Escaping Docker container using waitid() – CVE-2017-5123
twistlock.com
twistlock.com
Obviously, giving containers unecessary CAP privileges in unwise, but if you are practicing sound security best practices then there would be multiple layers of defense between you and this CVE. I think a strong AppArmor profile and SecComp profile would also make this CVE moot.
Edit: Also, this exploit relies on you being able to fork up to a certain pid value. You can and should take advantage of Linux’s per cgroup ulimit functionality. No container needs more than 255 threads (even if they do you can make special exceptions for such applications).
Edit2: Additionally this CVE relies on the getuid syscall being available, there is no reason to give a container this syscall, you should block it, ala this guide: https://rhelblog.redhat.com/2016/10/17/secure-your-container...
I have to say I’m more than a little dissapointed in Twistlock for not pointing out what countermeasures you can employ against this and other CVEs.
It's not just a matter of enabling user ns. There is no support at the vfs layer for uid/gid mapping. This means in order to use it, images must be chowned with the remapped ID's. Per-container mappings are not supported for this reason (it would require copying and chowning the entire image for each container mapping).
Do you care to qualify your statement about CRI-O?
Unless you are a giant enterprise shop with the resources to staff a decent sized K8s team, you should use the hosted solutions.
For example the work that was done on their seccomp filtering and apparmor profiles.
My guess would be that it's as User namespacing can introduce some issues (e.g. where mounting host filesystems), that they've decided the trade-offs aren't worth it.
Also looking at the CRI-O trello (https://trello.com/c/Ak2yMcpf/714-epic-cri-o-support-for-use...) it seems like user namespaces isn't even an option there yet?
On the other hand the container runtime is straightforward. I recently discovered that one can run a docker image with a bash script and the unshare command and get a very tight security setup. That explains proliferation of various alternatives to Docker to run its images.
Disclaimer: I worked at Docker for 3 years.
I just enabled user namespaces after reading this post. Broke Jenkins and there doesn't appear to be an easy solution. I mount the docker socket in the Jenkins container, which is not an option with user namespaces as the user Jenkins now runs as does not have permission to access the socket.
It seems to be possible to provide this user access to the socket through a socket proxy, but since all containers use the same user this seems to defeat the purpose of using namespaces in the first place.
Cherry on top: although `docker run` supports running containers with custom userns settings, docker swarm, which I use to run Jenkins, does not.
So as far as I can tell my only options are:
1. Go back to not using user namespaces 2. Make the docker daemon on the host available over HTTP, which is really something I was trying to avoid...
Anyone have a more elegant solution?
We're working on a solution that would please most people for docker containers and services called the Docker Entitlements: https://github.com/moby/libentitlement
These Entitlements are high-level privileges for containers and services that could be baked in images, same way as macOS/iOS apps. These permissions would allow to create custom {seccomp+capabilities+namespaces+apparmor+...} profiles (effectively security profiles) for a better granularity in app sandbox configuration by app developers and ops.
The current POC has `docker run`, `docker service create` and even build mechanism working. The integration is actively being worked on and PRs are being prepared.
The issue you mentioned is already opened here: https://github.com/moby/libentitlement/issues/44
Feel free to have a look at it and open issues/participate or reach out through Github as I'm the lead and would love to discuss use-cases :)
In the mean time, I make sure that all my containers runs as non-root with max security restrictions. The exception so far was sshd from OpenSSH and mostly due to incorrect porting from OpenBSD in portable ssh.
This would obviously only work for minimal containers (i.e. ones that don't contain a distribution), but software has to be pretty much built for such a case (e.g. statically linked, no dependencies on common tooling — popular with Go, but your Python application won't work edit: unless you copy all the layers, that is).
You can read the docs here: https://docs.docker.com/engine/security/userns-remap/#prereq..., and note that it stores image/container layers in subdirectories under /var/lib/docker.
Tl;dr: user namespaces are inherently incompatible with many of the usability features Docker brings over other solutions, while they're not particularly useful for many popular use cases (no shared hosting, minor differences in consequence between escalating to the root of the container and its host - though that's an assumption frequently wrongly made).
The gist is that a container is further sandboxed by the kernel that is agnostic of the higher level security precautions. It’s not perfect by itself, but used in conjunction with other features like AppArmor or SELinux and SecComp it can make a container virtually sandboxed.
The container that wasn't!
(I get the gist of it, just tongue in cheek)
Huh? Lots of legitimate things will break without working getuid().
