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diff --git a/qemu_mode/README.md b/qemu_mode/README.md index d28479d9..b4b5e7bf 100644 --- a/qemu_mode/README.md +++ b/qemu_mode/README.md @@ -1,184 +1,194 @@ # High-performance binary-only instrumentation for afl-fuzz - (See ../README.md for the general instruction manual.) +For the general instruction manual, see [README.md](../README.md). ## 1) Introduction 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. +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-cc. -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 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 current QEMU (shipped as qemuafl) is from -Andrea Fioraldi. Special thanks to abiondo that re-enabled TCG chaining. +The idea and much of the initial implementation comes from Andrew Griffiths. The +actual implementation on current QEMU (shipped as qemuafl) is from Andrea +Fioraldi. Special thanks to abiondo that re-enabled TCG chaining. -## 2) How to use qemu_mode +## 2) How to use QEMU mode -The feature is implemented with a patched QEMU. 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. +The feature is implemented with a patched QEMU. 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). +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. +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. +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). +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. +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 binaries the forkserver -handshake may fail due to the lack of reserved memory. Fix it with +Note: when targeting the i386 architecture, on some binaries the forkserver +handshake may fail due to the lack of reserved 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. +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. +If you want to specify a different path for libraries (e.g., to run an arm64 +binary on x86_64) use `QEMU_LD_PREFIX`. ## 3) Deferred initialization -As for LLVM mode (refer to its README.md for mode details) QEMU mode supports -the deferred initialization. +As for LLVM mode (refer to +[instrumentation/README.llvm.md](../instrumentation/README.llvm.md) 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.) +This can be enabled by 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. ## 4) Persistent mode -AFL++'s QEMU mode now supports also persistent mode for x86, x86_64, arm -and aarch64 targets. -This increases the speed by several factors, however it is a bit of work to set -up - but worth the effort. +AFL++'s QEMU mode now supports also persistent mode for x86, x86_64, arm, and +aarch64 targets. This increases the speed by several factors, however, it is a +bit of work to set up - but worth the effort. -Please see the extra documentation for it: [README.persistent.md](README.persistent.md) +For more information, see [README.persistent.md](README.persistent.md). ## 5) Snapshot mode As an extension to persistent mode, qemuafl can snapshot and restore the memory -state and brk(). Details are in the persistent mode readme. +state and brk(). For details, see [README.persistent.md](README.persistent.md). -The env var that enables the ready to use snapshot mode is AFL_QEMU_SNAPSHOT and -takes a hex address as a value that is the snapshot entrypoint. +The env var that enables the ready to use snapshot mode is `AFL_QEMU_SNAPSHOT` +and takes a hex address as a value that is the snapshot entry point. -Snapshot mode can work restoring all the writeable pages, that is typically slower than -fork() mode but, on the other hand, it can scale better with multicore. -If the AFL++ Snapshot kernel module is loaded, qemuafl will use it and, in this -case, the speed is better than fork() and also the scaling capabilities. +Snapshot mode can work restoring all the writeable pages, that is typically +slower than fork() mode but, on the other hand, it can scale better with +multicore. If the AFL++ snapshot kernel module is loaded, qemuafl will use it +and, in this case, the speed is better than fork() and also the scaling +capabilities. ## 6) Partial instrumentation You can tell QEMU to instrument only a part of the address space. -Just set AFL_QEMU_INST_RANGES=A,B,C... +Just set `AFL_QEMU_INST_RANGES=A,B,C...`. -The format of the items in the list is either a range of addresses like 0x123-0x321 -or a module name like module.so (that is matched in the mapped object filename). +The format of the items in the list is either a range of addresses like +0x123-0x321 or a module name like module.so (that is matched in the mapped +object filename). -Alternatively you can tell QEMU to ignore part of an address space for instrumentation. +Alternatively, you can tell QEMU to ignore part of an address space for +instrumentation. -Just set AFL_QEMU_EXCLUDE_RANGES=A,B,C... +Just set `AFL_QEMU_EXCLUDE_RANGES=A,B,C...`. -The format of the items on the list is the same as for AFL_QEMU_INST_RANGES, and excluding ranges -takes priority over any included ranges or AFL_INST_LIBS. +The format of the items on the list is the same as for `AFL_QEMU_INST_RANGES` +and excluding ranges takes priority over any included ranges or `AFL_INST_LIBS`. ## 7) CompareCoverage CompareCoverage is a sub-instrumentation with effects similar to laf-intel. -You have to set `AFL_PRELOAD=/path/to/libcompcov.so` together with -setting the AFL_COMPCOV_LEVEL you want to enable it. +You have to set `AFL_PRELOAD=/path/to/libcompcov.so` together with setting the +`AFL_COMPCOV_LEVEL` you want to enable it. -AFL_COMPCOV_LEVEL=1 is to instrument comparisons with only immediate -values / read-only memory. +`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 +`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 +`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, arm and aarch64 targets. +Integer comparison instructions are currently instrumented only on the x86, +x86_64, arm, and aarch64 targets. Recommended, but not as good as CMPLOG mode (see below). ## 8) CMPLOG mode -Another new feature is CMPLOG, which is based on the redqueen project. -Here all immediates in CMP instructions are learned and put into a dynamic -dictionary and applied to all locations in the input that reached that -CMP, trying to solve and pass it. -This is a very effective feature and it is available for x86, x86_64, arm -and aarch64. +Another new feature is CMPLOG, which is based on the redqueen project. Here all +immediates in CMP instructions are learned and put into a dynamic dictionary and +applied to all locations in the input that reached that CMP, trying to solve and +pass it. This is a very effective feature and it is available for x86, x86_64, +arm, and aarch64. -To enable it you must pass on the command line of afl-fuzz: - -c /path/to/your/target +To enable it, you must pass on the command line of afl-fuzz: + +``` +-c /path/to/your/target +``` ## 9) Wine mode -AFL++ QEMU can use Wine to fuzz Win32 PE binaries. Use the -W flag of afl-fuzz. +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. +Note that some binaries require user interaction with the GUI and must be +patched. -For examples look [here](https://github.com/andreafioraldi/WineAFLplusplusDEMO). +For examples, look +[here](https://github.com/andreafioraldi/WineAFLplusplusDEMO). ## 10) Notes on linking -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 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). +- 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. +- 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 +Setting `AFL_INST_LIBS=1` can be used to circumvent the .text detection logic and instrument every basic block encountered. ## 11) Benchmarking 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 +afl-clang-fast 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: +injected by afl-clang-fast, 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 @@ -186,36 +196,37 @@ match. ## 12) Other features -With `AFL_QEMU_FORCE_DFL` you force QEMU to ignore the registered signal +With `AFL_QEMU_FORCE_DFL`, you force QEMU to ignore the registered signal handlers of the target. ## 13) Gotchas, feedback, bugs If you need to fix up checksums or do other cleanups on mutated test cases, see -`afl_custom_post_process` in custom_mutators/examples/example.c for a viable solution. +`afl_custom_post_process` in custom_mutators/examples/example.c 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. +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. +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>. ## 14) Alternatives: static rewriting -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. +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. -Checkout the "Fuzzing binary-only targets" section in our main README.md and -the docs/binaryonly_fuzzing.md document for more information and hints. +For more information and hints, check out +[docs/fuzzing_binary-only_targets.md](../docs/fuzzing_binary-only_targets.md). \ No newline at end of file |