FRIDA MODE

The purpose of FRIDA mode is to provide an alternative binary only fuzzer for AFL just like that provided by QEMU mode. The intention is to provide a very similar user experience, right down to the options provided through environment variables.

Whilst AFLplusplus already has some support for running on FRIDA here this requires the code to be fuzzed to be provided as a shared library, it cannot be used to fuzz executables. Additionally, it requires the user to write a small harness around their target code of interest. FRIDA mode instead takes a different approach to avoid these limitations. In Frida mode binary programs are instrumented, similarly to QEMU mode.

Current Progress

As FRIDA mode is new, it is missing a lot of features. The design is such that it should be possible to add these features in a similar manner to QEMU mode and perhaps leverage some of its design and implementation.

Compatibility

Currently FRIDA mode supports Linux and macOS targets on both x86/x64 architecture and aarch64. Later releases may add support for aarch32 and Windows targets as well as embedded linux environments.

FRIDA has been used on various embedded targets using both uClibc and musl C runtime libraries, so porting should be possible. However, the current build system does not support cross compilation.

Getting Started

To build everything run make. To build for x86 run make 32. Note that in x86 bit mode, it is not necessary for afl-fuzz to be built for 32-bit. However, the shared library for frida_mode must be since it is injected into the target process.

Various tests can be found in subfolders within the test/ directory. To use these, first run make to build any dependencies. Then run make qemu or make frida to run on either QEMU of FRIDA mode respectively. To run frida tests in 32-bit mode, run make ARCH=x86 frida. When switching between architectures it may be necessary to run make clean first for a given build target to remove previously generated binaries for a different architecture.

Usage

FRIDA mode added some small modifications to afl-fuzz and similar tools in AFLplusplus. The intention was that it behaves identically to QEMU, but it uses the ‘O’ switch rather than ‘Q’. Whilst the options ‘f’, ‘F’, ‘s’ or ‘S’ may have made more sense for a mode powered by FRIDA Stalker, they were all taken, so instead we use ‘O’ in hommage to the author of FRIDA.

Similarly, the intention is to mimic the use of environment variables used by QEMU where possible (by replacing s/QEMU/FRIDA/g). Accordingly, the following options are currently supported:

• AFL_FRIDA_DEBUG_MAPS - See AFL_QEMU_DEBUG_MAPS
• AFL_FRIDA_EXCLUDE_RANGES - See AFL_QEMU_EXCLUDE_RANGES
• AFL_FRIDA_INST_RANGES - See AFL_QEMU_INST_RANGES
• AFL_FRIDA_PERSISTENT_ADDR - See AFL_QEMU_PERSISTENT_ADDR
• AFL_FRIDA_PERSISTENT_CNT - See AFL_QEMU_PERSISTENT_CNT
• AFL_FRIDA_PERSISTENT_HOOK - See AFL_QEMU_PERSISTENT_HOOK
• AFL_FRIDA_PERSISTENT_RET - See AFL_QEMU_PERSISTENT_RET

To enable the powerful CMPLOG mechanism, set -c 0 for afl-fuzz.

Scripting

One of the more powerful features of FRIDA mode is it's support for configuration by JavaScript, rather than using environment variables. For details of how this works see here.

Performance

Additionally, the intention is to be able to make a direct performance comparison between the two approaches. Accordingly, FRIDA mode includes various test targets based on the libpng benchmark used by fuzzbench and integrated with the StandaloneFuzzTargetMain from the llvm project. These tests include basic fork-server support, persistent mode and persistent mode with in-memory test-cases. These are built and linked without any special modifications to suit FRIDA or QEMU. The test data provided with libpng is used as the corpus.

The intention is to add support for FRIDA mode to the FuzzBench project and perform a like-for-like comparison with QEMU mode to get an accurate appreciation of its performance.

Design

FRIDA mode is supported by using LD_PRELOAD (DYLD_INSERT_LIBRARIES on macOS) to inject a shared library (afl-frida-trace.so) into the target. This shared library is built using the frida-gum devkit from the FRIDA project. One of the components of frida-gum is Stalker, this allows the dynamic instrumentation of running code for AARCH32, AARCH64, x86 and x64 architectures. Implementation details can be found here.

