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Ubuntu Security Podcast

Episode 152

28 min • 11 mars 2022

Overview

It’s a big week for kernel security vulnerabilities - we cover Dirty Pipe and fixes for the latest microarchitectural side channel issues, plus we bring you the first in a 3 part series on hardening your Ubuntu systems against malicious attackers.

This week in Ubuntu Security Updates

34 unique CVEs addressed

[USN-5312-1] HAProxy vulnerability [00:46]

  • 1 CVEs addressed in Focal (20.04 LTS), Impish (21.10)
  • CPU based DoS via the Set-Cookie2 header - obsolete HTTP response header used to send cookies from the server to the user - possible infinite loop when parsing responses which contained this header

[USN-5300-2, USN-5300-3] PHP vulnerabilities [01:24]

[USN-5311-1] containerd vulnerability [01:41]

  • 1 CVEs addressed in Bionic (18.04 LTS), Focal (20.04 LTS), Impish (21.10)
  • Able to access read-only copies of files from the host via specially crafted container image

[USN-5314-1] Firefox vulnerabilities [02:11]

  • 2 CVEs addressed in Bionic (18.04 LTS), Focal (20.04 LTS), Impish (21.10)
  • 2 critical impact (as defined by Mozilla) vulns - both UAFs
  • Mozilla reported seeing reports of both being exploited in the wild

[USN-5313-1] OpenJDK vulnerabilities [02:36]

[USN-5310-2] GNU C Library vulnerabilities [02:56]

[USN-5316-1] Redis vulnerability [03:09]

  • 1 CVEs addressed in Focal (20.04 LTS), Impish (21.10)
  • Redis contains a scripting interface using Lua and implements a sandbox for this to try and avoid scripts running arbitrary Lua code
  • Upstream has a vendored copy of lua but in Ubuntu + Debian the redis package links against the system installed liblua
    • this is a good thing as it means that when say a vuln appears in Lua itself we only have to patch Lua to fix other applications like redis whereas otherwise we would also have to patch the embedded/vendored copy of lua in redis and release a redis update for every Lua vulnerability as well
  • As such we also want it to use the system lua libs of cjson and bitop
  • Included a small custom piece of code to have it load those instead of the ones that would usually be shipped in redis itself
  • Discovered that this shim code failed to set the package variable and as such left this global variable uninitialised - an attacker with the ability to execute a Lua script could then cause Lua to load the full system liblua unsandboxed and hence then use this to execute other arbitrary commands on the host
  • Note in general it doesn’t look like upstream Redis consider the existing sandbox to be a security boundary so recommend to only give trusted users the permission to EVAL Lua in redis

[USN-5317-1] Linux kernel vulnerabilities [05:34]

  • 5 CVEs addressed in Focal (20.04 LTS), Impish (21.10)
  • Thanks to Thadeu Cascardo from the kernel team for coordinating all the work on these fixes
  • https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/DirtyPipe
    • https://dirtypipe.cm4all.com/
    • Similar to “Dirty Cow” but easier to exploit - one of the more high profile vulnerabilities in recent times - due to mishandling of the page cache within the Linux kernel, a malicious process could abuse the pipe and splice system calls to cause the kernel to overwrite contents of arbitrary files even when a user had no write permission to the particular file (even on immutable and RO-filesystems)
    • Very simple error due to the failure to initialize the flags element within a pipe buffer when handling pipe data within the kernel - fix is 2 lines of code to initialise this to 0
    • Flaw exists back to 4.9 but this is thought only to be exploitable since the 5.8 kernel which refactored this code
    • As such for now have patched for the 5.13 kernels in 21.10 + 20.04 LTS but will also patch in the future during regular development cycle for the older kernels like 5.4 in 20.04 LTS as well as an additional hardening measure
  • https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/BHI
    • Latest set of hardware microarchitectural issues - in the same vein as the original Spectre flaws from Jan 2018 (4 years ago!)
    • Set of vulnerabilities affecting both Intel and ARM processors which can allow unprivileged user to leak (read) memory from kernel / other applications
    • Requires the ability to execute a “gadget” in the kernel to do the speculative execution - and the only way known to get one of these is to inject it as BPF code
    • As a result this update also disabled unprivileged eBPF loading as well to close off this attack vector

