Reversing malware analysis training part4 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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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.
www.SecurityXploded.com

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.

Who am I #1
Amit Malik (sometimes DouBle_Zer0,DZZ)
 Member Cysinfo
 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

 Intro to x86-32
 Assembly Language
 Instructions
 Stack Operations
 Calling conventions
 Demo

 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)

 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.

 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

 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.

 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

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

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

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

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

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

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

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

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

 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.

 __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 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

 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

 #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;
 }

 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)

Reversing malware analysis training part4 assembly programming basics

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