A Practical approach of tailoring Linux Kernel. Junghwan Kang /

Transcription

1 A Practical approach of tailoring Linux Kernel Junghwan Kang / ultract@nsr.re.kr

2 Contents Introduction Related Work Design Implementation Demo Evaluation Conclusion

3 Introduction

4 Introduction Linux Kernel is being used on various devices & platforms IoT!

5 Introduction A lot of Features in Linux kernel ver 4.7.x in Debian jessie More than 30 Architectures More than 300 Feature Groups Feature groups : configuration menu defined in Kconfig More than 20,000 Configuration Options More than 20,000,000 Lines of Source Code Supports for variety of features widen attack surface CVE : Buffer Overflow in USB/IP CVE : Arbitrary code execution in Qualcomm media codecs CVE : Use-after-free in kvm_ioctl_create_device CVE : Arbitrary code execution Synaptics touchscreen driver CVE : Race Condition in n_hdlc Driver CVE : a double-free in DCCP Protocol

6 Introduction So, We need to tailor Linux kernel!

7 Introduction But, So many Configuration options More than 20,000 Options!! Configuration is hard due to the excessive number and choices However, There are preceding solutions!! (Kernel Tailoring Tool) Undertaker-tailor Very good idea! But, not working well (Make Option) make localmodconfig Very useful! But, still having unnecessary configuration options So, A better approach? Minimizing Attack Surface & Reliable Kernel Tailoring

8 Related Work

9 Related Work - Undertaker-tailor A result of studies below VAMOS(Variability Management in Operating Systems) Project of FAU Automatic OS Kernel TCB Reduction by Leveraging Compile-Time Configurability, HotDep '12 Attack Surface Metrics and Automated Compile-Time OS Kernel Tailoring, NDSS '13 Using Ftrace feature of Linux kernel(config_ftrace) Tracing All memory addresses of kernel events & executed functions Workflow of Undertaker-tailor

10 Related Work - Undertaker-tailor Undertaker-tailor Wrap-up: it is novel but incomplete Upside Use of ftrace, configuration formulae, and SAT Solver Downside Does not work An overhaul is needed L

11 Related Work - Undertaker-tailor Traced Configuration Options Try kernel tailoring! Nothing in Tailored Configuration!! Equal to ARCH=x86_64 make allnoconfig

12 Related Work - Undertaker-tailor Needed more deep analysis by myself Enable debug mode Faced some error messages during tailoring Floating point exception Out of memory, Contradiction found by the SAT solver Then, I found some bugs & fixed it Reported to a maintainer of undertaker-tailor J

13 Related Work - Undertaker-tailor Step 1. undertaker-tailor shell script Fixed!, Wrong if condition of 64bit & 32bit 64bit 32bit Because of KASLR (Kernel Address Space Layout Randomization)

14 Related Work - Undertaker-tailor Step 2. Debug undertaker-tailor by gdb Segmentation fault!! Create core dump file & Backtrace

15 Related Work - Undertaker-tailor Step 2. Debug undertaker-tailor by gdb undertaker.ignore

16 Related Work - Undertaker-tailor Step 3. Blacklist & Whitelist Outdated Configuration Options Contradiction found by the SAT solver Must be initialized!

17 Related Work - Undertaker-tailor Finally, Undertaker-tailor was working well J But, The tailored kernel didn t boot up L According to the paper, I should be able to get a working kernel by setting blacklist & whitelist Well, How can I check out each configuration options???

18 Related Work - Kconfig option Various kernel make options

19 Related Work - Kconfig option Kernel tailoring by make localmodconfig

20 Related Work Compare undertaker-tailor & make localmodconfig Used Linux kernel 4.7 based on Debian Jessie backport Default.config Enabled : 1,713 Modules : 3,064 Disabled : 1,602 Working well! But, Fat Tailored.config by localmodconfig Enabled : 1,158 Modules : 109 Disabled : 2,326 Working well! But, A little fat Tailored.config by undertaker-tailor Enabled : 337 Modules : 71 Disabled : 1333 Not Working!! But, thin

21 Design

22 Design I suggest a approach of tailoring Linux kernel! Automated Method Minimum Attack Surface Reliability & Availability In curiosity, I just wanted to know which configuration options are essential for a successful boot J

23 Design Architecture

24 Design Workflow Preparation Kernel tailoring Finish

25 Design Fill Configuration Options Transfusing the stable.config into the unstable.config But, numerous configuration options of a difference set(a-b) About 800 Configuration Options Grouping Configuration Options(Sort & Merge) About 300 Groups Excepting each of the Groups in filled.config one by one A Tailored.config by localmodconfig B Tailored.config by undertaker-tailor Working well! Not Working!!

