Security Model
gVisor was created in order to provide additional defense against the exploitation of kernel bugs by untrusted userspace code. In order to understand how gVisor achieves this goal, it is first necessary to understand the basic threat model.
Threats: The Anatomy of an Exploit
An exploit takes advantage of a software or hardware bug in order to escalate privileges, gain access to privileged data, or disrupt services. All of the possible interactions that a malicious application can have with the rest of the system (attack vectors) define the attack surface. We categorize these attack vectors into several common classes.
System API
An operating system or hypervisor exposes an abstract System API in the form of system calls and traps. This API may be documented and stable, as with Linux, or it may be abstracted behind a library, as with Windows (i.e. win32.dll or ntdll.dll). The System API includes all standard interfaces that application code uses to interact with the system. This includes high-level abstractions that are derived from low-level system calls, such as system files, sockets and namespaces.
Although the System API is exposed to applications by design, bugs and race conditions within the kernel or hypervisor may occasionally be exploitable via the API. This is common in part due to the fact that most kernels and hypervisors are written in C, which is well-suited to interfacing with hardware but often prone to security issues. In order to exploit these issues, a typical attack might involve some combination of the following:
- Opening or creating some combination of files, sockets or other descriptors.
- Passing crafted, malicious arguments, structures or packets.
- Racing with multiple threads in order to hit specific code paths.
For example, for the Dirty Cow privilege escalation bug, an
application would open a specific file in /proc or use a specific ptrace
system call, and use multiple threads in order to trigger a race condition when
touching a fresh page of memory. The attacker then gains control over a page of
memory belonging to the system. With additional privileges or access to
privileged data in the kernel, an attacker will often be able to employ
additional techniques to gain full access to the rest of the system.
While bugs in the implementation of the System API are readily fixed, they are also the most common form of exploit. The exposure created by this class of exploit is what gVisor aims to minimize and control, described in detail below.
System ABI
Hardware and software exploits occasionally exist in execution paths that are not part of an intended System API. In this case, exploits may be found as part of implicit actions the hardware or privileged system code takes in response to certain events, such as traps or interrupts. For example, the recent POPSS flaw required only native code execution (no specific system call or file access). In that case, the Xen hypervisor was similarly vulnerable, highlighting that hypervisors are not immune to this vector.
Side Channels
Hardware side channels may be exploitable by any code running on a system: native, sandboxed, or virtualized. However, many host-level mitigations against hardware side channels are still effective with a sandbox. For example, kernels built with retpoline protect against some speculative execution attacks (Spectre) and frame poisoning may protect against L1 terminal fault (L1TF) attacks. Hypervisors may introduce additional complications in this regard, as there is no mitigation against an application in a normally functioning Virtual Machine (VM) exploiting the L1TF vulnerability for another VM on the sibling hyperthread.
Other Vectors
The above categories in no way represent an exhaustive list of exploits, as we focus only on running untrusted code from within the operating system or hypervisor. We do not consider other ways that a more generic adversary may interact with a system, such as inserting a portable storage device with a malicious filesystem image, using a combination of crafted keyboard or touch inputs, or saturating a network device with ill-formed packets.
Furthermore, high-level systems may contain exploitable components. An attacker need not escalate privileges within a container if there’s an exploitable network-accessible service on the host or some other API path. A sandbox is not a substitute for a secure architecture.
Goals: Limiting Exposure
gVisor’s primary design goal is to minimize the System API attack vector through multiple layers of defense, while still providing a process model. There are two primary security principles that inform this design. First, the application’s direct interactions with the host System API are intercepted by the Sentry, which implements the System API instead. Second, the System API accessible to the Sentry itself is minimized to a safer, restricted set. The first principle minimizes the possibility of direct exploitation of the host System API by applications, and the second principle minimizes indirect exploitability, which is the exploitation by an exploited or buggy Sentry (e.g. chaining an exploit).
The first principle is similar to the security basis for a Virtual Machine (VM). With a VM, an application’s interactions with the host are replaced by interactions with a guest operating system and a set of virtualized hardware devices. These hardware devices are then implemented via the host System API by a Virtual Machine Monitor (VMM). The Sentry similarly prevents direct interactions by providing its own implementation of the System API that the application must interact with. Applications are not able to directly craft specific arguments or flags for the host System API, or interact directly with host primitives.
For both the Sentry and a VMM, it’s worth noting that while direct interactions are not possible, indirect interactions are still possible. For example, a read on a host-backed file in the Sentry may ultimately result in a host read system call (made by the Sentry, not by passing through arguments from the application), similar to how a read on a block device in a VM may result in the VMM issuing a corresponding host read system call from a backing file.
An important distinction from a VM is that the Sentry implements a System API based directly on host System API primitives instead of relying on virtualized hardware and a guest operating system. This selects a distinct set of trade-offs, largely in the performance, efficiency and compatibility domains. Since transitions in and out of the sandbox are relatively expensive, a guest operating system will typically take ownership of resources. For example, in the above case, the guest operating system may read the block device data in a local page cache, to avoid subsequent reads. This may lead to better performance but lower efficiency, since memory may be wasted or duplicated. The Sentry opts instead to defer to the host for many operations during runtime, for improved efficiency but lower performance in some use cases.
