Click here to close now.

Welcome!

Linux Containers Authors: Sanjeev Sharma, Carmen Gonzalez, Liz McMillan, Elizabeth White, Pat Romanski

Related Topics: Linux Containers

Linux Containers: Article

Stop Malicious Code Execution at the Kernel Level

An in-depth look at the DigSig solution

This article presents a Linux kernel module capable of verifying digital signatures of ELF binaries before running them. This kernel module is available under the GPL license at http://sourceforge.net/projects/disec, and has been successfully tested for kernel 2.5.66 and above.

Why Check the Signature of Your Binaries Before Running Them?
The problem with blindly running executables is that you are never sure they actually do what you think they are supposed to do (and nothing more). Viruses spread so much on Microsoft Windows systems mainly because users are frantic to execute whatever they receive, especially if the title is appealing. The LoveLetter virus, with over 2.5 million machines infected, is a famous illustration of this. Yet Linux is unfortunately not immune to malicious code either. By executing unknown and untrusted code, users are exposed to a wide range of Unix worms, viruses, trojans, backdoors, and so on. To prevent this, a possible solution is to digitally sign binaries you trust, and have the system check their digital signature before running them: if the signature cannot be verified, the binary is declared corrupt and the operating system will not let it run.

Related Work
There have already been several initiatives in this domain, such as Tripwire, BSign, Cryptomark, and IBM's Signed Executables, but we believe the DigSig project is the first to be both easily accessible to all (available on Sourceforge, under the GPL license) and to operate at the kernel level (see Table 1).

 

The DigSig Solution
To avoid reinventing the wheel, we based our solution on the existing open source project BSign, a Debian userspace binary signing package. BSign signs the binaries and embeds the signature in the binary itself. Then, at kernel level, DigSig verifies these signatures at execution time and denies execution if the signature is invalid.

Typically, in our approach, binaries are not signed by vendors, rather we hand over control of the system to the local administrator, who is responsible for signing all binaries he or she trusts with his or her private key. Then, those binaries are verified with the corresponding public key. This means you can still use your favorite (signed) binaries: no change in habits. Basically, DigSig guarantees only two things: (1) if you signed a binary, nobody other than you can modify that binary without being detected, and (2) nobody can run a binary that is not signed or is badly signed. Of course, you should be careful not to sign untrusted code: if malicious code is signed, all security benefits are lost.

How Do I Use DigSig?
DigSig is fairly simple to use. First, you need to sign all binaries you trust with BSign (version 0.4.5 or higher). Then you need to load DigSig with the public key that corresponds to the private key used to sign the binaries.

The following shows step by step how to sign the executable "ps":

$ cp 'which ps' ps-test
$ bsign -s ps-test // Sign the binary
$ bsign -V ps-test // Verify the validity of the signature

Next, install the DigSig kernel module. To do so, a recent kernel version is required (2.5.66 or higher), compiled with security options enabled (CONFIG_SECURITY=y). To compile DigSig, assuming your kernel source directory is /usr/src/linux-2.5.66, you do:

$ cd digsig
$ make -C /usr/src/linux-2.5.66 SUBDIRS=$PWD modules
$ cd digsig/tools && make

This builds the DigSig kernel module (digsig_verif.ko), and you're probably already halfway through the command to load it, but wait! If you are not cautious about the following point, you might secure your machine so well you'll basically freeze it. As a matter of fact, once DigSig is loaded, verification of binary signatures is activated. At that time, binaries will be able to run only if their signature is successfully verified. In all other cases (invalid signature, corrupted file, no signature...), execution of the binary will be denied. Consequently, if you forget to sign an essential binary such as /sbin/reboot, or /sbin/rmmod, you'll be most embarrassed to reboot the system if you have to. Therefore, for testing purposes, we recommend you initially run DigSig in debug mode. To do this, compile DigSig with the DSI_DIGSIG_DEBUG and DSI_DEBUG flags set in the Makefile:

EXTRA_CFLAGS += -DDSI_DEBUG -DDSI_DIGSIG_DEBUG -I $(obj)

In debug mode, DigSig lets unsigned binaries run. This state is ideal to test DigSig, and also list the binaries you need to sign to get a fully operational system.

