Merge branch 'master' into ip-forwarding

- Merges aleksej-paschenko's with HEAD
- Adds vfs2 support for ip_forward

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+# gVisor Networking Security
+
+In our
+[first blog post](https://gvisor.dev/blog/2019/11/18/gvisor-security-basics-part-1/),
+we covered some secure design principles and how they guided the architecture of
+gVisor as a whole. In this post, we will cover how these principles guided the
+networking architecture of gVisor, and the tradeoffs involved. In particular, we
+will cover how these principles culminated in two networking modes, how they
+work, and the properties of each.
+
+## gVisor's security architecture in the context of networking
+
+Linux networking is complicated. The TCP protocol is over 40 years old, and has
+been repeatedly extended over the years to keep up with the rapid pace of
+network infrastructure improvements, all while maintaining compatibility. On top
+of that, Linux networking has a fairly large API surface. Linux supports
+[over 150 options](https://github.com/google/gvisor/blob/960f6a975b7e44c0efe8fd38c66b02017c4fe137/pkg/sentry/strace/socket.go#L476-L644)
+for the most common socket types alone. In fact, the net subsystem is one of the
+largest and fastest growing in Linux at approximately 1.1 million lines of code.
+For comparison, that is several times the size of the entire gVisor codebase.
+
+At the same time, networking is increasingly important. The cloud era is
+arguably about making everything a network service, and in order to make that
+work, the interconnect performance is critical. Adding networking support to
+gVisor was difficult, not just due to the inherent complexity, but also because
+it has the potential to significantly weaken gVisor's security model.
+
+As outlined in the previous blog post, gVisor's
+[secure design principles](https://gvisor.dev/blog/2019/11/18/gvisor-security-basics-part-1/#design-principles)
+are:
+
+1.  Defense in Depth: each component of the software stack trusts each other
+    component as little as possible.
+1.  Least Privilege: each software component has only the permissions it needs
+    to function, and no more.
+1.  Attack Surface Reduction: limit the surface area of the host exposed to the
+    sandbox.
+1.  Secure by Default: the default choice for a user should be safe.
+
+gVisor manifests these principles as a multi-layered system. An application
+running in the sandbox interacts with the Sentry, a userspace kernel, which
+mediates all interactions with the Host OS and beyond. The Sentry is written in
+pure Go with minimal unsafe code, making it less vulnerable to buffer overflows
+and related memory bugs that can lead to a variety of compromises including code
+injection. It emulates Linux using only a minimal and audited set of Host OS
+syscalls that limit the Host OS's attack surface exposed to the Sentry itself.
+The syscall restrictions are enforced by running the Sentry with seccomp
+filters, which enforce that the Sentry can only use the expected set of
+syscalls. The Sentry runs as an unprivileged user and in namespaces, which,
+along with the seccomp filters, ensure that the Sentry is run with the Least
+Privilege required.
+
+gVisor's multi-layered design provides Defense in Depth. The Sentry, which does
+not trust the application because it may attack the Sentry and try to bypass it,
+is the first layer. The sandbox that the Sentry runs in is the second layer. If
+the Sentry were compromised, the attacker would still be in a highly restrictive
+sandbox which they must also break out of in order to compromise the Host OS.
+
+To enable networking functionality while preserving gVisor's security
+properties, we implemented a
+[userspace network stack](https://github.com/google/gvisor/tree/master/pkg/tcpip)
+in the Sentry, which we creatively named Netstack. Netstack is also written in
+Go, not only to avoid unsafe code in the network stack itself, but also to avoid
+a complicated and unsafe Foreign Function Interface. Having its own integrated
+network stack allows the Sentry to implement networking operations using up to
+three Host OS syscalls to read and write packets. These syscalls allow a very
+minimal set of operations which are already allowed (either through the same or
+a similar syscall). Moreover, because packets typically come from off-host (e.g.
+the internet), the Host OS's packet processing code has received a lot of
+scrutiny, hopefully resulting in a high degree of hardening.
+
+![Figure 1](/assets/images/2020-04-02-networking-security-figure1.png "Network and gVisor.")
+
+## Writing a network stack
+
+Netstack was written from scratch specifically for gVisor. Because Netstack was
+designed and implemented to be modular, flexible and self-contained, there are
+now several more projects using Netstack in creative and exciting ways. As we
+discussed, a custom network stack has enabled a variety of security-related
+goals which would not have been possible any other way. This came at a cost
+though. Network stacks are complex and writing a new one comes with many
+challenges, mostly related to application compatibility and performance.
+
+Compatibility issues typically come in two forms: missing features, and features
+with behavior that differs from Linux (usually due to bugs). Both of these are
+inevitable in an implementation of a complex system spanning many quickly
+evolving and ambiguous standards. However, we have invested heavily in this
+area, and the vast majority of applications have no issues using Netstack. For
+example,
+[we now support setting 34 different socket options](https://github.com/google/gvisor/blob/815df2959a76e4a19f5882e40402b9bbca9e70be/pkg/sentry/socket/netstack/netstack.go#L830-L1764)
+versus
+[only 7 in our initial git commit](https://github.com/google/gvisor/blob/d02b74a5dcfed4bfc8f2f8e545bca4d2afabb296/pkg/sentry/socket/epsocket/epsocket.go#L445-L702).