What can a sandbox do?
An application in a gVisor sandbox is permitted to do most things a standard container can do: for example, applications can read and write files mapped within the container, make network connections, etc. As described above, gVisor’s primary goal is to limit exposure to bugs and exploits while still allowing most applications to run. Even so, gVisor will limit some operations that might be permitted with a standard container. Even with appropriate capabilities, a user in a gVisor sandbox will only be able to manipulate virtualized system resources (e.g. the system time, kernel settings or filesystem attributes) and not underlying host system resources.
While the sandbox virtualizes many operations for the application, we limit the sandbox’s own interactions with the host to the following high-level operations:
- Communicate with a Gofer process via a connected socket. The sandbox may receive new file descriptors from the Gofer process, corresponding to opened files. These files can then be read from and written to by the sandbox.
- Make a minimal set of host system calls. The calls do not include the creation of new sockets (unless host networking mode is enabled) or opening files. The calls include duplication and closing of file descriptors, synchronization, timers and signal management.
- Read and write packets to a virtual ethernet device. This is not required if host networking is enabled (or networking is disabled).
System ABI, Side Channels and Other Vectors
gVisor relies on the host operating system and the platform for defense against hardware-based attacks. Given the nature of these vulnerabilities, there is little defense that gVisor can provide (there’s no guarantee that additional hardware measures, such as virtualization, memory encryption, etc. would actually decrease the attack surface). Note that this is true even when using hardware virtualization for acceleration, as the host kernel or hypervisor is ultimately responsible for defending against attacks from within malicious guests.
gVisor similarly relies on the host resource mechanisms (cgroups) for defense against resource exhaustion and denial of service attacks. Network policy controls should be applied at the container level to ensure appropriate network policy enforcement. Note that the sandbox itself is not capable of altering or configuring these mechanisms, and the sandbox itself should make an attacker less likely to exploit or override these controls through other means.
Principles: Defense-in-Depth
For gVisor development, there are several engineering principles that are employed in order to ensure that the system meets its design goals.
- No system call is passed through directly to the host. Every supported call has an independent implementation in the Sentry, that is unlikely to suffer from identical vulnerabilities that may appear in the host. This has the consequence that all kernel features used by applications require an implementation within the Sentry.
- Only common, universal functionality is implemented. Some filesystems, network devices or modules may expose specialized functionality to user space applications via mechanisms such as extended attributes, raw sockets or ioctls. Since the Sentry is responsible for implementing the full system call surface, we do not implement or pass through these specialized APIs.
- The host surface exposed to the Sentry is minimized. While the system call surface is not trivial, it is explicitly enumerated and controlled. The Sentry is not permitted to open new files, create new sockets or do many other interesting things on the host.
Additionally, we have practical restrictions that are imposed on the project to minimize the risk of Sentry exploitability. For example:
- Unsafe code is carefully controlled. All unsafe code is isolated in files that end with “unsafe.go”, in order to facilitate validation and auditing. No file without the unsafe suffix may import the unsafe package.
- No CGo is allowed. The Sentry must be a pure Go binary.
- External imports are not generally allowed within the core packages. Only limited external imports are used within the setup code. The code available inside the Sentry is carefully controlled, to ensure that the above rules are effective.
Finally, we recognize that security is a process, and that vigilance is critical. Beyond our security disclosure process, the Sentry is fuzzed continuously to identify potential bugs and races proactively, and production crashes are recorded and triaged to similarly identify material issues.
FAQ
Is this more or less secure than a Virtual Machine?
The security of a VM depends to a large extent on what is exposed from the host kernel and userspace support code. For example, device emulation code in the host kernel (e.g. APIC) or optimizations (e.g. vhost) can be more complex than a simple system call, and exploits carry the same risks. Similarly, the userspace support code is frequently unsandboxed, and exploits, while rare, may allow unfettered access to the system.
Some platforms leverage the same virtualization hardware as VMs in order to provide better system call interception performance. However, gVisor does not implement any device emulation, and instead opts to use a sandboxed host System API directly. Both approaches significantly reduce the original attack surface. Ultimately, since gVisor is capable of using the same hardware mechanism, one should not assume that the mere use of virtualization hardware makes a system more or less secure, just as it would be a mistake to make the claim that the use of a unibody alone makes a car safe.
Does this stop hardware side channels?
In general, gVisor does not provide protection against hardware side channels, although it may make exploits that rely on direct access to the host System API more difficult to use. To minimize exposure, you should follow relevant guidance from vendors and keep your host kernel and firmware up-to-date.
Is this just a ptrace sandbox?
No: the term “ptrace sandbox” generally refers to software that uses the Linux ptrace facility to inspect and authorize system calls made by applications, enforcing a specific policy. These commonly suffer from two issues. First, vulnerable system calls may be authorized by the sandbox, as the application still has direct access to some System API. Second, it’s impossible to avoid time-of-check, time-of-use race conditions without disabling multi-threading.
In gVisor, the platforms that use ptrace operate differently. The stubs that are traced are never allowed to continue execution into the host kernel and complete a call directly. Instead, all system calls are interpreted and handled by the Sentry itself, who reflects resulting register state back into the tracee before continuing execution in userspace. This is very similar to the mechanism used by User-Mode Linux (UML).