Reversing & Malware Analysis Training Part 4 - Assembly Programming Basics

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The Content, Demonstration, Source Code and Programs presented here
is "AS IS" without any warranty or conditions of any kind. Also the
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currently working.

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responsible for any damage or loss caused due to use or misuse of the
information presented here.

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Acknowledgement
 Special thanks to null & Garage4Hackers community for their extended
support and cooperation.
 Thanks to all the Trainers who have devoted their precious time and
countless hours to make it happen.


Reversing & Malware Analysis Training

This presentation is part of our Reverse Engineering & Malware
Analysis Training program. Currently it is delivered only during our local
meet for FREE of cost.

For complete details of this course, visit our Security Training page.


Who am I #1
Amit Malik (sometimes DouBle_Zer0,DZZ)
 Member SecurityXploded & Garage4Hackers
 Security Researcher
 RE, Exploit Analysis/Development, Malware Analysis
 Email: m.amit30@gmail.com


Who am I #2
Swapnil Pathak
 Member SecurityXploded
 Security Researcher
 RE, Malware Analysis, Network Security
 Email: swapnilpathak101@gmail.com


Presentation Outline
 Intro to x86-32

 Assembly Language

 Instructions

 Stack Operations

 Calling conventions

 Demo


x86-32
 32 bit instruction set architectures based on Intel 8086 CPU
 Address a linear address space up to 4GB
 8, 32 bit General Purpose Registers (GPR)
 6,16 bit Segment Registers
 EFLAGS and EIP register
 Control Registers (CR0-CR4) (16 bits)
 Memory Management Registers Descriptor Table Registers
(GDTR, IDTR, LDTR)
 Debug Registers ( DR0-DR7)


Registers Usage - RE
 Register

 Storage Locations.

 Much faster access compare to memory locations.

 EAX: Accumulator , mostly stores return values from functions (APIs)

 EBX: Base index (for use with arrays)

 ECX: Counter

 EDX: Data/general

 ESI: Source index for string operations.


Registers Usage – RE Cont.
 EDI: Destination index for string operations.

 ESP: Stack pointer for top address of the stack.

 EBP: Stack base pointer for holding the address of the current stack frame.

 EIP: Instruction pointer. Holds the program counter, the next instruction
address.

 Segment registers:

 Used to address particular segments of memory ( code, data, stack )

!) CS: Code !!) SS: Stack

!!!) ES: Extra !V) DS: Data V) FS, GS


Registers – 32 bit (X86)


(R/E)Flags Register
 Bit field of states
 Status Flags
 Carrry (CF) : set when an arithmetic carry/borrow has been generated
out of the MSB.
 Zero (ZF) : set when an arithmetic operation result is zero and reset
otherwise.
 Sign (SF) : set when an arithmetic operation set the MSB i.e. the result
value was negative.
 Trap (TF ) : when set permits operation of processor in single-step.
Mostly used by debuggers.
 Interrupt (IF) : determines whether the CPU should handle maskable
hardware interrupts.
 Direction (DF) : determines the direction (left-to-right or right-to-left)
of string processing.
 Overflow (OF) : indicates arithmetic overflow.


Assembly Language
 Low level programming language

 Symbolic representation of machine codes, constants.

 Assembly language program consist of sequence of process instructions
and meta statements

 Assembler translates them to executable instructions that are loaded into
memory and executed.

 Basic Structure

[label] : opcode operand1, operand2

opcode – mnemonic that symbolize instructions

 Example.

 MOV AL, 61h => 10110000 01100001

Instructions
ADD dst, src
- Adds the values of src and dst and stores the result into dst.
- For example ADD EAX, 1

SUB dst, src
- Subtracts src value from dst and stores the result in dst.
- For example SUB EAX, 1

CMP dst, src
- Subtracts src value from dst but does store the result in dst
- Mostly used to set/reset decision making bits in EFLAGS register
such as ZF
- For example CMP EAX, EBX


Instructions cont.
MOV dst, src
- Moves data from src (left operand) to destination (right operand)
- For example mov EDI, ESI
Note :
- Both operands cannot be memory locations.
- Both the operands must be of the same size

LEA dst, src
- Stands for Load Effective Address.
- Computes the effective address of src operand and stores it in dst operand.
- For example LEA ECX,[EBX + 5]
Note:
- Generally brackets denote value at memory locations.
- In case of LEA it does simple arithmetic and stores it in dst

Instructions cont.
XOR dst, src
- Performs a bitwise exclusive OR operation on the dst and src and
stores the result in dst.
- Each bit of the result is 1 if the corresponding bits of the operands
are different, 0 if the corresponding bit are same

Note :
- When used with same register clears the contents of the register
- Optimized way to clear the register. Better than MOV EAX, 0


Instructions cont.
REP
- Used with string operations
- Repeats a string instruction until ECX (counter register) value is equal to
zero.
- For example REP MOVS byte ptr DS:[EDI], DS:[ESI]

