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diff --git a/website/blog/2019-11-18-security-basics.md b/website/blog/2019-11-18-security-basics.md deleted file mode 100644 index 2256ee9d5..000000000 --- a/website/blog/2019-11-18-security-basics.md +++ /dev/null @@ -1,306 +0,0 @@ -# gVisor Security Basics - Part 1 - -This blog is a space for engineers and community members to share perspectives -and deep dives on technology and design within the gVisor project. Though our -logo suggests we're in the business of space exploration (or perhaps fighting -sea monsters), we're actually in the business of sandboxing Linux containers. -When we created gVisor, we had three specific goals in mind; _container-native -security_, _resource efficiency_, and _platform portability_. To put it simply, -gVisor provides _efficient defense-in-depth for containers anywhere_. - -This post addresses gVisor's _container-native security_, specifically how -gVisor provides strong isolation between an application and the host OS. Future -posts will address _resource efficiency_ (how gVisor preserves container -benefits like fast starts, smaller snapshots, and less memory overhead than VMs) -and _platform portability_ (run gVisor wherever Linux OCI containers run). -Delivering on each of these goals requires careful security considerations and a -robust design. - -## What does "sandbox" mean? - -gVisor allows the execution of untrusted containers, preventing them from -adversely affecting the host. This means that the untrusted container is -prevented from attacking or spying on either the host kernel or any other peer -userspace processes on the host. - -For example, if you are a cloud container hosting service, running containers -from different customers on the same virtual machine means that compromises -expose customer data. Properly configured, gVisor can provide sufficient -isolation to allow different customers to run containers on the same host. There -are many aspects to the proper configuration, including limiting file and -network access, which we will discuss in future posts. - -## The cost of compromise - -gVisor was designed around the premise that any security boundary could -potentially be compromised with enough time and resources. We tried to optimize -for a solution that was as costly and time-consuming for an attacker as -possible, at every layer. - -Consequently, gVisor was built through a combination of intentional design -principles and specific technology choices that work together to provide the -security isolation needed for running hostile containers on a host. We'll dig -into it in the next section! - -# Design Principles - -gVisor was designed with some common -[secure design](https://en.wikipedia.org/wiki/Secure_by_design) principles in -mind: Defense-in-Depth, Principle of Least-Privilege, Attack Surface Reduction -and Secure-by-Default[^1]. - -In general, Design Principles outline good engineering practices, but in the -case of security, they also can be thought of as a set of tactics. In a -real-life castle, there is no single defensive feature. Rather, there are many -in combination: redundant walls, scattered draw bridges, small bottle-neck -entrances, moats, etc. - -A simplified version of the design is below -([more detailed version](/docs/))[^2]: - - - -In order to discuss design principles, the following components are important to -know: - -* runsc - binary that packages the Sentry, platform, and Gofer(s) that run - containers. runsc is the drop-in binary for running gVisor in Docker and - Kubernetes. -* Untrusted Application - container running in the sandbox. Untrusted - application/container are used interchangeably in this article. -* Platform Syscall Switcher - intercepts syscalls from the application and - passes them to the Sentry with no further handling. -* Sentry - The "application kernel" in userspace that serves the untrusted - application. Each application instance has its own Sentry. The Sentry - handles syscalls, routes I/O to gofers, and manages memory and CPU, all in - userspace. The Sentry is allowed to make limited, filtered syscalls to the - host OS. -* Gofer - a process that specifically handles different types of I/O for the - Sentry (usually disk I/O). Gofers are also allowed to make filtered syscalls - to the Host OS. -* Host OS - the actual OS on which gVisor containers are running, always some - flavor of Linux (sorry, Windows/MacOS users). - -It is important to emphasize what is being protected from the untrusted -application in this diagram: the host OS and other userspace applications. - -In this post, we are only discussing security-related features of gVisor, and -you might ask, "What about performance, compatibility and stability?" We will -cover these considerations in future posts. - -## Defense-in-Depth - -For gVisor, Defense-in-Depth means each component of the software stack trusts -the other components as little as possible. - -It may seem strange that we would want our own software components to distrust -each other. But by limiting the trust between small, discrete components, each -component is forced to defend itself against potentially malicious input. And -when you stack these components on top of each other, you can ensure that -multiple security barriers must be overcome by an attacker. - -And this leads us to how Defense-in-Depth is applied to gVisor: no single -vulnerability should compromise the host. - -In the "Attacker's Advantage / Defender's Dilemma," the defender must succeed -all the time while the attacker only needs to succeed once. Defense in Depth -inverts this principle: once the attacker