(See ../docs/README for the general instruction manual.)
The code in this directory allows you to build a standalone feature that leverages the QEMU “user emulation” mode and allows callers to obtain instrumentation output for black-box, closed-source binaries. This mechanism can be then used by afl-fuzz to stress-test targets that couldn't be built with afl-gcc.
The usual performance cost is 2-5x, which is considerably better than seen so far in experiments with tools such as DynamoRIO and PIN.
The idea and much of the initial implementation comes from Andrew Griffiths. The actual implementation on QEMU 3 (shipped with afl++) is from Andrea Fioraldi. Special thanks to abiondo that re-enabled TCG chaining.
The feature is implemented with a patch to QEMU 3.1.1. The simplest way to build it is to run ./build_qemu_support.sh. The script will download, configure, and compile the QEMU binary for you.
QEMU is a big project, so this will take a while, and you may have to resolve a couple of dependencies (most notably, you will definitely need libtool and glib2-devel).
Once the binaries are compiled, you can leverage the QEMU tool by calling afl-fuzz and all the related utilities with -Q in the command line.
Note that QEMU requires a generous memory limit to run; somewhere around 200 MB is a good starting point, but considerably more may be needed for more complex programs. The default -m limit will be automatically bumped up to 200 MB when specifying -Q to afl-fuzz; be careful when overriding this.
In principle, if you set CPU_TARGET before calling ./build_qemu_support.sh, you should get a build capable of running non-native binaries (say, you can try CPU_TARGET=arm). This is also necessary for running 32-bit binaries on a 64-bit system (CPU_TARGET=i386). If you're trying to run QEMU on a different architecture you can also set HOST to the cross-compiler prefix to use (for example HOST=arm-linux-gnueabi to use arm-linux-gnueabi-gcc).
You can also compile statically-linked binaries by setting STATIC=1. This can be useful when compiling QEMU on a different system than the one you're planning to run the fuzzer on and is most often used with the HOST variable.
Note: when targetting the i386 architecture, on some bianries the forkserver handshake may fail due to the lack of reversed memory. Fix it with
export QEMU_RESERVED_VA=0x1000000
Note: if you want the QEMU helper to be installed on your system for all users, you need to build it before issuing ‘make install’ in the parent directory.
If you want to specify a different path for libraries (e.g. to run an arm64 binary on x86_64) use QEMU_LD_PREFIX.
As for LLVM mode (refer to its README for mode details) QEMU mode supports the deferred initialization.
This can be enabled setting the environment variable AFL_ENTRYPOINT which allows to move the forkserver to a different part, e.g. just before the file is opened (e.g. way after command line parsing and config file loading, etc.) which can be a huge speed improvement. Note that the specified address must be an address of a basic block.
QEMU mode supports also persistent mode for x86 and x86_64 targets. The environment variable to enable it is AFL_QEMU_PERSISTENT_ADDR=start addr. In this variable you must specify the address of the function that has to be the body of the persistent loop. The code in this function must be stateless like in the LLVM persistent mode. The return address on stack is patched like in WinAFL in order to repeat the execution of such function. Another modality to execute the persistent loop is to specify also the AFL_QEMU_PERSISTENT_RET=end addr env variable. With this variable assigned, instead of patching the return address, the specified instruction is transformed to a jump towards start addr. Note that the format of the addresses in such variables is hex.
Note that the base address of PIE binaries in QEMU user mode is 0x4000000000.
With the env variable AFL_QEMU_PERSISTENT_GPR you can tell QEMU to save the original value of general purpose registers and restore them in each cycle. This allows to use as persistent loop functions that make use of arguments on x86_64.
With AFL_QEMU_PERSISTENT_RETADDR_OFFSET you can specify the offset from the stack pointer in which QEMU can find the return address when start addr is hitted.
Use this mode with caution, probably it will not work at the first shot.
CompareCoverage is a sub-instrumentation with effects similar to laf-intel.
The option that enables QEMU CompareCoverage is AFL_COMPCOV_LEVEL. There is also ./libcompcov/ which implements CompareCoverage for *cmp functions (splitting memcmp, strncmp, etc. to make these conditions easier solvable by afl-fuzz). AFL_COMPCOV_LEVEL=1 is to instrument comparisons with only immediate values / read-only memory. AFL_COMPCOV_LEVEL=2 instruments all comparison instructions and memory comparison functions when libcompcov is preloaded. AFL_COMPCOV_LEVEL=3 has the same effects of AFL_COMPCOV_LEVEL=2 but enables also the instrumentation of the floating-point comparisons on x86 and x86_64 (experimental).
Integer comparison instructions are currently instrumented only on the x86, x86_64 and ARM targets.
Highly recommended.
AFL++ QEMU can use Wine to fuzz WIn32 PE binaries. Use the -W flag of afl-fuzz.
Note that some binaries require user interaction with the GUI and must be patched.
For examples look here.
The feature is supported only on Linux. Supporting BSD may amount to porting the changes made to linux-user/elfload.c and applying them to bsd-user/elfload.c, but I have not looked into this yet.
The instrumentation follows only the .text section of the first ELF binary encountered in the linking process. It does not trace shared libraries. In practice, this means two things:
Any libraries you want to analyze must be linked statically into the executed ELF file (this will usually be the case for closed-source apps).
Standard C libraries and other stuff that is wasteful to instrument should be linked dynamically - otherwise, AFL will have no way to avoid peeking into them.
Setting AFL_INST_LIBS=1 can be used to circumvent the .text detection logic and instrument every basic block encountered.
If you want to compare the performance of the QEMU instrumentation with that of afl-gcc compiled code against the same target, you need to build the non-instrumented binary with the same optimization flags that are normally injected by afl-gcc, and make sure that the bits to be tested are statically linked into the binary. A common way to do this would be:
$ CFLAGS=“-O3 -funroll-loops” ./configure --disable-shared $ make clean all
Comparative measurements of execution speed or instrumentation coverage will be fairly meaningless if the optimization levels or instrumentation scopes don't match.
If you need to fix up checksums or do other cleanup on mutated test cases, see experimental/post_library/ for a viable solution.
Do not mix QEMU mode with ASAN, MSAN, or the likes; QEMU doesn't appreciate the “shadow VM” trick employed by the sanitizers and will probably just run out of memory.
Compared to fully-fledged virtualization, the user emulation mode is NOT a security boundary. The binaries can freely interact with the host OS. If you somehow need to fuzz an untrusted binary, put everything in a sandbox first.
QEMU does not necessarily support all CPU or hardware features that your target program may be utilizing. In particular, it does not appear to have full support for AVX2 / FMA3. Using binaries for older CPUs, or recompiling them with -march=core2, can help.
Beyond that, this is an early-stage mechanism, so fields reports are welcome. You can send them to afl-users@googlegroups.com.
Statically rewriting binaries just once, instead of attempting to translate them at run time, can be a faster alternative. That said, static rewriting is fraught with peril, because it depends on being able to properly and fully model program control flow without actually executing each and every code path.
The best implementation is this one:
https://github.com/vanhauser-thc/afl-dyninst
The issue however is Dyninst which is not rewriting the binaries so that they run stable. A lot of crashes happen, especially in C++ programs that use throw/catch. Try it first, and if it works for you be happy as it is 2-3x as fast as qemu_mode.