26 Implementation

27 Implementation # of Using VMs

28 Implementation Environment H/W HP Z840, Intel Xeon E RAM 256GB, SSD 1TB, 1Gbps Ethernet Host OS Virtual Machine Development Language Ubuntu Desktop 64bit LTS VMware Workstation 12 4 Core CPU, RAM 4GB, HDD 15GB Gooroom , Linux Kernel Bash Script Etc undertaker-tailor cnee 3.19(Use Case Automation)

29 Implementation Source code files 754 SLOC by lizard File commands-in-host.sh commands-in-guest-a.sh use-case-a.sh kernel-tailoring.sh cnee_events.tar.gz commands-in-guest-b.sh use-case-b.sh Functions Kernel Tailoring Manager Configuration Filler Kernel Builder Set & Drive Tracing Machines Play Use Cases in Tracing Machines Make Tailored.config by undertaker-tailor cnee use cases file Set & Drive Testing Machines Play Use Cases & Check Stability of Testing Machine

30 Implementation Preparing Virtual Machine Auto Login /etc/lightdm/lightdm.conf, autologin-user=[user id] Sudoer /etc/sudoers, [user id] ALL=(ALL:ALL)NOPASSWD:ALL Disable Auto Screen Lock, Suspending, Hibernation Install vmtools(vix, vmrun) Create Snapshot & Clone VM Wait until saving state will be 100%! Located Testing VM Image in RAM Disk

31 Implementation Make Automated Use Cases by cnee Install cnee Package apt install xnee Record Use Case cnee --record --keyboard --mouse --time 5 --o./use_case_1.xnr Play Use Case cnee --replay --no-synchronise --force-core-replay -f./use_case_1.xnr Use Cases Execute Default Programs & System Setting Using Browser System Benchmark

32 Implementation Verify Tailored Linux kernel Boot State, success or failure? check exit code of vmrun & response time Play Use Cases Check System Log dmesg grep Failed to xxx journalctl /var/logs Kernel Module & ETC (compare kernel modules) lsmod (Internet connection) ping -c google.com (keyboard & mouse) /dev/input/by-path/, kdb, mouse

33 Implementation Fill Configuration Options make localmodconfig on tailored.config by undertaker-tailor Show dependency configuration options for vital kernel modules!!

34 Implementation Fill Configuration Options Make a Blacklist by bare hands CONFIG_XEN_XXX, CONFIG_KVM_XXX, CONFIG_CHROME_XXX Select a Configuration Group to Except CONFIG_XXX_XXX

35 Implementation Multi-VMs for Testing Bash Function & Run Background Check Working State of VMs Synchronizing by Atomic Operation mkdir /tmp/test.lock.d Essential Configuration Group Un-Essential Configuration Group Testing VMs VM1 VM2 VM3 VM4 Configuration Groups Synchronization VM5

36 Demo

37 Demo

38 Evaluation

39 Evaluation Test Condition # of Tracing VM : 1 # of Testing VM : 5 Total elapsed time : about 5 Hours Size of Kernel, Compressed kernel image Original : 3,969,824 byte (System.map : 3,145,248 byte) Tailored : 2,274,528 byte (System.map : 1,372,244 byte) Size of Kernel Module Original : 177,097,352 byte (# of Files : 3269) Tailored : 6,651,297 byte (# of Files : 105)

40 Evaluation Booting time systemd-analyze command Original kernel Tailored kernel Compare tailored.config undertaker-tailor s.config Enabled : 326 Modules : 71 Disabled : 1334 localmodconfig s.config Enabled : 1,158 Modules : 105 Disabled : 2,329 Final tailored.config Enabled : 536 Modules : 97 Disabled : 1539

41 Conclusion

42 Conclusion We saw an automatic system to tailor kernel for a custom Linux distro Working Tailored Kernel Minimizing Kernel Size or Attack Surface Shorten Boot up Time Required enough workloads or use cases in user level To identify vital kernel configurations for the applications & services Automatic systems or methods for tailoring Linux kernel are useful for users who don t have expert knowledges of Linux kernel Future work Testing & Adding other verification conditions Customizing for physical machines Design for other architectures

43 Question?

44 Appendix. Configuration Groups Essential Configuration Groups

45 Appendix. Configuration Groups Non-essential Configuration Groups

46