Hacking the Samsung NX300, part 2: Get Root!

Georg Lukas
Mon, 12 May 2014 18:05:26 +0200
hardware, net, reverse-engineering, samsung-nx

This is the second post in a series covering the Samsung NX300 "Smart" Camera. In the first post, we have analyzed how the camera is interacting with the outside world using NFC and WiFi. In this post, we will have a deeper look at the operating system running on the camera, execute some code and open a remote root shell. This process can be applied (with some adaptations) to different networked consumer electronics, including home routers, NAS boxes and Smart TVs. The third post will leveage that knowledge to add functionality.

Firmware: Looking for Loopholes

Experience shows that most firmware images provide an easy way to run a user-provided shell script on boot. This feature is often added by the "software engineers" during development, but it boils down to a local root backdoor. On a camera, the SD card would be a good place to search. Other devices might execute code from an USB flash drive or the built-in hard disk.

Usually, we have to start with the firmware update file (nx300.bin from this 241MB ZIP in our case), run binwalk on it, extract and mount the root file system and have our fun. In this case, however, the source archive from Samsung's OSS Release Center contains an unpacked root file system tree in TIZEN/project/NX300/image/rootfs, so we just examine that:

georg@megavolt:TIZEN/project/NX300/image/rootfs$ ls -l
total 72
drwxr-xr-x  4 4096 Oct 16  2013 bin/
drwxr-xr-x  3 4096 Oct 16  2013 data/
drwxr-xr-x  3 4096 Oct 16  2013 dev/
drwxr-xr-x 38 4096 Oct 16  2013 etc/
drwxr-xr-x  9 4096 Oct 16  2013 lib/
-rw-r--r--  1  203 Oct 16  2013 make_image.log
drwxr-xr-x  8 4096 Oct 16  2013 mnt/
drwxr-xr-x  3 4096 Oct 16  2013 network/
drwxr-xr-x 16 4096 Oct 16  2013 opt/
drwxr-xr-x  2 4096 Oct 16  2013 proc/
lrwxrwxrwx  1   13 Oct 16  2013 root -> opt/home/root/
drwxr-xr-x  2 4096 Oct 16  2013 sbin/
lrwxrwxrwx  1    8 Oct 16  2013 sdcard -> /mnt/mmc
drwxr-xr-x  2 4096 Oct 16  2013 srv/
drwxr-xr-x  2 4096 Oct 16  2013 sys/
drwxr-xr-x  2 4096 Oct 16  2013 tmp/
drwxr-xr-x 16 4096 Oct 16  2013 usr/
drwxr-xr-x 13 4096 Oct 16  2013 var/

make_image.log sounds like somebody forgot to clean up before shipping (this file is actually contained on the camera):

SBS logging begin
Wed Oct 16 14:27:21 KST 2013

WARNING: setting root UBIFS inode UID=GID=0 (root) and permissions to u+rwx,go+rx; use --squash-rino-perm or --nosquash-rino-perm to suppress this warning

If we can believe the /sdcard symlink, the SD card is mounted at /mnt/mmc. Usually, there are some scripts and tools referencing the directory, and we should start with them:

