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libdislocator, an abusive allocator

(See ../docs/README.md for the general instruction manual.)

This is a companion library that can be used as a drop-in replacement for the libc allocator in the fuzzed binaries. It improves the odds of bumping into heap-related security bugs in several ways:

  • It allocates all buffers so that they are immediately adjacent to a subsequent PROT_NONE page, causing most off-by-one reads and writes to immediately segfault,

  • It adds a canary immediately below the allocated buffer, to catch writes to negative offsets (won't catch reads, though),

  • It sets the memory returned by malloc() to garbage values, improving the odds of crashing when the target accesses uninitialized data,

  • It sets freed memory to PROT_NONE and does not actually reuse it, causing most use-after-free bugs to segfault right away,

  • It forces all realloc() calls to return a new address - and sets PROT_NONE on the original block. This catches use-after-realloc bugs,

  • It checks for calloc() overflows and can cause soft or hard failures of alloc requests past a configurable memory limit (AFL_LD_LIMIT_MB, AFL_LD_HARD_FAIL).

  • Optionally, in platforms supporting it, huge pages can be used by passing USEHUGEPAGE=1 to make.

  • Size alignment to max_align_t can be enforced with AFL_ALIGNED_ALLOC=1. In this case, a tail canary is inserted in the padding bytes at the end of the allocated zone. This reduce the ability of libdislocator to detect off-by-one bugs but also it make slibdislocator compliant to the C standard.

Basically, it is inspired by some of the non-default options available for the OpenBSD allocator - see malloc.conf(5) on that platform for reference. It is also somewhat similar to several other debugging libraries, such as gmalloc and DUMA - but is simple, plug-and-play, and designed specifically for fuzzing jobs.

Note that it does nothing for stack-based memory handling errors. The -fstack-protector-all setting for GCC / clang, enabled when using AFL_HARDEN, can catch some subset of that.

The allocator is slow and memory-intensive (even the tiniest allocation uses up 4 kB of physical memory and 8 kB of virtual mem), making it completely unsuitable for "production" uses; but it can be faster and more hassle-free than ASAN / MSAN when fuzzing small, self-contained binaries.

To use this library, run AFL like so:

AFL_PRELOAD=/path/to/libdislocator.so ./afl-fuzz [...other params...]

You have to specify path, even if it's just ./libdislocator.so or $PWD/libdislocator.so.

Similarly to afl-tmin, the library is not "proprietary" and can be used with other fuzzers or testing tools without the need for any code tweaks. It does not require AFL-instrumented binaries to work.

Note that the AFL_PRELOAD approach (which AFL internally maps to LD_PRELOAD or DYLD_INSERT_LIBRARIES, depending on the OS) works only if the target binary is dynamically linked. Otherwise, attempting to use the library will have no effect.