Debugging using radare2

Hi Y’ALL, before proceeding to solve other crackmes it is important to know how to debug in r2. This post is continuation of Solving IOLI Crackmes Using Radare2 [Part-I].

Let’s take the previous solved crackme0x02 and solve it by debugging it.

Opening binary

We use r2 -A crackme0x02 to open binary in r2 with analysing the binary on startup and can use other informational commands such as i,a,etc. To open binary in debugging mode, we have to use -d option.

r2 -Ad crackme0x02

The above one will open binary with analysing and starting in debugging mode.

root@n00b:~/Desktop/RE/radare2-workshop-2015/IOLI-crackme/bin-linux# r2 -Ad crackme0x03Process with PID 14660 started... = attach 14660 14660 bin.baddr 0x08048000 Using 0x8048000 asm.bits 32 glibc.fc_offset = 0x00148 [Invalid address from 0x080483ffith sym. and entry0 (aa) [x] Analyze all flags starting with sym. and entry0 (aa) [x] Analyze function calls (aac)[x] Analyze len bytes of instructions for references (aar) [x] Check for objc references Warning: aao experimental on 32bit binaries [x] Check for vtables [TOFIX: aaft can't run in debugger mode.ions (aaft) [x] Type matching analysis for all functions (aaft) [x] Propagate noreturn information [x] Use -AA or aaaa to perform additional experimental analysis. -- The Hard ROP Cafe [0xf7fcb0b0]>

Reason for starting at higher address

0xf7fcb0b0 is the starting address the r2 has placed its debug. But lets see what this address is , is this a function or not

[0xf7fcb0b0]> afl0x08048360 1 33 entry0 0x08048320 1 6 sym.imp.__libc_start_main 0x080483b0 6 47 sym.__do_global_dtors_aux 0x080483e0 4 50 sym.frame_dummy 0x080485a0 4 35 sym.__do_global_ctors_aux 0x08048590 1 5 sym.__libc_csu_fini 0x080485c4 1 26 sym._fini 0x08048520 4 99 sym.__libc_csu_init 0x0804846e 4 42 sym.test 0x08048595 1 4 sym.__i686.get_pc_thunk.bx 0x08048414 4 90 sym.shift 0x08048498 1 128 main 0x08048350 1 6 sym.imp.printf 0x08048330 1 6 sym.imp.scanf 0x080482f8 1 23 sym._init 0x08048384 3 33 fcn.08048384 0x08048340 1 6 sym.imp.strlen [0xf7fcb0b0]>

oops , we see that all functions starts with address 0x08 and our starting seek is 0xf7 . ooooh, okay let’s dig deeper regd this. So when we see the memory layout of c binaries , we can understand.

As we see in the image, the binary text will be in low address and thats why the addresses of the functions starts with 0x08 whereis the stack address have higher address that is it starts with 0xf and in our case 0xf7fcb0b0 should be a stack address. But we all know that the stack addresses is read/write but how it can execute in stack . If we analyse the memory maps we can well understand

[0xf7fcb0b0]> dm0x08048000 - 0x08049000 - usr 4K s r-x /root/Desktop/RE/radare2-workshop-2015/IOLI-crackme/bin-linux/crackme0x03 /root/Desktop/RE/radare2-workshop-2015/IOLI-crackme/bin-linux/crackme0x03 ; map.root_Desktop_RE_radare2_workshop_2015_IOLI_crackme_bin_linux_crackme0x03.r_x 0x08049000 - 0x0804b000 - usr 8K s rw- /root/Desktop/RE/radare2-workshop-2015/IOLI-crackme/bin-linux/crackme0x03 /root/Desktop/RE/radare2-workshop-2015/IOLI-crackme/bin-linux/crackme0x03 ; map.root_Desktop_RE_radare2_workshop_2015_IOLI_crackme_bin_linux_crackme0x03.rw 0xf7fc6000 - 0xf7fc9000 - usr 12K s r-- [vvar] [vvar] ; map.vvar_.r 0xf7fc9000 - 0xf7fca000 - usr 4K s r-x [vdso] [vdso] ; map.vdso_.r_x 0xf7fca000 - 0xf7fcb000 - usr 4K s r-- /usr/lib32/ld-2.29.so /usr/lib32/ld-2.29.so 0xf7fcb000 - 0xf7fe7000 * usr 112K s r-x /usr/lib32/ld-2.29.so /usr/lib32/ld-2.29.so ; map.usr_lib32_ld_2.29.so.r_x 0xf7fe7000 - 0xf7ff1000 - usr 40K s r-- /usr/lib32/ld-2.29.so /usr/lib32/ld-2.29.so ; map.usr_lib32_ld_2.29.so.r 0xf7ff2000 - 0xf7ff4000 - usr 8K s rw- /usr/lib32/ld-2.29.so /usr/lib32/ld-2.29.so ; map.usr_lib32_ld_2.29.so.rw0xffdc6000 - 0xffde8000 - usr 136K s rw- [stack] [stack] ; map.stack_.rw[0xf7fcb0b0]>