Dynamic instrumentation is used to augment the target application with similar coverage information to that inserted by afl-gcc or afl-clang. The shared library is also linked to the compiler-rt component of AFLplusplus to feedback this coverage information to AFL++ and also provide a fork server. It also makes use of the FRIDA prefetch support to feedback instrumented blocks from the child to the parent using a shared memory region to avoid the need to regenerate instrumented blocks on each fork.

Whilst FRIDA allows for a normal C function to be used to augment instrumented code, FRIDA mode instead makes use of optimized assembly instead on AARCH64 and x86/64 targets. By injecting these small snippets of assembly, we avoid having to push and pop the full register context. Note that since this instrumentation is used on every basic block to generate coverage, it has a large impact on performance.

CMPLOG support also adds code to the assembly, however, at present this code makes use of a basic C function and is yet to be optimized. Since not all instances run CMPLOG mode and instrumentation of the binary is less frequent (only on CMP, SUB and CALL instructions) performance is not quite so critical.

Advanced configuration options

• AFL_FRIDA_INST_DEBUG_FILE - File to write raw assembly of original blocks and their instrumented counterparts during block compilation.

***

Creating block for 0x7ffff7953313:
        0x7ffff7953313  mov qword ptr [rax], 0
        0x7ffff795331a  add rsp, 8
        0x7ffff795331e  ret

Generated block 0x7ffff75e98e2
        0x7ffff75e98e2  mov qword ptr [rax], 0
        0x7ffff75e98e9  add rsp, 8
        0x7ffff75e98ed  lea rsp, [rsp - 0x80]
        0x7ffff75e98f5  push rcx
        0x7ffff75e98f6  movabs rcx, 0x7ffff795331e
        0x7ffff75e9900  jmp 0x7ffff75e9384


***

• AFL_FRIDA_INST_JIT - Enable the instrumentation of Just-In-Time compiled code. Code is considered to be JIT if the executable segment is not backed by a file.
• AFL_FRIDA_INST_NO_OPTIMIZE - Don't use optimized inline assembly coverage instrumentation (the default where available). Required to use AFL_FRIDA_INST_TRACE.
• AFL_FRIDA_INST_NO_PREFETCH - Disable prefetching. By default the child will report instrumented blocks back to the parent so that it can also instrument them and they be inherited by the next child on fork.
• AFL_FRIDA_INST_SEED - Sets the initial seed for the hash function used to generate block (and hence edge) IDs. Setting this to a constant value may be useful for debugging purposes, e.g. investigating unstable edges.
• AFL_FRIDA_INST_TRACE - Log to stdout the address of executed blocks, implies AFL_FRIDA_INST_NO_OPTIMIZE.
• AFL_FRIDA_INST_TRACE_UNIQUE - As per AFL_FRIDA_INST_TRACE, but each edge is logged only once, requires AFL_FRIDA_INST_NO_OPTIMIZE.
• AFL_FRIDA_OUTPUT_STDOUT - Redirect the standard output of the target application to the named file (supersedes the setting of AFL_DEBUG_CHILD)
• AFL_FRIDA_OUTPUT_STDERR - Redirect the standard error of the target application to the named file (supersedes the setting of AFL_DEBUG_CHILD)
• AFL_FRIDA_PERSISTENT_DEBUG - Insert a Breakpoint into the instrumented code at AFL_FRIDA_PERSISTENT_HOOK and AFL_FRIDA_PERSISTENT_RET to allow the user to detect issues in the persistent loop using a debugger.

gdb \
		--ex 'set environment AFL_FRIDA_PERSISTENT_ADDR=XXXXXXXXXX' \
		--ex 'set environment AFL_FRIDA_PERSISTENT_RET=XXXXXXXXXX' \
		--ex 'set environment AFL_FRIDA_PERSISTENT_DEBUG=1' \
		--ex 'set environment AFL_DEBUG_CHILD=1' \
		--ex 'set environment LD_PRELOAD=afl-frida-trace.so' \
		--args <my-executable> [my arguments]