[USN-5318-1] Linux kernel vulnerabilities

[USN-5319-1] Linux kernel vulnerabilities

Goings on in Ubuntu Security Community

Camila discusses Ubuntu hardening (part 1) [10:20]

  • In response to a topic on discourse.ubuntu.com re Basic security advice for running your own server - Camila has prepared a 3 part series on steps you can take to harden your Ubuntu machines against attack - part 1 focuses on hardening at install time, part 2 is post install steps and part 3 looks at additional measures you can take once the machine is deployed - today we bring you the first part in this series

Transcript

Hello listener! Welcome to another episode of the Ubuntu Security Podcast where I, Camila, will be talking to you all more about one subject or another involving the Ubuntu Linux distribution and cyber security in general. Today’s episode is a response to a request. A request from someone that wants to learn more about how it is possible to create an Ubuntu system, which will be running some type of service, in a secure manner. After all, we do live in times where threats that were only physical have migrated to the digital world as well, so just having a server set up with all ports open and no access control set is no longer an option for those that wish to use the almighty Internet to provide some type of service. Heck! The concern should exist even if you don’t wish to have an Internet facing server, but simply if you own a computer…or a smartphone…or a smart TV…or a car. Or anything really. We are all connected by our WiFis, whether we want it or not, so taking care of our own digital perimeter has become something essential, and something that we all should be applying to not get spammed nor scammed in the days of today. So, since I do love me some lists, let’s talk about, in a chronological list format, what measures you can apply to your Ubuntu OS and what tools can you use in this same OS to make it safer, hardened against the cold and harsh wave of 0’s and 1’s that might be traveling out there through fiber optics cables just waiting to hack into your system.

Let’s start with the basics and talk about what can be done with the tools that you already have when you have an Ubuntu Linux coming fresh out of the bootable USB stick you used to format your computer. Actually, if we are indeed doing this, let’s do it for real: we will go back even further, and talk about the basics that can be done not only after a fresh install, but also, while you are installing your system. Let’s get prepared for the Ubuntu big bang and talk about what needs to happen before our binary universe can start to exist and securely function inside our CPUs and hard drives.

During an Ubuntu install you will make a few choices, such as whether or not you want to encrypt data in your disk. If you are not the one installing your own system, and you have an already running basic Ubuntu system in a cloud service platform, for example, this might not be something possible for you. However, if you do have the chance to apply this, it is a hardening measure that can be used to protect all data being saved in your hard drive. Of course, we need to consider that not all situations might fit well with this, as, for example, a server that forces the system to reboot frequently would require a password every time at system startup, something that one might not want to do, or be available to do, every single time, specially considering a situation where a completely automated system is the main goal to be achieved here. It is also important to consider that encrypting your hard drive might affect general file I/O performance, since data being read from the disk needs to be decrypted everytime, before being presented to the user or to the system for further processing, and data that will be written to the system needs to be encrypted before it is sent permanently to the hard drive. However, if none of those cases concerns you at all, the question here might be: why NOT encrypt your hard drive? If your hardware allows it, making the process fast, it might even be worth it despite the delay you can have due to the necessary encryption and decryption operations being performed. Either way, your data can be protected from those that might want to access it without authorization. Do not kid yourself by thinking that hackers will always stay behind a screen, as there are the very bold who might just think that by stealing your hard drive they will get what they need. Without a password, though, hackers can connect the disk to whatever computer they like, but the data will remain encoded and unreadable. Remember though, full disk encryption will NOT protect data in transit, also known as data you sent through the wires or through the air, via the World Wide Web, to other devices around the world. Disk encryption, as the name suggests, is local to the disk which is associated with your own device. Oh, also do be aware that the password that is used to encrypt the disk cannot be lost or else, you might be your own worst enemy and lose your data which becomes nearly impossible to crack cipher text.

