mmap() to null: disassembly

Showing posts with label disassembly. Show all posts

Tuesday, April 16, 2013

Travels from the prison of $ to the realm of # [PART 1]

Beginning Notes:
1. A modest amount of core knowledge is assumed.
2. A read of "Smashing The Stack for Fun and Profit" by Aleph One would be recommeded.
3. Point '2' doesn't guarante anything and for further queries refer to point '1' or ask away under comments.

To get the # symbol at a console is a local exploit's dream. But in the current privelaged land of root, it is not uncommon to face challanges. Lets take a simple example: a setuid root binary, data read from a file and echoed onto the stdout. Lets make this a little easier: the binary is not stripped. Lets make this a little more easier: ASLR & Stack Cookies are disabled... for now. At the same time, it's a 64 bit system, NX is enabled [Hardware Enforced] & we don't have the code.

Enough talking, lets get our hands dirty...

A sample run:

$> ./test
usage: ./test file_name
$> echo "hello" > foo
$> ./test foo
Data Read: hello

looks good, the program takes a filename as input, reads the file and echos the file contents on stdout.

lets try to spice it up a bit, run the program with input incremented by 100 characters each time:
for the character stream generation, we shall use good ol' python.

$> python -c "print 'A'*100" > foo && ./test foo
Data Read: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA... [OUTPUT TRUNCATED]

$> python -c "print 'A'*200" > foo && ./test foo
Data Read: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA...

$> python -c "print 'A'*300" > foo && ./test foo
Data Read: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA...

Seems good till now, maybe the code isn't vulnerable, but that wouldn't be fun would it ;)
lets keep trying...

$> python -c "print 'A'*400" > foo && ./test foo
Data Read: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA... [OUTPUT TRUNCATED]

$> python -c "print 'A'*500" > foo && ./test foo
Data Read: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA... [OUTPUT TRUNCATED]
Maybe this is a lost cause, we might as well go and check our email...
"Patience my padawan learner"

$> python -c "print 'A'*600" > foo && ./test foo
Data Read: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA...
Segmentation Fault (core dumped)

bullseye! segfault, the buffer doesn't seem to be unlimited afterall.
Looking at all the attempts, we can easily say that the buffer lies between 500 to 600 bytes.

Lets fireup GDB for a bit of code and stack analysis.

$> gdb test
GNU gdb (GDB) 7.2
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
For bug reporting instructions, please see:
...
Reading symbols from /home/sandman/test...(no debugging symbols found)...done.
(gdb) run foo
Starting program: /home/sandman/test foo
Data Read: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA...
Program received signal SIGSEGV, Segmentation fault.
0x00000000004007af in echo_me ()
Seems like the code segfaults in the function echo_me(), a little bit of disassembly required (thankfully the binary is not stripped)...

(gdb) disas echo_me
Dump of assembler code for function echo_me:
   0x00000000004006d4 <+0>:    push   %rbp
   0x00000000004006d5 <+1>:    mov    %rsp,%rbp
   0x00000000004006d8 <+4>:    sub    $0x240,%rsp
   0x00000000004006df <+11>:    mov    %rdi,-0x238(%rbp)
   0x00000000004006e6 <+18>:    mov    $0x4008ec,%edx
   0x00000000004006eb <+23>:    mov    -0x238(%rbp),%rax
   0x00000000004006f2 <+30>:    mov    %rdx,%rsi
   0x00000000004006f5 <+33>:    mov    %rax,%rdi
   0x00000000004006f8 <+36>:    callq 0x4005b0
   0x00000000004006fd <+41>:    mov    %rax,0x20053c(%rip)        # 0x600c40
   0x0000000000400704 <+48>:    mov    0x200535(%rip),%rax        # 0x600c40
   0x000000000040070b <+55>:    test   %rax,%rax
   0x000000000040070e <+58>:    jne    0x400724
   0x0000000000400710 <+60>:    mov    $0x4008ee,%edi
   0x0000000000400715 <+65>:    callq 0x400580
   0x000000000040071a <+70>:    mov    $0xffffffff,%edi
   0x000000000040071f <+75>:    callq 0x4005a0
   0x0000000000400724 <+80>:    mov    0x200515(%rip),%rax        # 0x600c40
   0x000000000040072b <+87>:    mov    $0x2,%edx
   0x0000000000400730 <+92>:    mov    $0x0,%esi
   0x0000000000400735 <+97>:    mov    %rax,%rdi
   0x0000000000400738 <+100>:    callq 0x400590
   0x000000000040073d <+105>:    mov    0x2004fc(%rip),%rax        # 0x600c40
   0x0000000000400744 <+112>:    mov    %rax,%rdi
   0x0000000000400747 <+115>:    callq 0x400570
   0x000000000040074c <+120>:    mov    %eax,-0x4(%rbp)
   0x000000000040074f <+123>:    mov    0x2004ea(%rip),%rax        # 0x600c40
   0x0000000000400756 <+130>:    mov    $0x0,%edx
   0x000000000040075b <+135>:    mov    $0x0,%esi
   0x0000000000400760 <+140>:    mov    %rax,%rdi
   0x0000000000400763 <+143>:    callq 0x400590
   0x0000000000400768 <+148>:    mov    0x2004d1(%rip),%rdx        # 0x600c40
   0x000000000040076f <+155>:    mov    -0x4(%rbp),%ecx
   0x0000000000400772 <+158>:    lea    -0x230(%rbp),%rax
   0x0000000000400779 <+165>:    mov    %ecx,%esi
   0x000000000040077b <+167>:    mov    %rax,%rdi
   0x000000000040077e <+170>:    callq 0x4005d0
   0x0000000000400783 <+175>:    mov    0x2004b6(%rip),%rax        # 0x600c40
   0x000000000040078a <+182>:    mov    %rax,%rdi
   0x000000000040078d <+185>:    callq 0x4005e0
   0x0000000000400792 <+190>:    mov    $0x4008fa,%eax
   0x0000000000400797 <+195>:    lea    -0x230(%rbp),%rdx
   0x000000000040079e <+202>:    mov    %rdx,%rsi
   0x00000000004007a1 <+205>:    mov    %rax,%rdi
   0x00000000004007a4 <+208>:    mov    $0x0,%eax
   0x00000000004007a9 <+213>:    callq 0x400560
   0x00000000004007ae <+218>:    leaveq
=> 0x00000000004007af <+219>:    retq
End of assembler dump.
(gdb)

