Merge pull request #2513 from amscanne:website-integrated

PiperOrigin-RevId: 311184385

1 files changed, 319 insertions, 0 deletions

diff --git a/website/blog/2019-11-18-security-basics.md b/website/blog/2019-11-18-security-basics.md
new file mode 100644
index 000000000..ed6d97ffe
--- /dev/null
+++ b/website/blog/2019-11-18-security-basics.md
@@ -0,0 +1,319 @@
+# 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 principles](https://www.owasp.org/index.php/Security_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/architecture_guide/))[^2]:
+
+--------------------------------------------------------------------------------
+
+![Figure 1](/assets/images/2019-11-18-security-basics-figure1.png)
+
+Figure 1: Simplified design of gVisor.
+
+--------------------------------------------------------------------------------
+
+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.
+
+--------------------------------------------------------------------------------
+
+![Figure 2](/assets/images/2019-11-18-security-basics-figure2.png)
+
+Figure 2: runsc components and their privileges.
+
+--------------------------------------------------------------------------------
+
+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/boot/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.
+
+--------------------------------------------------------------------------------
+
+![Figure 3](/assets/images/2019-11-18-security-basics-figure3.png)
+
+Figure 3: Reduction of Attack Surface of the Syscall Table. Note that the
+Senty's Syscall Emulation Layer keeps the Containerized Process from ever
+calling the Host OS.
+
+--------------------------------------------------------------------------------
+
+## 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://www.owasp.org/index.php/Security_by_Design_Principles](https://www.owasp.org/index.php/Security_by_Design_Principles)
+[^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)
+[^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.
+[^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)
+[^9]: [https://en.wikipedia.org/wiki/9P_(protocol)](https://en.wikipedia.org/wiki/9P_\(protocol\))
+[^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)