docs: add Security chapter to the documentation

This new chapter in the QEMU documentation covers the security
requirements that QEMU is designed to meet and principles for securely
deploying QEMU.

It is just a starting point that can be extended in the future with more
information.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-id: 20190509121820.16294-3-stefanha@redhat.com
Message-Id: <20190509121820.16294-3-stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 131 insertions, 0 deletions

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+@node Security
+@chapter Security
+
+@section Overview
+
+This chapter explains the security requirements that QEMU is designed to meet
+and principles for securely deploying QEMU.
+
+@section Security Requirements
+
+QEMU supports many different use cases, some of which have stricter security
+requirements than others.  The community has agreed on the overall security
+requirements that users may depend on.  These requirements define what is
+considered supported from a security perspective.
+
+@subsection Virtualization Use Case
+
+The virtualization use case covers cloud and virtual private server (VPS)
+hosting, as well as traditional data center and desktop virtualization.  These
+use cases rely on hardware virtualization extensions to execute guest code
+safely on the physical CPU at close-to-native speed.
+
+The following entities are untrusted, meaning that they may be buggy or
+malicious:
+
+@itemize
+@item Guest
+@item User-facing interfaces (e.g. VNC, SPICE, WebSocket)
+@item Network protocols (e.g. NBD, live migration)
+@item User-supplied files (e.g. disk images, kernels, device trees)
+@item Passthrough devices (e.g. PCI, USB)
+@end itemize
+
+Bugs affecting these entities are evaluated on whether they can cause damage in
+real-world use cases and treated as security bugs if this is the case.
+
+@subsection Non-virtualization Use Case
+
+The non-virtualization use case covers emulation using the Tiny Code Generator
+(TCG).  In principle the TCG and device emulation code used in conjunction with
+the non-virtualization use case should meet the same security requirements as
+the virtualization use case.  However, for historical reasons much of the
+non-virtualization use case code was not written with these security
+requirements in mind.
+
+Bugs affecting the non-virtualization use case are not considered security
+bugs at this time.  Users with non-virtualization use cases must not rely on
+QEMU to provide guest isolation or any security guarantees.
+
+@section Architecture
+
+This section describes the design principles that ensure the security
+requirements are met.
+
+@subsection Guest Isolation
+
+Guest isolation is the confinement of guest code to the virtual machine.  When
+guest code gains control of execution on the host this is called escaping the
+virtual machine.  Isolation also includes resource limits such as throttling of
+CPU, memory, disk, or network.  Guests must be unable to exceed their resource
+limits.
+
+QEMU presents an attack surface to the guest in the form of emulated devices.
+The guest must not be able to gain control of QEMU.  Bugs in emulated devices
+could allow malicious guests to gain code execution in QEMU.  At this point the
+guest has escaped the virtual machine and is able to act in the context of the
+QEMU process on the host.
+
+Guests often interact with other guests and share resources with them.  A
+malicious guest must not gain control of other guests or access their data.
+Disk image files and network traffic must be protected from other guests unless
+explicitly shared between them by the user.
+
+@subsection Principle of Least Privilege
+
+The principle of least privilege states that each component only has access to
+the privileges necessary for its function.  In the case of QEMU this means that
+each process only has access to resources belonging to the guest.
+
+The QEMU process should not have access to any resources that are inaccessible
+to the guest.  This way the guest does not gain anything by escaping into the
+QEMU process since it already has access to those same resources from within
+the guest.
+
+Following the principle of least privilege immediately fulfills guest isolation
+requirements.  For example, guest A only has access to its own disk image file
+@code{a.img} and not guest B's disk image file @code{b.img}.
+
+In reality certain resources are inaccessible to the guest but must be
+available to QEMU to perform its function.  For example, host system calls are
+necessary for QEMU but are not exposed to guests.  A guest that escapes into
+the QEMU process can then begin invoking host system calls.
+
+New features must be designed to follow the principle of least privilege.
+Should this not be possible for technical reasons, the security risk must be
+clearly documented so users are aware of the trade-off of enabling the feature.
+
+@subsection Isolation mechanisms
+
+Several isolation mechanisms are available to realize this architecture of
+guest isolation and the principle of least privilege.  With the exception of
+Linux seccomp, these mechanisms are all deployed by management tools that
+launch QEMU, such as libvirt.  They are also platform-specific so they are only
+described briefly for Linux here.
+
+The fundamental isolation mechanism is that QEMU processes must run as
+unprivileged users.  Sometimes it seems more convenient to launch QEMU as
+root to give it access to host devices (e.g. @code{/dev/net/tun}) but this poses a
+huge security risk.  File descriptor passing can be used to give an otherwise
+unprivileged QEMU process access to host devices without running QEMU as root.
+It is also possible to launch QEMU as a non-root user and configure UNIX groups
+for access to @code{/dev/kvm}, @code{/dev/net/tun}, and other device nodes.
+Some Linux distros already ship with UNIX groups for these devices by default.
+
+@itemize
+@item SELinux and AppArmor make it possible to confine processes beyond the
+traditional UNIX process and file permissions model.  They restrict the QEMU
+process from accessing processes and files on the host system that are not
+needed by QEMU.
+
+@item Resource limits and cgroup controllers provide throughput and utilization
+limits on key resources such as CPU time, memory, and I/O bandwidth.
+
+@item Linux namespaces can be used to make process, file system, and other system
+resources unavailable to QEMU.  A namespaced QEMU process is restricted to only
+those resources that were granted to it.
+
+@item Linux seccomp is available via the QEMU @option{--sandbox} option.  It disables
+system calls that are not needed by QEMU, thereby reducing the host kernel
+attack surface.
+@end itemize