successfully compromises any given -software component, they are immediately faced with needing to compromise a -subsequent, distinct layer in order to move laterally or acquire more privilege. - -For example, the untrusted container is isolated from the Sentry. The Sentry is -isolated from host I/O operations by serving those requests in separate -processes called Gofers. And both the untrusted container and its associated -Gofers are isolated from the host process that is running the sandbox. - -An additional benefit is that this generally leads to more robust and stable -software, forcing interfaces to be strictly defined and tested to ensure all -inputs are properly parsed and bounds checked. - -## Least-Privilege - -The principle of Least-Privilege implies that each software component has only -the permissions it needs to function, and no more. - -Least-Privilege is applied throughout gVisor. Each component and more -importantly, each interface between the components, is designed so that only the -minimum level of permission is required for it to perform its function. -Specifically, the closer you are to the untrusted application, the less -privilege you have. - - - -This is evident in how runsc (the drop in gVisor binary for Docker/Kubernetes) -constructs the sandbox. The Sentry has the least privilege possible (it can't -even open a file!). Gofers are only allowed file access, so even if it were -compromised, the host network would be unavailable. Only the runsc binary itself -has full access to the host OS, and even runsc's access to the host OS is often -limited through capabilities / chroot / namespacing. - -Designing a system with Defense-in-Depth and Least-Privilege in mind encourages -small, separate, single-purpose components, each with very restricted -privileges. - -## Attack Surface Reduction - -There are no bugs in unwritten code. In other words, gVisor supports a feature -if and only if it is needed to run host Linux containers. - -### Host Application/Sentry Interface: - -There are a lot of things gVisor does not need to do. For example, it does not -need to support arbitrary device drivers, nor does it need to support video -playback. By not implementing what will not be used, we avoid introducing -potential bugs in our code. - -That is not to say gVisor has limited functionality! Quite the opposite, we -analyzed what is actually needed to run Linux containers and today the Sentry -supports 237 syscalls[^3]<sup>,</sup>[^4], along with the range of critical -/proc and /dev files. However, gVisor does not support every syscall in the -Linux kernel. There are about 350 syscalls[^5] within the 5.3.11 version of the -Linux kernel, many of which do not apply to Linux containers that typically host -cloud-like workloads. For example, we don't support old versions of epoll -(epoll_ctl_old, epoll_wait_old), because they are deprecated in Linux and no -supported workloads use them. - -Furthermore, any exploited vulnerabilities in the implemented syscalls (or -Sentry code in general) only apply to gaining control of the Sentry. More on -this in a later post. - -### Sentry/Host OS Interface: - -The Sentry's interactions with the Host OS are restricted in many ways. For -instance, no syscall is "passed-through" from the untrusted application to the -host OS. All syscalls are intercepted and interpreted. In the case where the -Sentry needs to call the Host OS, we severely limit the syscalls that the Sentry -itself is allowed to make to the host kernel[^6]. - -For example, there are many file-system based attacks, where manipulation of -files or their paths, can lead to compromise of the host[^7]. As a result, the -Sentry does not allow any syscall that creates or opens a file descriptor. All -file descriptors must be donated to the sandbox. By disallowing open or creation -of file descriptors, we eliminate entire categories of these file-based attacks. - -This does not affect functionality though. For example, during startup, runsc -will donate FDs the Sentry that allow for mapping STDIN/STDOUT/STDERR to the -sandboxed application. Also the Gofer may donate an FD to the Sentry, allowing -for direct access to some files. And most files will be remotely accessed -through the Gofers, in which case no FDs are donated to the Sentry. - -The Sentry itself is only allowed access to specific -[whitelisted syscalls](https://github.com/google/gvisor/blob/master/runsc/config/config.go). -Without networking, the Sentry needs 53 host syscalls in order to function, and -with networking, it uses an additional 15[^8]. By limiting the whitelist to only -these needed syscalls, we radically reduce the amount of host OS attack surface. -If any attempts are made to call something outside the whitelist, it is -immediately blocked and the sandbox is killed by the Host OS. - -### Sentry/Gofer Interface: - -The Sentry communicates with the Gofer through a local unix domain socket (UDS) -via a version of the 9P protocol[^9]. The UDS file descriptor is passed to the -sandbox during initialization and all communication between the Sentry and Gofer -happens via 9P. We will go more into how Gofers work in future posts. - -### End Result - -So, of the 350 syscalls in the Linux kernel, the Sentry needs to implement only -237 of them to support containers. At most, the Sentry only needs to call 68 of -the host Linux syscalls. In other words, with gVisor, applications get the vast -majority (and growing) functionality of Linux containers for only 68 possible -syscalls to the Host OS. 350 syscalls to 68 is attack surface reduction. - - - -## Secure-by-default - -The default choice for a user should