A Stack Frame

1. CAP6135: Malware and Software Vulnerability Analysis Buffer Overflow : Example of Using GDB to Check Stack MemoryCliff ZouSpring 2011

2. A Stack Frame Parameters Return Address Calling Stack Pointer Local Variables BP SP+offset SP Addresses 00000000 SP: stack pointer BP: base/frame pointer Calling stack pointer: previous function’s SP

3. Using GDB to Check Stack • GDB tutorial: • http://gemma.apple.com/mac/library/documentation/DeveloperTools/gdb/gdb/gdb_7.html • http://www.yolinux.com/TUTORIALS/GDB-Commands.html#GDB_COMMAND_LINE_ARGS • When compile the c code, use “gcc –g …..” so that Gdb can match source code line number with code • Some knowledge: http://en.wikipedia.org/wiki/X86_assembly_language • Register eip: instruction pointer, the current position of next executable instruction • Register ebp: stack pointer, the top of the current stack, used for addressing local variable

4. Related Gdb Commands: • List: list the source code and each execution’s corresponding line number • Break linenumber: set breakpoint at the linenumber • Run argv: run the execution code with the parameter argv • Next: execute the next line of code • Backtrace: show trace of all function calls in stack • Info frame: List address, language, address of arguments/local variables and which registers were saved in frame. • This will show where the return address is saved • Return address is in Register EIP • Calling stack pointer is in Register EBP • x &variable: show the address and value of a local variable (in hex format) • x address: print binary representation of 4 bytes of memory pointed to by address.

5. Example of Using GDB #include <stdio.h> void foo(char * input){ int a1=11; int a2=22; char buf[7]; strcpy(buf, input); } void main(int argc, char **argv){ foo(argv[1]); } Question: What does the stack look like before strcpy()?

6. czou@eustis:~/buffer-code$ setarch i686 –R gdb ./gdb-example (gdb) list 1 #include <stdio.h> 2 void foo(char * input){ 3 int a1=11; 4 int a2=22; 5 char buf[7]; 6 strcpy(buf, input); 7 } 8 void main(int argc, char **argv){ 9 foo(argv[1]); 10 } (gdb) break 6 Breakpoint 1 at 0x80483e9: file gdb-example.c, line 6. (gdb) run Starting program: /home/czou/buffer-code/gdb-example Breakpoint 1, foo (input=0x0) at gdb-example.c:6 6 strcpy(buf, input); (gdb)

7. (gdb) info frame Stack level 0, frame at 0xbffff7d0: eip = 0x80483e9 in foo (gdb-example.c:6); saved eip 0x804842f called by frame at 0xbffff7e0 source language c. Arglist at 0xbffff7c8, args: input=0x0 Locals at 0xbffff7c8, Previous frame's sp is 0xbffff7d0 Saved registers: ebp at 0xbffff7c8, eip at 0xbffff7cc (gdb) x &a1 0xbffff7b8: 0x0000000b (gdb) x &a2 0xbffff7b4: 0x00000016 (gdb) x buf 0xbffff7bc: 0x080482ec (gdb) x 0xbffff7b4 0xbffff7b4: 0x00000016

8. Several Tips on Project 1 • Be sure to use the Makefile to generate executable of both exploit program and target program • Be sure to use “setarch i686 -R” in front of every execution, including both Gdb and ./exploit • You can use “break foo” to set breakpoint upon entering foo() function. • Fill the shell executable code (in the string array shellcode[]) byte-by-byte into the buffer for your modified return address to execute.

9. Two Techniques for Generating Stack Overflow Codes

10. NOPs • Most CPUs have a No-Operation instruction – it does nothing but advance the instruction pointer. • Usually we can put a bunch of these ahead of our program (in the string). • As long as the new return-address points to a NOP we are OK.

11. Using NOPs Real program (exec /bin/ls or whatever) new return address Can point anywhere in here nop instructions

12. Estimating the stack size • We can also guess at the location of the return address relative to the overflowed buffer. • Put in a bunch of new return addresses!

13. Estimating the Location new return address new return address new return address new return address new return address new return address Real program nop instructions