looking at the disassembly, its quite easy to get an idea of the code by a simple follow of the important @plt [procedure linkage table] calls...

function echo_me(){
    fopen(file)            | open the file
    fseek(till EOF)            --|
    ftell(file_des)                 |--> to calculate the length of the file
    fseek(till BOF)            --|
    fgets(the string from the file) | get the string from the file and store it in the buffer [the vulnerability lies here]
    fclose(file)                    | close the file
    printf(the string)        | print the string
}

looking at all this it is evident what the programmer has done. A buffer with a static size was defined, the file size was calculated and was read into the buffer. Finally, the buffer is displayed as the output.

Thursday, January 31, 2013

Infra-Webcam Mod Part 1.

Due to a recent break-in near at our apartments, the whole security infrastructure went through an overhaul. Going through all the new features, I came across one that caught my eye, Night-Vision Security Cameras. Upon further investigation, I found these were actually Infrared cameras.

The cameras themselves looked like webcams dotted with red LEDs. Now, I had seen them before but never wondered their purpose or capability. A simple Google search uncovered a ton of info and the part that totally took me by surprise was the fact that these could be engineered at home using simple everyday camera hardware.

Now, I'm not gonna write down a tutorial or go much in depth but will give some insight on how I made mine.

The idea is simple. A normal camera has 3 components, the Lens, The IR filter and the CCD chip. All one has to do is to remove the IR filter and replace it with a "Visible Light filter" (Hint: Kodak).

Anyway, below are a few pics I took while disassembling and modding.

The Camera (Source: Local flea market, Vendor: PC-Touch)

The PCB with the Lens, IR FLTR, CCD.

IR Filter (Note to self: Next time take the picture before breaking the thing :-P)

CCD Chip (Lens, IR FLTR removed)

Camera Lens with the light filter (Exposed camera film[Even Gradient])

Reassembled Module

Results in the next post.

Update[1/2/2013]: Damn! the drivers are 32 bit. Lets see if Linux does the job.

Wednesday, July 11, 2012

Old Cell Phones == treasure;

I guess many people don't realize before dumping their old phones that they are also throwing away a treasure trove of micro hardware that can be extracted and used in multiple DIY projects. I don't blame them as most people don't even know how does the vibration in their phones work.

So recently while going through some old 2006< cellphones, I decided to salvage such components. I recovered some really neat gems:

Micromotors [Both types: External and Sealed]
Mics
Speakers
2MP camera modules
LCDs [Color 64K & 256K 160ppi & B/W 100ppi]
Micro DC-in female jacks

Now to think of some things I could do with them!?? Bristlebots anyone ;-)

Sunday, March 13, 2011

Dell Studio 1535 cleaning/disassembly

The temperature sensors on procyon [My Dell Studio 1535] laptop were constantly hitting abnormal values recently. CPU kept on idling at around 60C, even after my previous post on the fix for lm_sensors configuration applied. So I knew that it was time I opened the girl up and do her some good'ol fashioned cleaning. So I borrowed a cam, took out my toolbox, acquired some Thermal Compound [Shin-Etsu Microsi's G-751 Thermal Paste (thanks Shray!)] from a good friend, added some Pink Floyd on my playlist and got to work. Here are a few pics of the internals for anyone's viewing pleasure since I could not find any decent teardown images of the same model on the web. Enjoy....