> you should block it, ala this guide
getuid() doesn't require any capabilities, so it can't be blocked by taking them away.
Doing it correctly should be the case using the default settings. Defaulting to an insecure setup is a bug.
There's a reason most users choose Ubuntu over OpenBSD as their workstation. I would put good money on the reason is because it's "secure enough" without getting too heavy handed on production use cases.
However, I do agree that there has to be a balance. Most tooling I write tends to lean more towards the "good user experience" side first, and then document the production use case. Either that or release two separate (but similar) products; one for developers, one for operations teams. Docker's doing that with the Community Edition/Enterprise Edition, but I still think the Community Edition is far too heavy-handed when it comes to things like pulling images from "insecure" registries.
The problem is a lot of these insecure defaults will be rolled to production by a "developer" only to get hacked later. Because the marketing and friction-less dev is more important then sane default security
> Over a quarter of MongoDB databases left open to the internet have been ransacked by online extortionists.[0]
They "forgot"/"did not know" the password is not set. This was for db tech.
Now you want your average dev, who is getting "forced" to use docker more and more. To know how to setup container tech secure?
But hey, I need to get this on my CV and it is just click click install. So what can go wrong?
[0]: http://www.zdnet.com/article/mongodb-ransacked-now-27000-dat...
User namespacing in docker is enabled at the daemon level, not per container, so all containers share the same offset. This would ensure that a root user in the container would escape to a different uid on the host, but doesn't prevent someone from moving sideways through the containers on the same host.
Note that enabling this will break the developer workflow of mounting files from the host into the container. I believe files will show up with the wrong ownership inside the container.
The author devised a particular exploit, but his example was hardly the only way to leverage the vulnerability. Being able to write to kernel memory is about as huge a vulnerability as you can get. Just because you can't think of a way to leverage a vulnerability doesn't mean an attacker can't; your failure of imagination is not evidence that it cannot be done.
2) Plenty of containers need more than 255 threads. Like, pretty much any Java server. In any event, this particular exploit doesn't necessarily require hundreds or thousands of simultaneous processes.
3) Blocking getuid is even worse than blocking waitid. Block getuid and you'll break glibc and god knows what. In any event, it would be futile as the real and effective UIDs are passed to the process through the auxiliary process vector when the kernel executes the process.
4) You're missing the forest for the trees. The real moral of the story is this: "In 2017 alone, 434 linux kernel exploits where found". Unless you're prepared to pour over every published exploit, 24/7, meticulously devise countermeasures, and be prepared to run effectively crippled software, you really shouldn't be relying on containers to isolate high-value assets. I wouldn't rely on VMs, either, as the driver infrastructure of hypervisors has also proven fertile ground for exploits.
I think your being overly alarmist. You have to trust someone else’s code at some point, otherwise you’ll be paralyzed by non-productivity.
As I understand it, a kuid_t is the UID in the root namespace, so setting your cred->uid to 0 gets you considered as equivalent to root in the container host.
Also, don't think that limited exposure to kernel memory saves you - take a look at the sudo "vudo" exploit from 2001, in which a single byte that was erroneously overwritten with 0, and then put back, turned out to be exploitable. http://phrack.org/issues/57/8.html (And in general, don't confuse the lack of public existence of an exploit with a proof that a thing isn't exploitable in a certain way.)
> Also I think your confusing Java threads for system threads they are not the same.
Current versions of the HotSpot JVM (where by "current" I mean "since about 1.1") create one OS thread per Java thread: http://openjdk.java.net/groups/hotspot/docs/RuntimeOverview.... "The basic threading model in Hotspot is a 1:1 mapping between Java threads (an instance of java.lang.Thread) and native operating system threads. The native thread is created when the Java thread is started, and is reclaimed once it terminates." Plus there are some other OS threads for the runtime itself.
> I think your being overly alarmist. You have to trust someone else’s code at some point, otherwise you’ll be paralyzed by non-productivity.
Sure, but you can choose which code to trust, and how to structure your systems to take advantage of the code you trust and not the code you don't. Putting mutually-distrusted things on physically separate Linux machines on the same network is a pretty good architecture: I trust that the Linux kernel is relatively low on CVEs that let TCP packets from a remote machine overwrite kernel memory.
You know, you say that...
Oh? On mainstream JVMs, a Java thread is the same as the thing you could create with pthreads. What do you mean by "system threads"?