Still talking about disk configurations during the installation process, do consider creating a swap partition when setting up your system. The swap partition is essentially used by the Ubuntu System as if it were RAM. Therefore, if your RAM is filled up completely, the swap partition, which is actually a part of the hard drive, will be used rather than the RAM memory space to perform operations. necessary A swap partition can also be used to make more RAM space available during a certain point in processing time, said space being provided for data that is more relevant or is being used more frequently. Data that is being less used, less referenced, can therefore be moved to swap space instead of being left in the ever busy, constantly used RAM. The swap will act as an extension of your RAM, but do note, it is not as efficient as RAM, since it is actually your hard drive pretending to be something that it is not: a volatile memory device. Setting up a swap partition, however, can be very useful to increase performance in your server. As previously mentioned, swap space can be used to store data that is not all that frequently accessed, opening up space in RAM for more regularly accessed information. Since data in the swap is not being used constantly, the delay you would have when performing I/O operations on it becomes less of an issue, and you essentially gain more RAM space to process whatever your server needs to process. And, you know, even if people do forget it sometimes, remembering about it only when they suffer a massive DoS attack, availability is one of the 3 pillars of cyber security, so preparing for that in order to guarantee a system with better performance is valuable. Another big advantage of having swap lies in the fact that you as a system administrator might have more time to react to possible memory issues when your server is facing them. When you run out of memory and you don’t have swap, you risk having your system suddenly crash and not only losing all data that was in RAM, but having your service be out of reach for whoever knows how long. You can also have OOM killer go and kill your most important process because you are…running out of memory…and it doesn’t even have the courtesy of asking you if you are ok with it. Just rude! If you set up your swap space to at least the size of your largest process though and you monitor your system, you are able to detect possible issues by analyzing swap space usage, and then you can most likely avoid many undesired service and system crashes. However, do not forget: setting swap can boost your system performance, as it can hinder it if you don’t implement things correctly. Your main volatile memory source should be your RAM, and the swap partition will not be a substitute for it. Therefore, if you have little RAM and over encumber your system, you won’t make it any faster by using swap, as the hard drive will be used to process that overflowing amount of data that should be being processed primarily by your RAM. The idea is to use swap as a complimentary performance measure to your appropriate RAM sized system. If using swap memory, don’t forget to configure how this extra memory space will be used together with your RAM, by setting the ‘swapiness’ metric, for example, which will tell the kernel how aggressively it will swap memory pages in the system when necessary. Once again, setting too much of a high value might make your system inefficient as you start making your kernel believe that the harddrive is actually RAM - the perfect disguise - but setting a low value might also not give you the best performance possible. Each case will be its own, so know your system and your needs, and act accordingly.