georg@megavolt:TIZEN/project/NX300/image/rootfs$ grep /mnt/mmc -r .
./etc/fstab:/dev/mmcblk0    /mnt/mmc        exfat   noauto,user,umask=1000 0 0
./etc/fstab:/dev/mmcblk0p1  /mnt/mmc        exfat   noauto,user,umask=1000 0 0
./usr/sbin/pivot_rootfs_ubi.sh:umount /oldroot/mnt/mmc
./usr/sbin/rcS.pivot:   mkdir -p /mnt/mmc
./usr/sbin/rcS.pivot:       mount -t vfat -o noatime,nodiratime $card_path /mnt/mmc
./usr/bin/inspkg.sh:    mount -t vfat /dev/mmcblk0 /mnt/mmc
./usr/bin/inspkg.sh:    mount -t vfat /dev/mmcblk0p1 /mnt/mmc
./usr/bin/inspkg.sh:mount -t vfat /dev/mmcblk0p1 /mnt/mmc
./usr/bin/inspkg.sh:find /mnt/mmc -name "*$1*.deb" -exec dpkg -i {} \; 2> /dev/null
./usr/bin/ubi_initial.sh:mount -t vfat /dev/mmcblk0p1 /mnt/mmc
./usr/bin/ubi_initial.sh:cd /mnt/mmc
./usr/bin/remount_mmc.sh:   nr_mnt_dev=`/usr/bin/stat -c %d /mnt/mmc` #/opt/storage
./usr/bin/remount_mmc.sh:       umount /mnt/mmc 2> /dev/null
./usr/bin/remount_mmc.sh:           /bin/mount -t vfat /dev/mmcblk${i}p1 /mnt/mmc -o uid=0,gid=0,dmask=0000,fmask=0000,iocharset=iso8859-1,utf8,shortname=mixed
./usr/bin/remount_mmc.sh:               /bin/mount -t vfat /dev/mmcblk${i} /mnt/mmc -o uid=0,gid=0,dmask=0000,fmask=0000,iocharset=iso8859-1,utf8,shortname=mixed
[ stripped a bunch of binary matches in /usr/bin and /usr/lib ]

What we have here are some usual Linux boot-up configuration files (fstab, rcS.pivot, ubi_initial.sh), a very interesting script that installs any Debian packages from the SD card (inspkg.sh), and ~50 shared libraries and executable binaries with the /mnt/mmc string hardcoded inside.

Package Installer Script

Let us have a look at inspkg.sh first:

#! /bin/sh

echo $1
if [ "$#" = "2" ]
then
    if [ "$2" = "0" ]
    then
    echo -e "mount mmcblk0.."
    mount -t vfat /dev/mmcblk0 /mnt/mmc
    else
    echo -e "mount mmcblk0p1..."
    mount -t vfat /dev/mmcblk0p1 /mnt/mmc
    fi
else
echo -e "mount mmcblk0p1..."
mount -t vfat /dev/mmcblk0p1 /mnt/mmc
fi

find /mnt/mmc -name "*$1*.deb" -exec dpkg -i {} \; 2> /dev/null

echo -e "sync...."
sync

This is a shell script that takes one or two arguments. The first one is the package name to look for (the find command will find and install all .deb files containing the first argument in their name). The second argument is used to mount the correct partition of the SD card. Surely we can use this script to install dropbear, gcc or moon-buggy. Now we only need to figure out how (or from where) this script is run:

georg@megavolt:TIZEN/project/NX300/image/rootfs$ grep -r inspkg.sh .
georg@megavolt:TIZEN/project/NX300/image/rootfs$

Whoops. There are no references to it in the firmware. It was merely a red herring, and we need to find another way in.

The Magic Binary Blob

In /usr/bin, the most interesting file is di-camera-app-nx300, making references to /mnt/mmc/Demo/NX300_Demo.mp4, /mnt/mmc/SYSTEM/Device.xml and a bunch of WAV files in /mnt/mmc/sounds/ that seem to correspond to UI actions (up, down, ..., delete, ev, wifi).

This is obviously the magic binary blob controlling the really interesting functions (like the UI, the shutter, and the image processor). Most consumer electronics branded as "Open Source" contain some kind of Linux runtime which is only used to execute one large binary. That binary in turn encloses all the things you want to tinker with, but it is not provided with source code, still leaving you at the mercy of the manufacturer.