The dm command will gives us the memory map regions allocated for the particular binary(crackme0x02) and it varies and it is allocated by the operating system for the processes. The * denotes the current execution region. The binary loaded in the current execution region is ld library which is loaded dynamically. Also permissions for the memory regions is also showed in 5th column and in our ld memory region it is r-x , so it has read and execute permission. Since there may be several entry functions for binaries , r2 will keep its first debugging in ld memory region and thats the reason we will start at higher address . Learn more about LD preload to understand more on why ld.so is loaded.

debugging commands

It is important to know basic debugging commands in r2

d? - debugging commands help

All debugging commands starts with d.

db - set breakpointdc - continueds - step intodso - step overdcu - set breakpoint and continue which will hit the breakpoint at the function.

analysing memory commands starts with p

pfz @ - print value at addr as string(ends with 00)pxw @ - print word from addrpx[b|w|q] - print byte|word|quadwordp?,p??,p??? - help

Visual Mode

It will be difficult to debug without seeing cursor on the line executing by r2 and we can see that in visual mode.

v - open visual modep - shift to different visual modes s - step into S - step overq - quit visual mode: - run r2 commands(you dont have to quit in order to run r2 commands)

It will look like this

[0xf7fcb0b0 [xAdvc]0 0% 255 /root/Desktop/RE/radare2-workshop-2015/IOLI-crackme/bin-linux/crackme0x03]> pd $r @ loc._end+-268955504 # 0xf7fcb0b0 ;-- eip: 0xf7fcb0b0 89e0 mov eax, esp 0xf7fcb0b2 83ec0c sub esp, 0xc 0xf7fcb0b5 50 push eax 0xf7fcb0b6 e8e50c0000 call 0xf7fcbda0 ;[1] 0xf7fcb0bb 83c410 add esp, 0x10 0xf7fcb0be 89c7 mov edi, eax 0xf7fcb0c0 e8dbffffff call 0xf7fcb0a0 ;[2] 0xf7fcb0c5 81c33b7f0200 add ebx, 0x27f3b 0xf7fcb0cb 8b8334f8ffff mov eax, dword [ebx - 0x7cc] 0xf7fcb0d1 5a pop edx 0xf7fcb0d2 8d2484 lea esp, [esp + eax*4] 0xf7fcb0d5 29c2 sub edx, eax 0xf7fcb0d7 52 push edx 0xf7fcb0d8 8b8340000000 mov eax, dword [ebx + 0x40] 0xf7fcb0de 8d749408 lea esi, [esp + edx*4 + 8] 0xf7fcb0e2 8d4c2404 lea ecx, [esp + 4] 0xf7fcb0e6 89e5 mov ebp, esp 0xf7fcb0e8 83e4f0 and esp, 0xfffffff0 0xf7fcb0eb 83ec0c sub esp, 0xc 0xf7fcb0ee 55 push ebp 0xf7fcb0ef 56 push esi 0xf7fcb0f0 51 push ecx 0xf7fcb0f1 52 push edx 0xf7fcb0f2 50 push eax 0xf7fcb0f3 31ed xor ebp, ebp 0xf7fcb0f5 e876f00000 call 0xf7fda170 ;[3]