LOOP
- Similar to loops in high level languages
- Used to execute sequence of instructions multiple times.
- For example
MOV ECX, 10
Test : INC EBX
INC EAX
LOOP Test

Instructions cont.
TEST dst, src
- Performs bitwise logical and between dst and src
- Updates the Zero flag bit of the EFLAGS register
- Mostly used to check if the return value of the function is not zero
- For example TEST EAX, EAX

INT 3h
- Breakpoint instruction
- Used by debuggers to stop execution of the program at particular
instruction


Instructions cont.
CALL address
- Performs two functions
- Push address of the next instruction on stack (return address)
- Jump to the address specified by the instruction
- For example CALL dword ptr [EAX+4]

RET
- Transfers the control to the address previously pushed on the stack
by CALL instruction
- Mostly denotes the end of the function


Instructions cont.
Jump instructions
- Categorized as conditional and unconditional
- Unconditional jump instructions
- JMP (Far Jump) – E9 – (Cross segments)
- JMP ( Short Jump ) – EB – (-127 to 128 bytes)
- JMP ( Near Jump ) – E9 – (in a segment)
- For example JMP EAX

- Conditional jump instructions
- Jumps according to bit flags set in the EFLAGS register
- JC, JNC, JZ, JNZ, JS, JNS, JO, JNO
- Unsigned comparisons JA, JAE, JB, JBE
- Signed comparisons JG, JGE, JL, JLE
- Usually followed by CMP instruction


Instructions cont.
PUSH operand
- Pushes operand on the stack
- Decrements the stack pointer register by operand size
- For example PUSH EAX

POP operand
- Stores the value pointed by the stack pointer in operand
- Increments the stack pointer register by operand size
- For example POP EAX

Note: POP/PUSH EIP is an invalid instruction

PUSHF, POPF


Calling Conventions
 Describes how the arguments are passed and values returned by functions.
 Steps performed when a function is called
 Arguments are passed to the called function
 Program execution is transferred to the address of the called function
 Called function starts with lines of code that prepare stack and registers for use within
the function. Also known as function prologue.
○ For e.g.
push ebp
mov ebp, esp
or with enter instruction
 Called function ends with lines of code that restore stack and registers set initially. Also
known as function epilogue.
○ For e.g.
mov esp, ebp
pop ebp
ret
or with leave instruction
 Passed arguments are removed from the stack, known as stack cleanup. Can be
performed by both calling function or called function depending on the calling
convention used.


Calling conventions cont.
 __cdecl (C calling convention)
 Arguments are passed from right to left and placed on the stack
 Stack cleanup is performed by the caller
 Return values are stored in EAX register
 Standard calling convention used by C compilers

 __stdcall (Standard calling convention)
 Arguments are passed from right to left and placed on the stack
 Stack cleanup is performed by the called function
 Return values are stored in EAX register
 Standard calling convention for Microsoft Win32 API

 __fastcall (Fast calling convention)
 Arguments passed are stored in registers for faster access

 Thiscall
 Arguments are passed from right to left and placed on the stack. this pointer placed in
ECX
- Standard calling convention for calling member functions of C++ classes

Stack operations
 Stack is a LIFO (Last In First Out) type data structure

 Stacks grows downward in memory, from higher memory address
to lower memory address

 PUSH decrement the stack pointer i.e ESP

 POP Increment the stack pointer i.e ESP

 Each function has its own stack frame

 Function prologue setup the stack frame for each function

 Local variable of a function are stored into its stack frame


Stack #1


Stack #2


Stack #3
 Each function creates its own stack.
 Caller function stack: known as parent stack.
 Called function stack: known as child stack.
For e.g.
main(){ ASM Pseudo:
sum(); _main:
} 123: push ebp
124: mov ebp,esp
125: sub esp,val
126: call _sum
127: mov esp,ebp
128: pop ebp
129: ret


Stack #4


Stack #5


Stack #6


DEMO (Source Code)
 #include <stdio.h>
 /*
 Author: Amit Malik
 http://www.securityxploded.com - Compile in Dev C++
 */
 int mysum(int,int);
 int main()
 {
 int a,b,s;
 a = 5;
 b = 6;
 s = mysum(a,b); // call mysum function
 printf("sum is: %d",s);
 getchar();
 }
 int mysum(int l, int m) // mysum function
 {
 int c;
 c = l + m;
 return c;
 }


x86-64 Intro.
 64 bit instruction set architectures based on Intel 8086 CPU
 Address a linear address space up to 16TB
 16, 64 bit General Purpose Registers (GPR)
 6, 16 bit Segment Registers
 RFLAGS and RIP register
 Control Registers (CR0-CR4) and CR8 (16 bits)
 Memory Management Registers Descriptor Table Registers
(GDTR, IDTR, LDTR) size expanded to 10 bytes
 Debug Registers ( DR0-DR7)


Reference
 Complete Reference Guide for Reversing & Malware
Analysis Training


Thank You !


Reversing & Malware Analysis Training Part 4 - Assembly Programming Basics

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