As expected, this program comes out of nowhere. There are traces of the di-camera-app-nx300 Debian package (version 0.2.387) being installed:

Package: di-camera-app-nx300
Status: install ok installed
Priority: extra
Section: misc
Installed-Size: 87188
Maintainer: Sookyoung Maeng <[snip]@samsung.com>, Jeounggon Yoo <[snip]@samsung.com>
Architecture: armel
Source: di-camera-app
Version: 0.2.387
Depends: libappcore-common-0, libappcore-efl-0, libaul-1, libbundle-0, libc6 (>= 2.4),
    libdevman-0, libdlog-0, libecore, libecore-evas, libecore-file, libecore-input,
    libecore-x, libedje (>= 0.9.9.060+svn20100304), libeina (>= 1.0.0.001+svn20100831),
    libelm, libevas (>= 0.9.9.060+svn20100203), libgcc1 (>= 1:4.4.0),
    libglib2.0-0 (>= 2.12.0), libmm-camcorder, libmm-player, libmm-sound-0,
    libmm-utility, libnetwork-0, libnl2 (>= 2.0), libslp-pm-0, libslp-utilx-0,
    libstdc++6 (>= 4.5), libvconf-0, libwifi-wolf-client, libx11-6,
    libxrandr2 (>= 2:1.2.0), libxtst6, prefman, libproduction-mode,
    libfilelistmanagement, libmm-common, libmm-photo, libasl, libdcm,
    libcapture-fw-slpcam-nx300, libvideo-player-ext-engine, libhibernation-slpcam-0,
    sys-mmap-manager, libstorage-manager, libstrobe, libdustreduction, libmm-slideshow,
    di-sensor, libdi-network-dlna-api, libproduction-commands, d4library,
    diosal
Description: Digital Imaging inhouse application for nx300

So this package is created from di-camera-app, which does not exist either, except "inhouse". Thank you Samsung for spoiling the fun... :-(

Besides of some start/stop scripts, the only other interesting reference to this magic binary blob is in TIZEN/build/exec.sh, which looks like a mixture of installation and startup script:

#!/bin/sh
#

cp -f *.so /usr/lib
cp -f di-camera-app-nx300 /usr/bin
sync
sync
sync

sleep 1
cd /
startx; di-camera-app &

(Because with only one sync, you can never know, and two might still not be enough if you must be 300% sure the data has been written).

The camera app is accompanied by another magic binary blob for WiFi, smart-wifi-app-nx300 (Samsung should get an award for creative file names). However, there are no hints at possible code execution in either program, so we need to dig even deeper.

Searching Shared Libraries

The situation in /usr/lib is different, though. We can run strings on the files that mention the SD card mount point (limiting the output to the relevant lines):

georg@megavolt:TIZEN/project/NX300/image/rootfs$ for f in `grep -l /mnt/mmc *.so` ; do \
                echo "--- $f" ; strings $f | grep /mnt/mmc; done
[snip]
--- libmisc.so
/mnt/mmc
/mnt/mmc/autoexec.sh
--- libnetwork.so
/mnt/mmc/iperf.txt
--- libstorage.so
/usr/bin/iozone -A -s 40m -U /mnt/mmc -f /mnt/mmc/test -e > /tmp/card_result.txt
cp /tmp/card_result.txt /mnt/mmc
/mnt/mmc/auto_run.sh

Okay, this is starting to get hot! /mnt/mmc/autoexec.sh and /mnt/mmc/auto_run.sh are exactly what we have been looking for. We need to try one of them and see what happens!

autoexec.sh

To test our theory, we need to mount the camera via USB, and create the following autoexec.sh file in its root directory (Windows users watch out, the file needs to have Unix linebreaks!):

#!/bin/sh
LOG=/mnt/mmc/autoexec.log
date >> $LOG
id >> $LOG
echo "$PATH" >> $LOG
ps axfu >> $LOG
mount >> $LOG

Now we need to unmount the camera, turn it off and on again, wait some seconds, mount it, and check if we got lucky. Let's see... autoexec.log is there! Jackpot! Now we can analyze its contents, piece by piece:

Fri May  9 06:25:20 UTC 2014
uid=0(root) gid=0(root)
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/X11R6/bin:/sbin:/usr/local/bin:/usr/scripts

This output was just generated, it was running as root (yeah!), and the path looks rather boring.

