sentinel
• No alignment tricks interfering with ASLR like jemalloc, PartitionAlloc, etc.
• No usage of the legacy brk heap
• Aggressive sanity checks
  • Errors other than ENOMEM from mmap, munmap, mprotect and mremap treated as fatal, which can help to detect memory management gone wrong elsewhere in the process.

Randomness

The current implementation of random number generation for randomization-based mitigations is based on generating a keystream from a stream cipher (ChaCha8) in small chunks. A separate CSPRNG is used for each small size class, large allocations, etc. in order to fit into the existing fine-grained locking model without needing to waste memory per thread by having the CSPRNG state in Thread Local Storage. Similarly, it's protected via the same approach taken for the rest of the metadata. The stream cipher is regularly reseeded from the OS to provide backtracking and prediction resistance with a negligible cost. The reseed interval simply needs to be adjusted to the point that it stops registering as having any significant performance impact. The performance impact on recent Linux kernels is primarily from the high cost of system calls and locking since the implementation is quite efficient (ChaCha20), especially for just generating the key and nonce for another stream cipher (ChaCha8).

ChaCha8 is a great fit because it's extremely fast across platforms without relying on hardware support or complex platform-specific code. The security margins of ChaCha20 would be completely overkill for the use case. Using ChaCha8 avoids needing to resort to a non-cryptographically secure PRNG or something without a lot of scrunity. The current implementation is simply the reference implementation of ChaCha8 converted into a pure keystream by ripping out the XOR of the message into the keystream.

The random range generation functions are a highly optimized implementation too. Traditional uniform random number generation within a range is very high overhead and can easily dwarf the cost of an efficient CSPRNG.

Size classes

The zero byte size class is a special case of the smallest regular size class. It's allocated in a separate region with the memory left non-readable and non-writable.

The slab slot count for each size class is not yet finely tuned beyond choosing values avoiding internal fragmentation for slabs (i.e. avoiding wasted space due to page size rounding).

The choice of size classes is the same as jemalloc, but with a much different approach to the slabs containing them:

size classes are multiples of the quantum [16], spaced such that there are four size classes for each doubling in size, which limits internal fragmentation to approximately 20% for all but the smallest size classes

size class	worst case internal fragmentation	slab slots	slab size	worst case internal fragmentation for slabs
16	100%	256	4096	0.0%
32	46.875%	128	4096	0.0%
48	31.25%	85	4096	0.390625%
64	23.4375%	64	4096	0.0%
80	18.75%	51	4096	0.390625%
96	15.625%	42	4096	1.5625%
112	13.392857142857139%	36	4096	1.5625%
128	11.71875%	64	8192	0.0%
160	19.375%	51	8192	0.390625%
192	16.145833333333343%	64	12288	0.0%
224	13.839285714285708%	54	12288	1.5625%
256	12.109375%	64	16384	0.0%
320	19.6875%	64	20480	0.0%
384	16.40625%	64	24576	0.0%
448	14.0625%	64	28672	0.0%
512	12.3046875%	64	32768	0.0%
640	19.84375%	64	40960	0.0%
768	16.536458333333343%	64	49152	0.0%
896	14.174107142857139%	64	57344	0.0%
1024	12.40234375%	64	65536	0.0%
1280	19.921875%	16	20480	0.0%
1536	16.6015625%	16	24576	0.0%
1792	14.229910714285708%	16	28672	0.0%
2048	12.451171875%	16	32768	0.0%
2560	19.9609375%	8	20480	0.0%
3072	16.634114583333343%	8	24576	0.0%
3584	14.2578125%	8	28672	0.0%
4096	12.4755859375%	8	32768	0.0%
5120	19.98046875%	8	40960	0.0%
6144	16.650390625%	8	49152	0.0%
7168	14.271763392857139%	8	57344	0.0%
8192	12.48779296875%	8	65536	0.0%
10240	19.990234375%	6	61440	0.0%
12288	16.658528645833343%	5	61440	0.0%
14336	14.278738839285708%	4	57344	0.0%
16384	12.493896484375%	4	65536	0.0%