The critical vulnerability is that the cred pointer address is entirely under your control, so you get to poke at whatever kernel memory you want. The limitation is 1) locating the address of what you want to poke, and 2) being limited to a smallish ranges of values that you can write out.
Also, I'm not confusing Java threads with system threads. Most JVMs use a 1:1 threading model. And because on Linux a thread is just a process (which unfortunately still causes headaches with things like signals, setuid, etc), each thread has its own PID.
I'm not being alarmist, just realistic. Nobody is going to stop using Linux anytime soon. Nor am I. But the fact of the matter is that the Linux kernel is riddled with vulnerabilities. Something like the waitid vulnerability comes along at least 3 or 4 times a year, and that's just the published ones. (IMO, part of the reason is precisely because of complex features like user namespaces, which add tremendous complexity to the kernel. But that's a contentious point.)
At least for high-value assets (however you want to define that), people should just treat Linux as if it lacks secure process isolation entirely, absent a commitment to herculean efforts--extremely locked down seccomp, PNaCL-like sandboxing, etc for all your code that juggles tainted data. Even then, vulnerabilities like recvmmsg come along and ruin your day, but those are rare enough that it would be unfair to single-out Linux.
Not only is that pragmatic and reasonable, after 25 years of endless vulnerabilities of this sort I wouldn't trust the judgment of anyone who thought otherwise. And for what it's worth, I'd make much the same point about Windows, although I have much less experience on that platform.
Can you state what OS/software you believe has secure process isolation?
Would one of the BSDs have it or would I have to use some OS that's more exotic to run a process in isolation?
More importantly, while nowhere near as bad as Linux, macOS, or Windows, OpenBSD has at least one published severe local kernel vulnerability every year or two. In many cases those OpenBSD boxen I mentioned survived _despite_ being neglected by IT and not kept up-to-date; I know for a fact some were in a known, locally exploitable state for a not insignificant period of time. That makes me think a big part of their relative security is simply related to OpenBSD not being a common target of rootkits and worms. I have little doubt a sophisticated attacker could root an OpenBSD box from the shell, for example, if he was targeting that box. (My rule of thumb when isolating services is that anything running a web service using a non-trivial framework (PHP, NodeJS, etc) provides at least the equivalent to shell-level access to a targeted attacker. Among other things, that means even if I'm writing a privilege separated, locked-down, formally-verified backend daemon, I assume as a general rule that any data it's trying to protect isn't safe from that front-end web application unless it's running on separate hardware.)
While I don't think that security and convenience are necessarily mutually exclusive, as a practical matter they are largely mutually exclusive. Unless you're prepared to accept the burden and cost of using a specialized OS like seL4--and in particular use it in a way that preserves and leverages the stronger security guarantees--your best bet is simply to use separate servers when you want a significant degree of isolation assurance. Separate hardware is not sufficient (if all your boxes have Intel ME cards, or have firmware pushed from a puppet server, or share an engineer's account whose hacked desktop is logging SSH keys, passwords, and Yubikey PINs), but it's largely necessary. This is true whether you're concerned with targeted or opportunistic attacks, but _especially_ opportunistic attacks, which are by far the most common and, in many respects, an important element to targeted attacks.
Separate hardware is a uniquely simple, high-dividend solution. But the point is to be realistic about the actual robustness of your solutions, to be able to easily identify and mitigate risks, so you can make more informed security decisions. And it all depends on what you're protecting and what sort of investment you're capable of making. Just endeavor to predicate your decisions on accurate and informed assessments. Among other things, that means being honest about and understanding your own lack of knowledge and experience. Similarly, continuity and long-term maintenance are critical to security, which means you need to be honest about institutional capabilities (or for a private server, what you're prepared to track and upgrade 3 years from now.)
Linux, OpenBSD, co-location, and cloud hosting can all be part of a perfectly robust strategy. And HSMs probably should be, too, which is basically just a way to attach a tiny, isolated, dedicated piece of hardware to a computer. But none of these options alone are likely to be reasonable choices, all things considered, especially in the organizational context.
I can confirm hundreds of threads for networked Java applications is normal behaviour.
This exploit relies on it. The vulnerability does not. The exploit happens to use getuid() along the way to using heap spraying, but the writeup is pretty clear that neither getuid() nor heap spraying is required.