Panel Mode

My favorite is panel mode , i use panel mode for debugging

V - shift to panel modes - step intoS - step overh,j,k,l - movement in panel mode? - help w - window mode (shift to next window, other window operations)e - create a new command in the panelD - open disassembler - open graph view

solving crackme0x02

Open binary using r2 -Ad crackme0x02

dcu main - set breakpoint at main and continue Change to panel mode by using v

root@n00b:~/Desktop/RE/radare2-workshop-2015/IOLI-crackme/bin-linux# r2 -Ad crackme0x02Process with PID 15189 started...= attach 15189 15189bin.baddr 0x08048000Using 0x8048000asm.bits 32glibc.fc_offset = 0x00148[Invalid address from 0x080483cfith sym. and entry0 (aa)[x] Analyze all flags starting with sym. and entry0 (aa)[x] Analyze function calls (aac)[x] Analyze len bytes of instructions for references (aar)[x] Check for objc referencesWarning: aao experimental on 32bit binaries[x] Check for vtables[TOFIX: aaft can't run in debugger mode.ions (aaft)[x] Type matching analysis for all functions (aaft)[x] Propagate noreturn information[x] Use -AA or aaaa to perform additional experimental analysis. -- Use radare2! Lemons included![0xf7ef80b0]> dcu mainContinue until 0x080483e4 using 1 bpsizehit breakpoint at: 80483e4[0x080483e4]>

Switch to panel mode by using v.