Our install happens on our disk, so, unfortunately I must tell you that once again we will be checking out disk settings we can consider when creating our hardened Ubuntu server. Cheers to our disks! Installing all of the system in one single partition tends to be a lot easier and a lot faster. However, we are not looking for easy here, we are looking for secure, so let’s get out of the one single partition and out of our comfort zones and possibly separate our system directories into different partitions. Having /boot in a separate partition is useful to avoid not being able to log into a system after the current kernel image has run across issues. The backup kernel images will be available and you might be able to do a quicker recovery that won’t require connecting an external device in, or removing your own in order to fix what has been broken in the OS. In case you encrypt your / (root) partition, you will need to perform this regardless, or else, your OS won’t boot. Encrypted code might be cool looking but it’s not exactly functional considering a situation where you need to know what are the basic instructions that will allow you to get the operating system up and running. Encrypting /boot together with / (root) would be the same as hearing the “ready, set, go” at a car race and staying stuck in place because you just remembered you put a boot in your wheel. The locked boot is stopping you from moving the car forward and getting it where you need it to be, and, considering /boot outside of the analogy, it’s stopping you from getting your computer to execute your operating system because it’s encrypted. Therefore, if you encrypt your hard drive, as previously suggested, you already get to escape from the old boring one partition scheme. That being said with this very convenient analogy, let’s get back to it and discuss the other partitioning options you might have and that you can apply to your system in order to make it more efficient and more secure, options which include, for example, putting /tmp in a separate partition. This is most likely a good call, especially considering that world-writable /tmp is a common target for attackers. Servers that might use /tmp for storage of, as the name suggests, temporary files could cause a self DoS in case this directory is filled up with various large files. If the directory is in a different partition, however, only that specific partition will fill up and not the entire system storage instead. Other processes using other directories in your system are unaffected and only the process filling up /tmp is terminated. It is also a lot easier to manage a filled up /tmp partition than it is the entire system. Plus, different permissions can be set for this specific partition later on, but we will discuss this soon enough, albeit not now. Separating the /home and the /var directories from the rest of the system also shares these advantages. Leaving these directories in their own separate “drawers” inside the closet that is our hard drive might be an interesting choice in order to avoid necessary space to be taken up by a file that might not be essential for the workings of the server. The /home directory will contain user files, and we don’t trust users, and the /var directory might get filled up completely with a huge amount of logs, for example. Filling up the logs might be an attack of choice made by some hacker out there, but if you created a separate partition, you were prepared for it. Having smaller partitions also makes for faster file searches in the system, which might be a valid performance boost for your IT infrastructure. If you plan to share resources through the network, have these resources be connected to a directory mounted in a separate partition, as you can have more permissive access control rules in the shared partition, but keep the rigorous one in all others that might contain sensitive information, which is in itself another advantage: different partitions, different permissions during mount time. However, we will go into more detail about this later on, as I already mentioned. The point here is: separate partitions are separate filesystems, and, therefore, the OS will not behave in the same way as it would if all data were to be stored under a single partition…a single filesystem. All of that being said, it will require more management than a system that has only one partition, and space usage might not be the most efficient when you establish limits to each directory. However, if it is feasible for your needs, it might be a good way to avoid some issues…security issues.

Up next, I say this everytime and I will never get tired of saying it: strong passwords, people! Strong passwords! Whenever creating the first user for your Ubuntu system, which will happen during the install process, do not use your birthday as your password. Or your dog’s name…or any 6 letter word followed by the digits that are the current year. Easy to remember, easy to hack. The first step to avoid being hacked is not wanting to be hacked, and forgive you me if I am being too blunt, but setting up lazy passwords and not expecting it to be a problem is like eating rotten food and expecting to not get sick: you can wish all you want, but the outcome will not be positive for you, my friend…and to your closest loved ones involved. So…strong passwords, please, and non-expired food.

Our system is installed. BIG. BANG. Our Ubuntu OS universe now exists after we set everything up so that it looks just right for our security needs. All is not done, however, since after the big-bang, the galaxy and more specifically Earth, had to go through a lot of steps before it was ready to host life, which is our main goal here: host life in the form of executable, network service providing code. We now have galaxies, stars, planets, and all necessary to maybe create life in the future, but first things first, we need our huge ball of fire to be tweaked a little bit, since life as we know it will not be born in such an unsafe, or might I say, insecure environment. Let’s then make it secure so that we can start thinking about giving it some life, or, in our case, installing some software, developing customized code, setting up frameworks, all that good stuff that makes developers go crazy with excitement.

I will, however, keep you on your toes, and continue talking more about this subject in another episode only! So stay tuned to the podcast to continue on this Ubuntu hardening journey with me, and while you wait for what is to come, feel free to share your thoughts in any of our social media platforms, as your opinion is always welcome! I await your return to the podcast in the following weeks so that we can once again share information, but for now I bid you all farewell and until next time! Bye!

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