Risks (rev)

theres this pretty cool isa called riscv it sounds awesome.

14KB
Risks
Open

We are given a binary which uses the riscv64 architecture. As such, we can run the binary using qemu-riscv64 -L /usr/riscv64-linux-gnu ./Risks

If you don't have qemu, you can install it using the following command: 

sudo apt update
sudo apt install -y qemu-user libc6-arm64-cross libc6-riscv64-cross libc6-ppc64-cross

The binary accepts text, and checks if it's the flag. We get "nuh uh" for an invalid flag. The decompiled main code is as follows:

undefined8 main(void)
{
  char cVar1;
  long lVar2;
  undefined8 len;
  long buf;
  long local_20;
  long local_18;
  
  buf = 0;
  len = 0;
  print("whats the flag: ");
  /* buf becomes the address of the buffer, local_18 is no. of bytes read incl terminating byte */
  local_18 = getline(&buf,&len,_stdin);
  *(undefined *)(local_18 + -1 + buf) = 0;
  local_20 = buf;
  FUN_00101ad4(buf);
  FUN_00101346(local_20);
  FUN_00100c8c(local_20);
  FUN_00100786(local_20);
  lVar2 = FUN_00100690(buf);
  if ((lVar2 == 0x20) && (cVar1 = check(local_20), cVar1 == '\x01')) {
    puts("good job!");
    return 0;
  }
  puts("nuh uh");
  return 0;
}

The program accepts a line from stdin, and sets the last byte to be a null byte. FUN_00101ad4, FUN_00101346, FUN_00100c8c and FUN_00100786 are all functions which perform a combination of addition and XOR operations on the input buffer, which is treated as a long arr[4]. For example, FUN_00101ad4 contains the following code snippet:

FUN_00100690 then checks that the input is 32 bytes long. The check function contains the following code:

We want to fulfil all conditions in order to enter the if block, and have 1 returned. This will cause "good job" to be printed by main.

We can use the following Z3 script to perform the addition and XOR operations, and impose the 4 restrictions upon the arithmetic results.

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