• AFL_FRIDA_STATS_FILE - Write statistics information about the code being instrumented to the given file name. The statistics are written only for the child process when new block is instrumented (when the AFL_FRIDA_STATS_INTERVAL has expired). Note that simply because a new path is found does not mean a new block needs to be compiled. It could simply be that the existing blocks instrumented have been executed in a different order.

stats
-----
Index:                          2
Pid:                            1815944
Time:                           2021-05-28 15:26:41
Blocks:                         1985
Instructions:                   9192
Avg Instructions / Block:       4

Call Immediates:                391 (4.25%)
Call Immediates Excluded:       65 (0.71%)
Call Register:                  0 (0.00%)
Call Memory:                    0 (0.00%)

Jump Immediates:                202 (2.20%)
Jump Register:                  10 (0.11%)
Jump Memory:                    12 (0.13%)

Conditional Jump Immediates:    1210 (13.16%)
Conditional Jump CX Immediate:  0 (0.00%)
Conditional Jump Register:      0 (0.00%)
Conditional Jump Memory:        0 (0.00%)

Returns:                        159 (0.00%)

Rip Relative:                   247 (0.00%)

• AFL_FRIDA_STATS_INTERVAL - The maximum frequency to output statistics information. Stats will be written whenever they are updated if the given interval has elapsed since last time they were written.
• AFL_FRIDA_STATS_TRANSITIONS - Also dump the internal stalker counters to stderr when the regular stats are written. Note that these stats are reset in the child each time a new fork occurs since they are not stored in shared memory. Unfortunately, these stats are internal to stalker, so this is the best we can do for now.

stats
-----
Index: 2
Pid:   1816794
Time:  2021-05-28 15:26:41


total_transitions: 786
        call_imms: 97
        call_regs: 0
        call_mems: 0
        post_call_invokes: 86
        excluded_call_imms: 29
        ret_slow_paths: 23

        jmp_imms: 58
        jmp_mems: 7
        jmp_regs: 26

        jmp_cond_imms: 460
        jmp_cond_mems: 0
        jmp_cond_regs: 0
        jmp_cond_jcxzs: 0

        jmp_continuations: 0

FASAN - Frida Address Sanitizer Mode

Frida mode also supports FASAN. The design of this is actually quite simple and very similar to that used when instrumenting applications compiled from source.

Address Sanitizer Basics

When Address Sanitizer is used to instrument programs built from source, the compiler first adds a dependency (DT_NEEDED entry) for the Address Sanitizer dynamic shared object (DSO). This shared object contains the main logic for Address Sanitizer, including setting and managing up the shadow memory. It also provides replacement implementations for a number of functions in standard libraries.

These replacements include things like malloc and free which allows for those allocations to be marked in the shadow memory, but also a number of other fuctions. Consider memcpy for example, this is instrumented to validate the paramters (test the source and destination buffers against the shadow memory. This is much easier than instrumenting those standard libraries since, first it would require you to re-compile them and secondly it would mean that the instrumentation would be applied at a more expensive granular level. Lastly, load-widening (typically found in highy optimized code) can also make this instrumentation more difficult.

Since the DSO is loaded before all of the standard libraries (in fact it insists on being first), the dynamic loader will use it to resolve imports from other modules which depend on it.

FASAN Implementation

FASAN takes a similar approach. It requires the user to add the Address Sanitizer DSO to the AFL_PRELOAD environment variable such that it is loaded into the target. Again, it must be first in the list. This means that it is not necessary to instrument the standard libraries to detect when an application has provided an incorrect argument to memcpy for example. This avoids issues with load-widening and should also mean a huge improvement in performance.

FASAN then adds instrumentation for any instrucutions which use memory operands and then calls into the __asan_loadN and __asan_storeN functions provided by the DSO to validate memory accesses against the shadow memory.

Collisions

FRIDA mode has also introduced some improvements to reduce collisions in the map. See here for details.

TODO

The next features to be added are Aarch32 support as well as looking at potential performance improvements. The intention is to achieve feature parity with QEMU mode in due course. Contributions are welcome, but please get in touch to ensure that efforts are deconflicted.