Hardened allocator designed for modern systems. It has integration into Android's Bionic libc and can be used externally with musl and glibc as a dynamic library for use on other Linux-based platforms. It will gain more portability / integration over time.
Go to file
2018-09-02 05:08:45 -04:00
test reorder offset test output 2018-08-26 04:51:47 -04:00
third_party move libdivide.h to third_party/libdivide.h 2018-08-30 03:03:19 -04:00
.gitignore initial commit 2018-08-23 15:42:41 -04:00
calculate_waste.py output markdown table from calculate_waste 2018-08-26 06:21:09 -04:00
chacha.c use ChaCha8-based CSPRNG 2018-08-25 23:24:00 -04:00
chacha.h use ChaCha8-based CSPRNG 2018-08-25 23:24:00 -04:00
CREDITS use much faster get_random_{type}_uniform approach 2018-08-30 02:08:05 -04:00
LICENSE initial commit 2018-08-23 15:42:41 -04:00
Makefile add stubs for Android extensions 2018-09-02 05:08:45 -04:00
malloc.c add stubs for Android extensions 2018-09-02 05:08:45 -04:00
malloc.h add stubs for Android extensions 2018-09-02 05:08:45 -04:00
memory.c add mmap MAP_FIXED wrapper for dropping slabs 2018-08-29 13:04:52 -04:00
memory.h add mmap MAP_FIXED wrapper for dropping slabs 2018-08-29 13:04:52 -04:00
pages.c mark page management error code paths as unlikely 2018-09-02 02:08:44 -04:00
pages.h split out code for managing page spans 2018-09-02 02:03:39 -04:00
preload.sh make preload script for testing a bit more robust 2018-08-30 09:08:33 -04:00
random.c use much faster get_random_{type}_uniform approach 2018-08-30 02:08:05 -04:00
random.h manually specialize the CSPRNG cache 2018-08-26 00:43:46 -04:00
README.md only use reserved memory for regions hash table 2018-09-01 10:23:06 -04:00
util.c initial commit 2018-08-23 15:42:41 -04:00
util.h fix builds with prefix 2018-08-29 15:07:36 -04:00

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.

Ports to Android (Bionic libc) and musl libc will be created later. The initial target is glibc for ease of development, since it supports replacing the malloc implementation at runtime using dynamic linking including via LD_PRELOAD.

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 paged 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
  • [in-progress] 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
  • [in-progress] Random canaries placed after each slab allocation to absorb and then later detect overflows/underflows
    • High entropy per-slab random values
    • Mangled into a unique value per slab slot (although not with a strong keyed hash due to performance limitations)
  • [in-progress] Some 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 decent memory management gone wrong elsewhere in the program

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%