PID     VSZ   RSS STAT  COMMAND
[stripped boring kernel threads and some columns]
  1    2988    52 Ss    init
139   11460  1348 S     /usr/bin/system_server
144    2652   188 Ss    dbus-daemon --system
181    3416   772 Ss    /usr/bin/power_manager
232   12268  4608 S<s+  /usr/bin/Xorg :0 -logfile /opt/var/log/Xorg.0.log -ac -noreset \
    -r +accessx 0 -config /usr/etc/X11/xorg.conf -configdir /usr/etc/X11/xorg.conf.d
243    2988    76 Ss    init
244    2988    56 Ss    init
245    2988    60 Ss+   init
246    2988    56 Ss+   init
247    2988     8 S     sh /usr/etc/X11/xinitrc
256   20200  2336 S      \_ /usr/bin/enlightenment -profile samsung \
    -i-really-know-what-i-am-doing-and-accept-full-responsibility-for-it
254   19876     8 S     /usr/bin/launchpad_preloading_preinitializing_daemon
255   12648   816 S     /usr/bin/ac_daemon
259    3600     8 S     dbus-launch --exit-with-session /usr/bin/enlightenment -profile samsung \
    -i-really-know-what-i-am-doing-and-accept-full-responsibility-for-it
260    2652     8 Ss    /usr/bin/dbus-daemon --fork --print-pid 5 --print-address 7 --session
267  690688 34760 Ssl   di-camera-app-nx300
404    2988   520 S      \_ sh -c /mnt/mmc/autoexec.sh
405    2988   552 S          \_ /bin/sh /mnt/mmc/autoexec.sh
408    2860   996 R              \_ ps axfu

Our script is executed by di-camera-app-nx300, there is enlightenment and dbus running, and i-really-know-what-i-am-doing-and-accept-full-responsibility-for-it.

The mount point list looks pretty standard as well for an embedded device, using UBIFS for flash memory and the exfat driver for the SD card:

rootfs on / type rootfs (rw)
ubi0!rootdir on / type ubifs (ro,relatime,bulk_read,no_chk_data_crc)
devtmpfs on /dev type devtmpfs (rw,relatime,size=47096k,nr_inodes=11774,mode=755)
none on /proc type proc (rw,relatime)
tmpfs on /tmp type tmpfs (rw,relatime)
tmpfs on /var/run type tmpfs (rw,relatime)
tmpfs on /var/lock type tmpfs (rw,relatime)
tmpfs on /var/tmp type tmpfs (rw,relatime)
tmpfs on /var/backups type tmpfs (rw,relatime)
tmpfs on /var/cache type tmpfs (rw,relatime)
tmpfs on /var/local type tmpfs (rw,relatime)
tmpfs on /var/log type tmpfs (rw,relatime)
tmpfs on /var/mail type tmpfs (rw,relatime)
tmpfs on /var/opt type tmpfs (rw,relatime)
tmpfs on /var/spool type tmpfs (rw,relatime)
tmpfs on /opt/var/log type tmpfs (rw,relatime)
sysfs on /sys type sysfs (rw,relatime)
/dev/ubi2_0 on /mnt/ubi2 type ubifs (ro,noatime,nodiratime,bulk_read,no_chk_data_crc)
/dev/ubi1_0 on /mnt/ubi1 type ubifs (rw,noatime,nodiratime,bulk_read,no_chk_data_crc)
/dev/mmcblk0 on /mnt/mmc type exfat (rw,nosuid,nodev,noatime,nodiratime,uid=5000,gid=6,fmask=0022,
        dmask=0022,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,namecase=0,errors=remount-ro)

Remote Access

The camera is not connected to your WiFi network by default, you have to launch one of the WiFi apps first. The most reliable one for experimenting in your (protected) home network is the Email app. After you launch it, the camera looks for WiFi networks (configure your own one here), and stays connected for a long time, keeping the X server (and anything you run via autoexec.sh) open.