Once this precaution has been taken it's time to load the DigSig module, with your public key as argument. BSign uses GnuPG keys to sign binaries, so retrieve your public key as follows:

$ gpg --export >> my_public_key.pub

Then log as root, and use the digsig.init script to load the module.

# ./digsig.init start my_public_key.pub
Testing if sysfs is mounted in /sys.
sysfs found
Loading Digsig module.
Loading public key.
Done.

This is it: signature verification is activated. You can check the signed ps executable (ps-test) works:

$./ps-test
$ su
Password:
# tail -f /var/log/messages
Sep 16 15:49:16 colby kernel: DSI-LSM MODULE - binary is ./ps-test
Sep 16 15:49:16 colby kernel: DSI-LSM MODULE - dsi_bprm_compute_creds: Found signature
section
Sep 16 15:49:16 colby kernel: DSI-LSM MODULE - dsi_bprm_compute_creds: Signature
verification successful

But, corrupted executables won't run:

$ ./ps-corrupt
bash: ./ps-corrupt: Operation not permitted
Sep 16 15:55:20 colby kernel: DSI-LSM MODULE - binary is ./ps-corrupt
Sep 16 15:55:20 colby kernel: DSI-LSM MODULE Error - dsi_bprm_compute_creds: Signatures
do not match for ./ps-corrupt

If the permissive debug mode is set, signature verification is skipped for unsigned binaries. Otherwise, the control is strictly enforced in the normal behavior:

$ ./ps
bash: ./ps: cannot execute binary file
# su
Password:
# tail -f /var/log/messages
Sep 16 16:05:10 colby kernel: DSI-LSM MODULE - binary is ./ps
Sep 16 16:05:10 colby kernel: DSI-LSM MODULE - dsi_bprm_compute_creds:
Signatures do not match

DigSig, Behind the Scenes
The core of DigSig lies in the LSM hooks placed in the kernel's routines for executing a binary. The starting point of any binary execution is a system call to sys_exec(), which triggers do_execve(). This is the transition between user space and kernel space.

The first LSM hook to be called is bprm_alloc_security, where a security structure is optionally attached to the linux_bprm structure that represents the task. DigSig does not use this hook as it doesn't need any specific security structure.

Then, the kernel tries to find a binary handler (search_binary_handler) to load the file. This is when the LSM hook bprm_check_security is called, and precisely when DigSig performs signature verification of the binary. If successful, load_elf_binary() gets called, which eventually calls do_mmap(), then the LSM hook file_mmap(), and finally bprm_free_security().

 

So, this is how DigSig enforces binary signature verification at kernel level. Now, a brief explanation of the signing mechanism of DigSig's userland counterpart: BSign. When signing an ELF binary, BSign stores the signature in a new section in the binary. To do so, it modifies the ELF's section header table to account for this new section, with the name "signature" and a user defined type 0x80736967 (which comes from the ASCII characters "s", "i", and "g"). You can check your binary's section header table with the command readelf -S <binary>. It then performs a SHA1 hash on the entire file, after having zeroed the additional signature section. Next it prefixes this hash with "#1; bsign v%s" where %s is the version number of BSign, and stores the result at the begining of the binary's signature section. Finally, BSign calls GnuPG to sign the signature section (containing the hash), and stores the signature at the current position of the signature section. A short compatibility note: GnuPG adds a 32-byte timestamp and a signature class identifier in the buffer it signs.

 

On a cryptographic point of view, DigSig needs to verify BSign's signatures, i.e., RSA signatures. More precisely, this consists in, on one side, hashing the binary with a one-way function (SHA-1) and padding the result (EMSA PKCS1 v1.5), and, on the other side, "decrypting" the signature with the public key and verifying this corresponds to the padded text.