+We are continuing to make good progress in this area.
+
+Performance issues typically come from TCP behavior and packet processing speed.
+To improve our TCP behavior, we are working on implementing the full set of TCP
+RFCs. There are many RFCs which are significant to performance (e.g.
+[RACK](https://tools.ietf.org/id/draft-ietf-tcpm-rack-03.html) and
+[BBR](https://tools.ietf.org/html/draft-cardwell-iccrg-bbr-congestion-control-00))
+that we have yet to implement. This mostly affects TCP performance with
+non-ideal network conditions (e.g. cross continent connections). Faster packet
+processing mostly improves TCP performance when network conditions are very good
+(e.g. within a datacenter). Our primary strategy here is to reduce interactions
+with the Go runtime, specifically the garbage collector (GC) and scheduler. We
+are currently optimizing buffer management to reduce the amount of garbage,
+which will lower the GC cost. To reduce scheduler interactions, we are
+re-architecting the TCP implementation to use fewer goroutines. Performance
+today is good enough for most applications and we are making steady
+improvements. For example, since May of 2019, we have improved the Netstack
+runsc
+[iperf3 download benchmark](https://github.com/google/gvisor/tree/master/test/benchmarks/network)
+score by roughly 15% and upload score by around 10,000X. Current numbers are
+about 17 Gbps download and about 8 Gbps upload versus about 42 Gbps and 43 Gbps
+for native (Linux) respectively.
+
+## An alternative
+
+We also offer an alternative network mode: passthrough. This name can be
+misleading as syscalls are never passed through from the app to the Host OS.
+Instead, the passthrough mode implements networking in gVisor using the Host
+OS's network stack. (This mode is called
+[hostinet](https://github.com/google/gvisor/tree/master/pkg/sentry/socket/hostinet)
+in the codebase.) Passthrough mode can improve performance for some use cases as
+the Host OS's network stack has had an enormous number of person-years poured
+into making it highly performant. However, there is a rather large downside to
+using passthrough mode: it weakens gVisor's security model by increasing the
+Host OS's Attack Surface. This is because using the Host OS's network stack
+requires the Sentry to use the Host OS's
+[Berkeley socket interface](https://en.wikipedia.org/wiki/Berkeley_sockets). The
+Berkeley socket interface is a much larger API surface than the packet interface
+that our network stack uses. When passthrough mode is in use, the Sentry is
+allowed to use
+[15 additional syscalls](https://github.com/google/gvisor/blob/b1576e533223e98ebe4bd1b82b04e3dcda8c4bf1/runsc/boot/filter/config.go#L312-L517).
+Further, this set of syscalls includes some that allow the Sentry to create file
+descriptors, something that
+[we don't normally allow](https://gvisor.dev/blog/2019/11/18/gvisor-security-basics-part-1/#sentry-host-os-interface)
+as it opens up classes of file-based attacks.
+
+There are some networking features that we can't implement on top of syscalls
+that we feel are safe (most notably those behind
+[ioctl](http://man7.org/linux/man-pages/man2/ioctl.2.html)) and therefore are
+not supported. Because of this, we actually support fewer networking features in
+passthrough mode than we do in Netstack, reducing application compatibility.
+That's right: using our networking stack provides better overall application
+compatibility than using our passthrough mode.
+
+That said, gVisor with passthrough networking still provides a high level of
+isolation. Applications cannot specify host syscall arguments directly, and the
+sentry's seccomp policy restricts its syscall use significantly more than a
+general purpose seccomp policy.
+
+## Secure by Default
+
+The goal of the Secure by Default principle is to make it easy to securely
+sandbox containers. Of course, disabling network access entirely is the most
+secure option, but that is not practical for most applications. To make gVisor
+Secure by Default, we have made Netstack the default networking mode in gVisor
+as we believe that it provides significantly better isolation. For this reason
+we strongly caution users from changing the default unless Netstack flat out
+won't work for them. The passthrough mode option is still provided, but we want
+users to make an informed decision when selecting it.
+
+Another way in which gVisor makes it easy to securely sandbox containers is by
+allowing applications to run unmodified, with no special configuration needed.
+In order to do this, gVisor needs to support all of the features and syscalls
+that applications use. Neither seccomp nor gVisor's passthrough mode can do this
+as applications commonly use syscalls which are too dangerous to be included in
+a secure policy. Even if this dream isn't fully realized today, gVisor's
+architecture with Netstack makes this possible.
+
+## Give Netstack a Try
+
+If you haven't already, try running a workload in gVisor with Netstack. You can
+find instructions on how to get started in our
+[Quick Start](/docs/user_guide/quick_start/docker/). We want to hear about both
+your successes and any issues you encounter. We welcome your contributions,
+whether that be verbal feedback or code contributions, via our
+[Gitter channel](https://gitter.im/gvisor/community),
+[email list](https://groups.google.com/forum/#!forum/gvisor-users),
+[issue tracker](https://gvisor.dev/issue/new), and
+[Github repository](https://github.com/google/gvisor). Feel free to express
+interest in an [open issue](https://gvisor.dev/issue/), or reach out if you
+aren't sure where to start.