[0x08048498]>v

File Settings Edit View Tools Search Emulate Debug Analyze Help Tab [1] [0x080483e4]┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐──────────────────────────────────────────────────────────────────────────────┐│[X] Disassembly (pd) [Cache] Off │[X] Stack (pxw 256@r:SP) [Cache] Off ││ ; DATA XREF from entry0 @ 0x8048347 │ 0xffab282c 0xf7d097e1 0x00000001 0xffab28c4 0xffab28cc .........(...(.. ││ ;-- eip: │ 0xffab283c 0xffab2854 0x00000001 0x00000000 0xf7ec2000 T(........... .. ││ ┌ 144: int main (int argc, char **argv, char **envp); │ 0xffab284c 0x00000000 0xf7f12000 0x00000000 0xf7ec2000 ..... ....... .. ││ │ ; var int32_t var_ch @ ebp-0xc │ 0xffab285c 0xf7ec2000 0x00000000 0xd678a6e9 0x210780f9 . ........x....! ││ │ ; var int32_t var_8h @ ebp-0x8 │ 0xffab286c 0x00000000 0x00000000 0x00000000 0x00000001 ................ ││ │ ; var int32_t var_4h @ ebp-0x4 │ 0xffab287c 0x08048330 0x00000000 0xf7efe450 0xf7ef92d0 0.......P....... ││ │ ; var int32_t var_sp_4h @ esp+0x4 │ 0xffab288c 0xf7f12000 0x00000001 0x08048330 0x00000000 . ......0....... ││ │ 0x080483e4 * 55 push ebp │ 0xffab289c 0x08048351 0x080483e4 0x00000001 0xffab28c4 Q............(.. ││ │ 0x080483e5 89e5 mov ebp, esp │ 0xffab28ac 0x08048480 0x080484f0 0xf7ef92d0 0xffab28bc .............(.. ││ │ 0x080483e7 83ec18 sub esp, 0x18 │ 0xffab28bc 0x0000001c 0x00000001 0xffab3f48 0x00000000 ........H?...... ││ │ 0x080483ea 83e4f0 and esp, 0xfffffff0 │ 0xffab28cc 0xffab3f56 0xffab3f66 0xffab3fb2 0xffab3fbd V?..f?...?...?.. ││ │ 0x080483ed b800000000 mov eax, 0 │ 0xffab28dc 0xffab3fd0 0xffab4006 0xffab401f 0xffab4053 .?...@[email protected]@.. ││ │ 0x080483f2 83c00f add eax, 0xf ; 15 │ 0xffab28ec 0xffab408d 0xffab40c7 0xffab40dd 0xffab40f0 .@...@...@...@.. ││ │ 0x080483f5 83c00f add eax, 0xf ; 15 │ 0xffab28fc 0xffab40fa 0xffab4132 0xffab4145 0xffab417f [email protected].. ││ │ 0x080483f8 c1e804 shr eax, 4 │ 0xffab290c 0xffab41ad 0xffab41e5 0xffab41f9 0xffab420e .A...A...A...B.. ││ │ 0x080483fb c1e004 shl eax, 4 │ 0xffab291c 0xffab4221 0xffab423d 0xffab424c 0xffab4280 !B..=B..LB...B.. ││ │ 0x080483fe 29c4 sub esp, eax │ ││ │ 0x08048400 c70424488504. mov dword [esp], str.IOLI_Crackme_Level_0x02 ; [0x8048548:4]=0x494c4f49 ; "IO│ ││ │ 0x08048407 e810ffffff call sym.imp.printf ;[1] ; int printf(const char *format) │ ││ │ 0x0804840c c70424618504. mov dword [esp], str.Password: ; [0x8048561:4]=0x73736150 ; "Password: " │ ││ │ 0x08048413 e804ffffff call sym.imp.printf ;[1] ; int printf(const char *format) │ ││ │ 0x08048418 8d45fc lea eax, [var_4h] │ ││ │ 0x0804841b 89442404 mov dword [var_sp_4h], eax │ ││ │ 0x0804841f c704246c8504. mov dword [esp], 0x804856c ; [0x804856c:4]=0x50006425 │ ││ │ 0x08048426 e8e1feffff call sym.imp.scanf ;[2] ; int scanf(const char *format) │ ││ │ 0x0804842b c745f85a0000. mov dword [var_8h], 0x5a ; 'Z' ; 90 │ ││ │ 0x08048432 c745f4ec0100. mov dword [var_ch], 0x1ec ; 492 │──────────────────────────────────────────────────────────────────────────────┐│ │ 0x08048439 8b55f4 mov edx, dword [var_ch] │[X] Registers (dr) [Cache] Off ││ │ 0x0804843c 8d45f8 lea eax, [var_8h] │ eax = 0xf7ec4548 ││ │ 0x0804843f 0110 add dword [eax], edx │ ebx = 0x00000000 ││ │ 0x08048441 8b45f8 mov eax, dword [var_8h] │ ecx = 0x210780f9 ││ │ 0x08048444 0faf45f8 imul eax, dword [var_8h] │ edx = 0xffab2854 ││ │ 0x08048448 8945f4 mov dword [var_ch], eax │ esi = 0xf7ec2000 ││ │ 0x0804844b 8b45fc mov eax, dword [var_4h] │ edi = 0xf7ec2000 ││ │ 0x0804844e 3b45f4 cmp eax, dword [var_ch] │ esp = 0xffab282c ││ │ ┌─< 0x08048451 750e jne 0x8048461 │ ebp = 0x00000000 ││ │ │ 0x08048453 c704246f8504. mov dword [esp], str.Password_OK_: ; [0x804856f:4]=0x73736150 ; "Password OK │ eip = 0x080483e4 ││ │ │ 0x0804845a e8bdfeffff call sym.imp.printf ;[1] ; int printf(const char *format) │ eflags = 0x00000246 ││ │ ┌──< 0x0804845f eb0c jmp 0x804846d │ oeax = 0xffffffff ││ │ │└─> 0x08048461 c704247f8504. mov dword [esp], str.Invalid_Password ; [0x804857f:4]=0x61766e49 ; "Invalid P│ ││ │ │ 0x08048468 e8affeffff call sym.imp.printf ;[1] ; int printf(const char *format) │ ││ │ │ ; CODE XREF from main @ 0x804845f │ ││ │ └──> 0x0804846d b800000000 mov eax, 0 │ ││ │ 0x08048472 c9 leave │ ││ └ 0x08048473 c3 ret │ ││ 0x08048474 90 nop │ ││ 0x08048475 90 nop │ ││ 0x08048476 90 nop │ ││ 0x08048477 90 nop │ ││ 0x08048478 90 nop │ ││ 0x08048479 90 nop │ ││ 0x0804847a 90 nop │ ││ 0x0804847b 90 nop │ │

This is how it looks like in panel mode and the line to be executed will be highlighted. As you can see there are three windows, the disassembly window and other two , one is stack value at the top right and other one is registers (bottom-rigth). Press s to step to next instruction and the registers, stacks will change accordingly.

Step till 0x0804844b and see the value of eax at the register window or use pxd @ eax to see the value from eax registers

This is all for debugging , so next we will continue on Part-II