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How to use the persistent mode in AFL++'s QEMU mode

1) Introduction

Persistent mode lets you fuzz your target persistently between two addresses - without forking for every fuzzing attempt. This increases the speed by a factor between x2 and x5, hence it is very, very valuable.

The persistent mode is currently only available for x86/x86_64, arm, and aarch64 targets.

2) How use the persistent mode

2.1) The START address

The start of the persistent loop has to be set with environment variable AFL_QEMU_PERSISTENT_ADDR.

This address can be the address of whatever instruction. Setting this address to the start of a function makes the usage simple. If the address is, however, within a function, either RET, OFFSET, or EXITS (see below in 2.2, 2.3, 2.6) have to be set. This address (as well as the RET address, see below) has to be defined in hexadecimal with the 0x prefix or as a decimal value.

If both RET and EXITS are not set, QEMU will assume that START points to a function and will patch the return address (on stack or in the link register) to return to START (like WinAFL).

Note: If the target is compiled with position independent code (PIE/PIC) qemu loads these to a specific base address. For amd64 bit you have to add 0x4000000000 (9 zeroes) and for 32 bit 0x40000000 (7 zeroes) to the address. For aarch64 it is usually 0x5500000000. On strange setups the base address set by QEMU for PIE executable may change. You can check it printing the process map using AFL_QEMU_DEBUG_MAPS=1 afl-qemu-trace TARGET-BINARY.

If this address is not valid, afl-fuzz will error during startup with the message that the forkserver was not found.

2.2) The RET address

The RET address is the last instruction of the persistent loop. The emulator will emit a jump to START when translating the instruction at RET. It is optional and only needed if the return should not be at the end of the function to which the START address points into, but earlier.

It is defined by setting AFL_QEMU_PERSISTENT_RET, and too 0x4000000000 has to be set if the target is position independent.

2.3) The OFFSET

This option is valid only for x86/x86_64 only, arm/aarch64 do not save the return address on stack.

If the START address is not the beginning of a function, and no RET has been set (so the end of the loop will be at the end of the function but START will not be at the beginning of it), we need an offset from the ESP pointer to locate the return address to patch.

The value by which the ESP pointer has to be corrected has to be set in the variable AFL_QEMU_PERSISTENT_RETADDR_OFFSET.

Now to get this value right here is some help:

  1. Use gdb on the target.
  2. Set a breakpoint to "main" (this is required for PIE/PIC binaries so the addresses are set up).
  3. "run" the target with a valid commandline.
  4. Set a breakpoint to the function in which START is contained.
  5. Set a breakpoint to your START address.
  6. "continue" to the function start breakpoint.
  7. Print the ESP value with print $esp and take note of it.
  8. "continue" the target until the second breakpoint.
  9. Again print the ESP value.
  10. Calculate the difference between the two values - and this is the offset.

2.4) Resetting the register state

It is very, very likely you need to restore the general purpose registers state when starting a new loop. Because of this 99% of the time you should set

AFL_QEMU_PERSISTENT_GPR=1

An example is when you want to use main() as persistent START:

int main(int argc, char **argv) {

  if (argc < 2) return 1;

  // do stuff

}

If you don't save and restore the registers in x86_64, the parameter argc will be lost at the second execution of the loop.

2.5) Resetting the memory state

This option restores the memory state using the AFL++ Snapshot LKM if loaded. Otherwise, all the writeable pages are restored.

To enable this option, set AFL_QEMU_PERSISTENT_MEM=1.

2.6) Reset on exit()

The user can force QEMU to set the program counter to START instead of executing the exit_group syscall and exit the program.

The environment variable is AFL_QEMU_PERSISTENT_EXITS.

2.7) Snapshot

AFL_QEMU_SNAPSHOT=address is just a "syntactical sugar" environment variable that is equivalent to the following set of variables:

AFL_QEMU_PERSISTENT_ADDR=address
AFL_QEMU_PERSISTENT_GPR=1
AFL_QEMU_PERSISTENT_MEM=1
AFL_QEMU_PERSISTENT_EXITS=1

3) Optional parameters

3.1) Loop counter value

The more stable your loop in the target, the longer you can run it, the more unstable it is the lower the loop count should be. A low value would be 100, the maximum value should be 10000. The default is 1000. This value can be set with AFL_QEMU_PERSISTENT_CNT.

This is the same concept as in the llvm_mode persistent mode with __AFL_LOOP().

3.2) A hook for in-memory fuzzing

You can increase the speed of the persistent mode even more by bypassing all the reading of the fuzzing input via a file by reading directly into the memory address space of the target process.

All this needs is that the START address has a register that can reach the memory buffer or that the memory buffer is at a known location. You probably need the value of the size of the buffer (maybe it is in a register when START is hit).

The persistent hook will execute a function on every persistent iteration (at the start START) defined in a shared object specified with AFL_QEMU_PERSISTENT_HOOK=/path/to/hook.so.

The signature is:

void afl_persistent_hook(struct ARCH_regs *regs,
                         uint64_t guest_base,
                         uint8_t *input_buf,
                         uint32_t input_buf_len);

Where ARCH is one of x86, x86_64, arm or arm64. You have to include path/to/qemuafl/qemuafl/api.h.

In this hook, you can inspect and change the saved GPR state at START.

You can also initialize your data structures when QEMU loads the shared object with:

int afl_persistent_hook_init(void);

If this routine returns true, the shared mem fuzzing feature of AFL++ is used and so the input_buf variables of the hook becomes meaningful. Otherwise, you have to read the input from a file like stdin.

An example that you can use with little modification for your target can be found here: utils/qemu_persistent_hook