The problem with MAC schemes is that, in practice, they lead to security people imposing random and arbitrary restrictions on general APIs in the name of the least privilege. In doing so, they break the orthogonality of general-purpose platform concepts and break the reductive mental model necessary to get anything done. It's a misunderstanding of what least privilege actually means.
Security is better achieved by creating clear, principled security domains and boundaries, then controlling access to these domains in a general and transparent way. Saying "you, unix process, you can call system call X, but not system call Y, because in my opinion, Y is risky", when neither X nor Y breaks through a security domain, is bad practice. So is arbitrarily capping the number of threads in a container.
> CVE-2017-5123 was published earlier this year on Oct 12 — it was a Linux kernel vulnerability in the waitid() syscall for 4.12-4.13 kernel versions.
Does this mean that kernel versions prior to 4.12 are not affected? That's what I understood from the related issue in the bug tracker https://bugzilla.redhat.com/show_bug.cgi?id=1500094
By the way, this is very important:
> In 2017 alone, 434 linux kernel exploits where found, and as you have seen in this post, kernel exploits can be devastating for containerized environments. This is because containers share the same kernel as the host, thus trusting the built-in protection mechanisms alone isn’t sufficient. Make sure your kernel is always updated on all of your production hosts.
Great article!
https://elixir.free-electrons.com/linux/v4.11/source/kernel/...
The code is significantly different but I still see a lack of access_ok(), so was the checking performed somewhere else that I didn't notice (I haven't looked closely at this part of the kernel before)?
It was patched in 4.13.7 after being introduced in the 4.13.0 merge window.
See https://lwn.net/Articles/736348/
This issue shouldn't have happened at all, but it was caught and patched very quickly, so relatively few real-world systems are or were affected.
which by itself is in a 4.12 vanilla code tree
https://github.com/torvalds/linux/commit/4c48abe91be03d191d0... (click the little '...' to expand tags it's in) or:
$ git tag --contains 4c48abe91be03d191d0c20cc755877da2cb35622
v4.13
And in order to avoid any zero-day exploits, always use dedicated machines, never use a VPS server.
- A vulnerability is a sofware bug that has particular behaviors and ramifications that allow it to be used maliciously.
- An exploit is a crafted piece of input data that is designed to trigger a vulnerability to execute arbitrary code, crash the target (Denial-of-Service), etc.
> In 2017 alone, 434 linux kernel exploits where found, and as you have seen in this post, kernel exploits can be devastating for containerized environments.
There are a few places in the article like this one where the correct terminology is vulnerability not exploit. cvedetails.com aggregates vulnerabilities. Places like exploit-db.com aggregate exploits people have written to take advantage of vulnerabilities to enable them to perform some unintended action against the target.
Edit: formatting
But to answer the spirit your question, each container runtime uses its own peculiar combination of such constructs. It's helpful to know that this attack allows you to break out of the combination used specifically by runc, and thereby to break out of any system relying on Docker (with the default runc execution driver.)
The author shows a concrete exploit of the kernel bug described in CVE-2017-5123 as he has developed it in the context of the docker container environment.
He shows how to use this bug to break out of docker, so he calls the blog post "Escaping Docker ...".
Which is IMHO the most interesting container runtime to write such an exploit for first because it is very widely deployed, but it might also just have been what the author is most familiar with or what was easiest to develop for him.
How would you reasonably talk about "Linux containers" without having a very exhaustive list of all existing implementations and testing all of them? If one of them is not affected you are now factually wrong.
Let's make it realistic and say he had used RedHat OpenShift as his target and example for the exploit. I'd be completely fine with the title referencing that exact product by name.
Why would he have to dance around what he is using in his demo? Maybe that concrete product has multiple layers of security or lacks them, or uses a certain version etc. He can only speak to what he himself was using and testing. "Escaping Docker container..." is the best short description (as you would need it for a title) of this demo exploit I can think of.
There's a long tradition by enterprise vendors large and small to market someone else's product as insecure, in order to create demand for their "improved, secured" version.
In this particular instance, Twistlock is selling Docker security by amplifying the meme of "insecure Docker". The Docker brand has visibility the target audience (Enterprise IT) so it's a good target for this kind of piggyback.
This type of FUD marketing happens all the time in many different markets, it's not specific to Docker.
In most use cases I see containers used for rapid and consistent deployment. The isolation benefit with multiple containers on a host is that if you install things with different library dependencies you don't run into conflicts. As such, the comparison for the common use case is just software installed directly on the host, which also is subject to this vuln.