PKCS#1 padding is pretty simple to implement, so we had no problems coding it. Concerning SHA-1 hashing, we used Linux's kernel CryptoAPI:

  • We allocate a crypto_tfm structure (crypto_alloc_tfm), and use it to initialize the hashing process (crypto_digest_init).
  • Then we read the binary block by block, and feed it to the hashing routine (crypto_digest_update).
  • Finally, we retrieve the hash (crypto_digest_final).
The trickiest part is most certainly the RSA verification because the CryptoAPI does not support asymetric algorithms (such as RSA) yet, so we had to implement it. The theory behind RSA is relatively simple: it consists in a modular exponentation (m^e mod n) using very large primes; however, in practice, everybody will agree that implementing an efficient big number library is tough work. So, instead of writing ours, we decided it would be safer to use an existing one and adapt it to kernel restrictions. We decided to port GnuPG's math library (which is actually derived from GMP, GNU's math library):
  • Only the RSA signature verification routines have been kept. For instance, functions to generate large primes have been erased.
  • Allocations on the stack have been limited to the strict minimum.
How Much Does It Slow the System Down?
We have performed two different kinds of benchmarks for DigSig: a benchmark of the real impact of DigSig for users (how much they feel the system is slowed down), and a more precise benchmark evaluating the exact overhead induced by our kernel module.

The first set of benchmarks has been performed by comparing how long it takes to run an executable with or without DigSig. To do so, we used the command "time" over fast to longer executions. The following benchmark has been run 20 times:

% time /bin/ls -Al   # times /bin/ls
% time ./digsig.init compile    # times compilation with gcc
% time tar jxvfp linux-2.6.0-test8.tar.bz2 # times tar

On a Pentium 4, 2.2GHz, with 512MB of RAM, with DigSig using GnuPG's math library, we obtained the results displayed in Table 2. They clearly show that the impact of DigSig is quite important for short executions (such as ls) but soon becomes completely negligible for longer executions such as compiling a project with gcc, or untarring sources with tar.

 

Second, we measured the exact overhead introduced by our kernel module. To do so, we basically compared jiffies at the beginning and at the end of bprm_check_security. In brief, jiffies represent the number of clock ticks since the system has booted, so they are a precise way to measure time in the Linux kernel. In our case, jiffies are in milliseconds. We have run each binary 30 times (see Table 3) for DigSig compiled with GnuPG.

 

The results show that, naturally, the digital signature verification overhead increases with the executable's size (which is not a surprise because it takes longer to hash all data).

Finally, to assist us in optimizing our code, we have run Oprofile, a system profiler for Linux, over DigSig (see Table 4). Results clearly indicate that the modular exponentiation routines are the most expensive, so this is where we should concentrate our optimization efforts for future releases. More particularly, we plan to port ASM code of math libraries to the kernel, instead of using pure C code.

 

Conclusion and Future Work
We've shown how DigSig can help you in mitigating the risk of running malicious code. Our future work will focus on two main areas: performance and features.

Obviously, as signature verification overhead impacts all binaries, it is important to optimize it. There are several paths we might follow such as caching signature verification, sporadically verifying signatures, or optimizing math libraries.

From a feature point of view, we recently implemented digital signature verification of shared libraries: if malicious code is inserted into a library, all executables (even signed ones) that link to this library are compromised, which is a severe limitation. This implementation is currently in the testing phase and will be released soon.

References

  • Wraight, C. (2003). "Securing Your Linux Environment." LinuxWorld Magazine, Vol 1, issue 2.
  • Tripwire: www.tripwire.com
  • Bsign: http://packages.qa.debian.org/b/bsign.html
  • Cryptomark: www.immunix.org/cryptomark.html
  • Van Doorn, L., Ballintijn, G., Arbaugh, W.A., Signed Executables for Linux, January 2003.
  • GnuPG: www.gnupg.org
  • OProfile: http://oprofile.sourceforge.net LinuxWorld Magazine www.LinuxWorld.com
  • More Stories By Makan Pourzandi

    Makan Pourzandi received his doctoral degree on parallel computing in 1995 from the University of Lyon, France. He works for Ericsson Research
    Canada in the Open Systems Research Department. His research domains are security, cluster computing, and component-based methods for
    distributed programming. He has more than 7 publications in International conferences with reference committees. Makan has delivered several talks
    at universities, international conferences, and Open Source forums. He is involved in several Open Source projects: Distributed Security
    Infrastructure (disec.sourceforge.net), and a contributer to the
    security requirements of the Open Source Development Lab (OSDL) Carrier Grade Linux (CGL).

    More Stories By Axelle Apvrille

    Axelle Apvrille currently works for Ericsson Research Canada in the Open Systems Research Department. Her
    research interests are cryptography, security protocols and distributed
    security. She received her computer science engineering degree in 1996
    at ENSEIRB, Bordeaux, France.

