Analysis of linux shellcode about miasm

[commial]

Can you supply the whole shellcode, or at least a minimalist example with the associated Python code ? That way, we will be able to reproduce and fix this issue.

In addition, please move your second question in a separate issue (and add the aforementioned code snippet).

[Summus-31c04089c3cd80]

This was the used shellcode : "\x31\xc0\x83\xc0\x29\x6a\x02\x5b\xcd\x80\x83\xe8\x04\xcd\x80"
I've since modified the python code , but it was a "basic" code, similar to this one : https://github.com/cea-sec/miasm/blob/master/example/jitter/x86_32.py (with the above given code part)

[serpilliere]

For the moment, put aside the libs. To have the same conditions, I first generate the following shellcode:

main:
    XOR EAX, EAX
    MOV AL, 1
    XOR EBX,EBX
    INT 0x80
    RET

So I assemble it:

python example/asm/shellcode.py x86_32 example/samples/x86_32_int80.S out.bin

And I emulate it:

$ python example/jitter/x86_32.py out.bin
RAX 0000000000000000 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000000 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000000
40000000 XOR        EAX, EAX
RAX 0000000000000000 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000000
40000002 MOV        AL, 0x1
RAX 0000000000000001 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000002
40000004 XOR        EBX, EBX
RAX 0000000000000001 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000004
40000006 INT        0x80
Traceback (most recent call last):
  File "example/jitter/x86_32.py", line 40, in <module>
    myjit.continue_run()
  File "/usr/local/lib/python2.7/dist-packages/miasm2/jitter/jitload.py", line 339, in continue_run
    return self.run_iterator.next()
  File "/usr/local/lib/python2.7/dist-packages/miasm2/jitter/jitload.py", line 311, in runiter_once
    assert(self.get_exception() == 0)
AssertionError

We have an error on the int 0x80. This instruction through an exception. The basic emulation code doesn't handle exceptions. To deal with it, you have to add an exception handler::

def exception_int(jitter):
    print 'interrupt!'
    return False
myjit.add_exception_handler(EXCEPT_INT_XX, exception_int)

(You have to import EXCEPT_INT_XX something like: from miasm2.jitter.csts import PAGE_READ, PAGE_WRITE, EXCEPT_INT_XX)

Result:

$ python xxx.py out.bin 
RAX 0000000000000000 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000000 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000000
40000000 XOR        EAX, EAX
RAX 0000000000000000 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000000
40000002 MOV        AL, 0x1
RAX 0000000000000001 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000002
40000004 XOR        EBX, EBX
RAX 0000000000000001 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000004
40000006 INT        0x80
interrupt!

And the shellcode stops (the return false makes the execution stop)

Now you have to implement your own system call for the int 0x80!

For example:

def exception_int(jitter):
    print 'interrupt!'
    print 'syscall num:', hex(jitter.cpu.EAX)
    jitter.cpu.EAX = 0x1337
    jitter.cpu.set_exception(0)
    return True

(Here, jitter.cpu.set_exception(0) resets the exception generated by the int 0x80)

And the result:

$ python xxx.py out.bin 
RAX 0000000000000000 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000000 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000000
40000000 XOR        EAX, EAX
RAX 0000000000000000 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000000
40000002 MOV        AL, 0x1
RAX 0000000000000001 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000002
40000004 XOR        EBX, EBX
RAX 0000000000000001 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000004
40000006 INT        0x80
interrupt!
syscall num: 0x1L
RAX 0000000000001337 RBX 0000000000000000 RCX 0000000000000000 RDX 0000000000000000
RSI 0000000000000000 RDI 0000000000000000 RSP 000000000123FFFC RBP 0000000000000000
zf 0000000000000001 nf 0000000000000000 of 0000000000000000 cf 0000000000000000
RIP 0000000040000008
40000008 RET        

See EAX is 1337 after the system call.

[serpilliere]

It seems your posted shellcode doesn't have any call so it doesn't match your issue.

As @commial said, move this sub issue in another topic.

[Summus-31c04089c3cd80]

Ok thank you !
I moved the question about call in another topic, this shellcode was about this issue :)

So is it possible to execute an external function in the miasm VM ?
If i implement a syscall in python and execute it in the exception_handler, the environment should be different from the one where is executed the shellcode, right ?

[serpilliere]

No, if you modify jitter.cpu.EAX, it modify the EAX in the current VM.
I may not have understand your request: What do you mean by "external" function?

[Summus-31c04089c3cd80]

For example : getuid. I'm asking if the return will be the same if it is called by the python code or if it is called directly by the shellcode (in the VM) ?

[serpilliere]

The Miasm VM is there to simulate the behavior of a real machine. So you can set EAX to any uid you want here.

[commial]

Note: the discussion on the second issue continues on #184.

[Summus-31c04089c3cd80]

Yeah that wasn't a good example, and i can not find another one.
The problem comes when the shellcode is expecting a specific value, but even in the VM this can not work since it is different from the target machine ...

[serpilliere]

Have you got a "good" example or the original shell code? (if you can share it with us)

[Summus-31c04089c3cd80]

I have not yet found a shellcode which generates a such problem.
So I'll close this issue, thank you for all the help you have given me !