More than one kernel exploit _per day_. Exploiting Linux is just a matter of finding one such vulnerability and using it. This can be done in a single day.
There's just no fixing megabytes of buggy kernel code.
It really drives home the need for a proper OS based on a verified, capability-enabled microkernel such as seL4.
Only if you're lucky. Most of these exploits probably took weeks to find and analyze properly, it's not like one person found more than one a day. They're found because whole teams are working with the linux kernel at the same time and either happen by them or actively look for them.
Linus is not entirely crazy. The Windows kernel SDK only supported C++ in the last decade and it has a lot of limitations.
As for stubbornness, C can be used safely with proper discipline. Kernel development does require a certain amount of experience and discipline, so arguably C can be used by kernel developers in mostly safe manner. That's why some view it as a feature: if you don't have the required discipline then just stick to userspace development.
I'm not sure I understand what interface you're referring to that was "unsafe" and subsequently "used in an unsafe manner". What is the "unsafe interface" here that was being used in an unsafe manner? It seems to me that the problem was that the pointer was not marked as __user? Which is awful, because shouldn't __user be the implicit default behavior for a pointer argument to a syscall? Why should the default behavior be the unsafe one you pretty much never want?
Before using a pointer to user-space one should check if access_ok() to it. The usual safe interfaces — copy_{to,from}_user(), put_user(), get_user() — always perform this check and fail with an error if the pointer is not an okay user-space pointer.
The commit that introduced the vulnerability [2] replaced the safe interface with unsafe ones, possibly for performance improvements. The code used put_user() function to set individual fields of a struct. Multiple calls to put_user() were replaced with multiple calls to unsafe_put_user() which does not perform access_ok() check every time. A check for NULL pointer was added before the stores. unsafe_put_user() still checks whether the address points to an actually mapped memory location, but does not verify whether the location is in user-space.
The commit was not really discussed in-depth on LKML [3] as it came from Al Viro who should know better, is one of the Sparse maintainers, etc. Some projects require human justifications for any usage of unsafe interfaces during code review (like, flagging a review with 'needs-check' or something that requires a sign-off by another human that the unsafe thing is actually safe). This may have been the case where it could matter, as the static analysis tool should not produce bogus warnings for interfaces which are designed to perform unsafe stuff. Though, it may also be useful to add a check to Sparse which will verify that unsafe_{get,put}_user() calls are preceded by an access_ok() call in the same function.
[1]: http://elixir.free-electrons.com/linux/v4.13.6/source/kernel... [2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin... [3]: https://lkml.org/lkml/2017/5/15/896
Why in the world does this class of vulnerabilities still exist in 2017? Why are kernel maintainers not writing some kind of C linter that makes sure every single pointer argument to every syscall is passed to a well-known function like access_ok (Linux) or ProbeForRead (Windows)? Literally all you need is a syntactic check; you don't even need to do any kind of semantic analysis... since all you want is to flag the code so someone can inspect each spot manually. Why is this not done?!
The generated machine code would be identical to what's in the kernel today, but it'd be both safer and cleaner. C++ still has to get over the bad gang-of-four-1990s-era-object-goo reputation it has among systems people.
Funny you mention this... https://news.ycombinator.com/item?id=16032324
Is there actually a counterpoint for this? Who is saying that containerization can be used for isolation?
if (!access_ok(VERIFY_WRITE, infop, sizeof(*infop)))
goto Efault;
Secondly, why does it use goto at all? I thought it has also been established not to use goto unless it is the only performant solution (and performance is important in that case). Sure Efault with probably kill the program but wouldn’t it still be better to use a function call considering that the desired resolution should be the same?
[1]: https://www.imperialviolet.org/2014/02/22/applebug.html
https://www.kernel.org/doc/html/v4.10/process/coding-style.h...
> Do not unnecessarily use braces where a single statement will do.
2. There is no built-in exception handling in C. `goto ERROR_HANDLING_CODE` is a common and well established pattern to handle exceptions in C, see e.g. [2].
[1] https://www.kernel.org/doc/html/v4.10/process/coding-style.h... [2] https://news.ycombinator.com/item?id=3883310
“Established” by whom? Certainly not by kernel developers — `goto` is very common in all kernels I have looked at (xnu, Linux, bsd)
By the way, gcc now has -Wmisleading-indentation, which is activated by default if you enable -Wall: https://news.ycombinator.com/item?id=10875449
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...