    More Stories By David Gordon

    David Gordon has a bachelor’s degree from the university of Sherbrooke. His interests include security and next-generation networks.

    More Stories By Vincent Roy

    Vincent Roy is an electrical engineering student at Sherbrooke University (Canada). He has been working on Linux kernel–related project since his first internship.

    Comments (1) View Comments

    Share your thoughts on this story.

    Add your comment
    You must be signed in to add a comment. Sign-in | Register

    In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


    Most Recent Comments
    jackie113 07/11/07 05:07:35 AM EDT

    PlayStation 3 is not only an expensive game console but also an excellent video player. It could play high-def Blu-ray movies in addition to standard DVDs with a Blu-ray drive

    www.mp4-converter.net/ps3-converter/

    @ThingsExpo Stories
    Internet of Things (IoT) will be a hybrid ecosystem of diverse devices and sensors collaborating with operational and enterprise systems to create the next big application. In their session at @ThingsExpo, Bramh Gupta, founder and CEO of robomq.io, and Fred Yatzeck, principal architect leading product development at robomq.io, discussed how choosing the right middleware and integration strategy from the get-go will enable IoT solution developers to adapt and grow with the industry, while at the same time reduce Time to Market (TTM) by using plug and play capabilities offered by a robust IoT ...
    17th Cloud Expo, taking place Nov 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy. Meanwhile, 94% of enterprises are using some form of XaaS – software, platform, and infrastructure as a service.
    SYS-CON Events announced today that Secure Infrastructure & Services will exhibit at SYS-CON's 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Secure Infrastructure & Services (SIAS) is a managed services provider of cloud computing solutions for the IBM Power Systems market. The company helps mid-market firms built on IBM hardware platforms to deploy new levels of reliable and cost-effective computing and high availability solutions, leveraging the cloud and the benefits of Infrastructure-as-a-Service (IaaS...
    The 17th International Cloud Expo has announced that its Call for Papers is open. 17th International Cloud Expo, to be held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, APM, APIs, Microservices, Security, Big Data, Internet of Things, DevOps and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit your speaking proposal today!
    "We have a tagline - "Power in the API Economy." What that means is everything that is built in applications and connected applications is done through APIs," explained Roberto Medrano, Executive Vice President at Akana, in this SYS-CON.tv interview at 16th Cloud Expo, held June 9-11, 2015, at the Javits Center in New York City.
    The 5th International DevOps Summit, co-located with 17th International Cloud Expo – being held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA – announces that its Call for Papers is open. Born out of proven success in agile development, cloud computing, and process automation, DevOps is a macro trend you cannot afford to miss. From showcase success stories from early adopters and web-scale businesses, DevOps is expanding to organizations of all sizes, including the world's largest enterprises – and delivering real results. Among the proven benefits, DevOps is corr...
    The basic integration architecture, as defined by ESBs, hasn’t changed for more than a decade. Most cloud integration providers still rely on an ESB architecture and their proprietary connectors. As a result, enterprise integration projects suffer from constraints of availability and reliability of these connectors that are not re-usable across other integration vendors. However, the rapid adoption of APIs and almost ubiquitous availability of APIs amongst most SaaS and Cloud applications are rapidly redefining traditional integration approaches and their reliance on proprietary connectors. ...
    The Internet of Things is not only adding billions of sensors and billions of terabytes to the Internet. It is also forcing a fundamental change in the way we envision Information Technology. For the first time, more data is being created by devices at the edge of the Internet rather than from centralized systems. What does this mean for today's IT professional? In this Power Panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists addressed this very serious issue of profound change in the industry.
    Internet of Things is moving from being a hype to a reality. Experts estimate that internet connected cars will grow to 152 million, while over 100 million internet connected wireless light bulbs and lamps will be operational by 2020. These and many other intriguing statistics highlight the importance of Internet powered devices and how market penetration is going to multiply many times over in the next few years.
    Today air travel is a minefield of delays, hassles and customer disappointment. Airlines struggle to revitalize the experience. GE and M2Mi will demonstrate practical examples of how IoT solutions are helping airlines bring back personalization, reduce trip time and improve reliability. In their session at @ThingsExpo, Shyam Varan Nath, Principal Architect with GE, and Dr. Sarah Cooper, M2Mi’s VP Business Development and Engineering, will explore the IoT cloud-based platform technologies driving this change including privacy controls, data transparency and integration of real time context wi...