After tinkering around with a static dropbear binary downloaded from the Internets (and binary-patching the references to dropbear_rsa_host_key and authorized_keys), I ran into a really silly problem:

[443] May 09 12:00:45 user 'root' has blank password, rejected

Running a Telnet Server

Around the same time, I realized one thing that I should have checked first:

lrwxrwxrwx    1    17 May 22  2013 /usr/sbin/telnetd -> ../../bin/busybox

Our firmware comes with busybox, and busybox comes with telnetd - an easy to deploy remote login service. After the realization settled, the first attempt looked like we almost did it:

georg@megavolt:~$ telnet nx300
Trying 192.168.0.147...
Connected to nx300.local.
Escape character is '^]'.
Connection closed by foreign host.

georg@megavolt:~$ telnet nx300
Trying 192.168.0.147...
telnet: Unable to connect to remote host: Connection refused

Wow, the telnet port was open, something was running, but we crashed it! Another two mount-edit-restart-mount cycles later, the issue was clear:

telnetd: can't find free pty

Fortunately, the solution is documented. Now we can log into the camera for sure?

georg@megavolt:~$ telnet nx300
Trying 192.168.0.147...
Connected to nx300.local.
Escape character is '^]'.


************************************************************
*                 SAMSUNG LINUX PLATFORM                   *
************************************************************

nx300 login: root
Login incorrect

Damn, Samsung! Why no login? Maybe we can circumvent this in some way? Does the busybox telnetd help provide any hints?

    -l LOGIN        Exec LOGIN on connect

Maybe we can replace the evil password-demanding login command with... a shell? Let us adapt our SD card autoexec.sh script to what we have gathered:

#!/bin/sh

mkdir -p /dev/pts
mount -t devpts none /dev/pts

telnetd -l /bin/bash -F > /mnt/mmc/telnetd.log 2>&1 &

Another mount-edit-restart cycle, and we are in:

georg@megavolt:~$ telnet nx300
Trying 192.168.0.147...
Connected to nx300.local.
Escape character is '^]'.


************************************************************
*                 SAMSUNG LINUX PLATFORM                   *
************************************************************

nx300:/# cat /proc/cpuinfo
Processor       : ARMv7 Processor rev 8 (v7l)
BogoMIPS        : 1395.91
Features        : swp half thumb fastmult vfp edsp neon vfpv3 tls
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x2
CPU part        : 0xc09
CPU revision    : 8

Hardware        : Samsung-DRIMeIV-NX300
Revision        : 0000
Serial          : 0000000000000000
nx300:/# free
         total       used       free     shared    buffers     cached
Mem:        512092     500600      11492          0        132      41700
-/+ buffers/cache:     458768      53324
Swap:        30716       8084      22632
nx300:/# df -h /
Filesystem                Size      Used Available Use% Mounted on
ubi0!rootdir            352.5M    290.8M     61.6M  83% /
nx300:/# ls -al /opt/sd0/DCIM/100PHOTO/
total 1584
drwxr-xr-x    2 root     root           520 May 22  2013 .
drwxr-xr-x    3 root     root           232 May 22  2013 ..
-rwxr-xr-x    1 root     root        394775 May 22  2013 SAM_0015.JPG
-rwxr-xr-x    1 root     root        335668 May 22  2013 SAM_0016.JPG   [Obama was here]
-rwxr-xr-x    1 root     root        357591 May 22  2013 SAM_0017.JPG
-rwxr-xr-x    1 root     root        291493 May 22  2013 SAM_0018.JPG
-rwxr-xr-x    1 root     root        232470 May 22  2013 SAM_0019.JPG
nx300:/#

Congratulations, you have gained network access to yet another Linux appliance! From here, you should be able to perform anything you want on the camera, except from the interesting things closed in the Samsung binaries.

Comments on HN

Full series:

part 1: Hacking the NX300

part 2: Get Root!

part 3: Firmware Mods