be safe. If users need to run a less secure -configuration of the sandbox for the sake of performance or application -compatibility, they must make the choice explicitly. - -An example of this might be a networking application that is performance -sensitive. Instead of using the safer, Go-based Netstack in the Sentry, the -untrusted container can instead use the host Linux networking stack directly. -However, this means the untrusted container will be directly interacting with -the host, without the safety benefits of the sandbox. It also means that an -attack could directly compromise the host through his path. - -These less secure configurations are **not** the default. In fact, the user must -take action to change the configuration and run in a less secure mode. -Additionally, these actions make it very obvious that a less secure -configuration is being used. - -This can be as simple as forcing a default runtime flag option to the secure -option. gVisor does this by always using its internal netstack by default. -However, for certain performance sensitive applications, we allow the usage of -the host OS networking stack, but it requires the user to actively set a -flag[^10]. - -# Technology Choices - -Technology choices for gVisor mainly involve things that will give us a security -boundary. - -At a higher level, boundaries in software might be describing a great many -things. It may be discussing the boundaries between threads, boundaries between -processes, boundaries between CPU privilege levels, and more. - -Security boundaries are interfaces that are designed and built so that entire -classes of bugs/vulnerabilities are eliminated. - -For example, the Sentry and Gofers are implemented using Go. Go was chosen for a -number of the features it provided. Go is a fast, statically-typed, compiled -language that has efficient multi-threading support, garbage collection and a -constrained set of "unsafe" operations. - -Using these features enabled safe array and pointer handling. This means entire -classes of vulnerabilities were eliminated, such as buffer overflows and -use-after-free. - -Another example is our use of very strict syscall switching to ensure that the -Sentry is always the first software component that parses and interprets the -calls being made by the untrusted container. Here is an instance where different -platforms use different solutions, but all of them share this common trait, -whether it is through the use of ptrace "a la PTRACE_ATTACH"[^11] or kvm's -ring0[^12]. - -Finally, one of the most restrictive choices was to use seccomp, to restrict the -Sentry from being able to open or create a file descriptor on the host. All file -I/O is required to go through Gofers. Preventing the opening or creation of file -descriptions eliminates whole categories of bugs around file permissions -[like this one](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-4557)[^13]. - -# To be continued - Part 2 - -In part 2 of this blog post, we will explore gVisor from an attacker's point of -view. We will use it as an opportunity to examine the specific strengths and -weaknesses of each gVisor component. - -We will also use it to introduce Google's Vulnerability Reward Program[^14], and -other ways the community can contribute to help make gVisor safe, fast and -stable. - -## Notes - -[^1]: [https://en.wikipedia.org/wiki/Secure_by_design](https://en.wikipedia.org/wiki/Secure_by_design) -[^2]: [https://gvisor.dev/docs/architecture_guide](https://gvisor.dev/docs/architecture_guide/) -[^3]: [https://github.com/google/gvisor/blob/master/pkg/sentry/syscalls/linux/linux64_amd64.go](https://github.com/google/gvisor/blob/master/pkg/sentry/syscalls/syscalls.go) - -<!-- mdformat off(mdformat formats this into multiple lines) --> -[^4]: Internally that is, it doesn't call to the Host OS to implement them, in fact that is explicitly disallowed, more on that in the future. -<!-- mdformat on --> - -[^5]: [https://elixir.bootlin.com/linux/latest/source/arch/x86/entry/syscalls/syscall_64.tbl#L345](https://elixir.bootlin.com/linux/latest/source/arch/x86/entry/syscalls/syscall_64.tbl#L345) -[^6]: [https://github.com/google/gvisor/tree/master/runsc/boot/filter](https://github.com/google/gvisor/tree/master/runsc/boot/filter) -[^7]: [https://en.wikipedia.org/wiki/Dirty_COW](https://en.wikipedia.org/wiki/Dirty_COW) -[^8]: [https://github.com/google/gvisor/blob/master/runsc/boot/config.go](https://github.com/google/gvisor/blob/master/runsc/boot/config.go) - -<!-- mdformat off(mdformat breaks this url by escaping the parenthesis) --> -[^9]: [https://en.wikipedia.org/wiki/9P_(protocol)](https://en.wikipedia.org/wiki/9P_(protocol)) -<!-- mdformat on --> - -[^10]: [https://gvisor.dev/docs/user_guide/networking/#network-passthrough](https://gvisor.dev/docs/user_guide/networking/#network-passthrough) -[^11]: [https://github.com/google/gvisor/blob/c7e901f47a09eaac56bd4813227edff016fa6bff/pkg/sentry/platform/ptrace/subprocess.go#L390](https://github.com/google/gvisor/blob/c7e901f47a09eaac56bd4813227edff016fa6bff/pkg/sentry/platform/ptrace/subprocess.go#L390) -[^12]: [https://github.com/google/gvisor/blob/c7e901f47a09eaac56bd4813227edff016fa6bff/pkg/sentry/platform/ring0/kernel_amd64.go#L182](https://github.com/google/gvisor/blob/c7e901f47a09eaac56bd4813227edff016fa6bff/pkg/sentry/platform/ring0/kernel_amd64.go#L182) -[^13]: [https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-4557](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-4557) -[^14]: [https://www.google.com/about/appsecurity/reward-program/index.html](https://www.google.com/about/appsecurity/reward-program/index.html) |