    WebRTC converts the entire network into a ubiquitous communications cloud thereby connecting anytime, anywhere through any point. In his session at WebRTC Summit,, Mark Castleman, EIR at Bell Labs and Head of Future X Labs, will discuss how the transformational nature of communications is achieved through the democratizing force of WebRTC. WebRTC is doing for voice what HTML did for web content.
    To many people, IoT is a buzzword whose value is not understood. Many people think IoT is all about wearables and home automation. In his session at @ThingsExpo, Mike Kavis, Vice President & Principal Cloud Architect at Cloud Technology Partners, discussed some incredible game-changing use cases and how they are transforming industries like agriculture, manufacturing, health care, and smart cities. He will discuss cool technologies like smart dust, robotics, smart labels, and much more. Prepare to be blown away with a glimpse of the future.
    Explosive growth in connected devices. Enormous amounts of data for collection and analysis. Critical use of data for split-second decision making and actionable information. All three are factors in making the Internet of Things a reality. Yet, any one factor would have an IT organization pondering its infrastructure strategy. How should your organization enhance its IT framework to enable an Internet of Things implementation? In his session at @ThingsExpo, James Kirkland, Red Hat's Chief Architect for the Internet of Things and Intelligent Systems, described how to revolutionize your archit...
    It is one thing to build single industrial IoT applications, but what will it take to build the Smart Cities and truly society-changing applications of the future? The technology won’t be the problem, it will be the number of parties that need to work together and be aligned in their motivation to succeed. In his session at @ThingsExpo, Jason Mondanaro, Director, Product Management at Metanga, discussed how you can plan to cooperate, partner, and form lasting all-star teams to change the world and it starts with business models and monetization strategies.
    SYS-CON Events announced today that BMC will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. BMC delivers software solutions that help IT transform digital enterprises for the ultimate competitive business advantage. BMC has worked with thousands of leading companies to create and deliver powerful IT management services. From mainframe to cloud to mobile, BMC pairs high-speed digital innovation with robust IT industrialization – allowing customers to provide amazing user experiences with optimized IT per...
    There will be 150 billion connected devices by 2020. New digital businesses have already disrupted value chains across every industry. APIs are at the center of the digital business. You need to understand what assets you have that can be exposed digitally, what their digital value chain is, and how to create an effective business model around that value chain to compete in this economy. No enterprise can be complacent and not engage in the digital economy. Learn how to be the disruptor and not the disruptee.
    The Internet of Things is not only adding billions of sensors and billions of terabytes to the Internet. It is also forcing a fundamental change in the way we envision Information Technology. For the first time, more data is being created by devices at the edge of the Internet rather than from centralized systems. What does this mean for today's IT professional? In this Power Panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists will addresses this very serious issue of profound change in the industry.
    Business as usual for IT is evolving into a "Make or Buy" decision on a service-by-service conversation with input from the LOBs. How does your organization move forward with cloud? In his general session at 16th Cloud Expo, Paul Maravei, Regional Sales Manager, Hybrid Cloud and Managed Services at Cisco, discusses how Cisco and its partners offer a market-leading portfolio and ecosystem of cloud infrastructure and application services that allow you to uniquely and securely combine cloud business applications and services across multiple cloud delivery models.
    In his General Session at 16th Cloud Expo, David Shacochis, host of The Hybrid IT Files podcast and Vice President at CenturyLink, investigated three key trends of the “gigabit economy" though the story of a Fortune 500 communications company in transformation. Narrating how multi-modal hybrid IT, service automation, and agile delivery all intersect, he will cover the role of storytelling and empathy in achieving strategic alignment between the enterprise and its information technology.
    Buzzword alert: Microservices and IoT at a DevOps conference? What could possibly go wrong? In this Power Panel at DevOps Summit, moderated by Jason Bloomberg, the leading expert on architecting agility for the enterprise and president of Intellyx, panelists peeled away the buzz and discuss the important architectural principles behind implementing IoT solutions for the enterprise. As remote IoT devices and sensors become increasingly intelligent, they become part of our distributed cloud environment, and we must architect and code accordingly. At the very least, you'll have no problem fillin...