mirror of
https://github.com/AFLplusplus/AFLplusplus.git
synced 2025-06-08 08:11:34 +00:00
71 lines
3.1 KiB
Markdown
71 lines
3.1 KiB
Markdown
# libdislocator, an abusive allocator
|
|
|
|
For the general instruction manual, see [docs/README.md](../../docs/README.md).
|
|
|
|
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.
|
|
|
|
- Optionally, in platforms supporting it, `named` pages can be used by passing
|
|
`USENAMEDPAGE=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 reduces the ability of libdislocator to detect
|
|
off-by-one bugs but also it makes libdislocator 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.
|