docs/system: Convert security.texi to rST format

security.texi is included from qemu-doc.texi but is not used
in the qemu.1 manpage. So we can do a straightforward conversion
of the contents, which go into the system manual.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-17-peter.maydell@linaro.org
Message-id: 20200226113034.6741-16-pbonzini@redhat.com

docs/system/index.rst

@@ -14,4 +14,5 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   security
    vfio-ap

docs/system/security.rst

@@ -0,0 +1,173 @@
+Security
+========
+
+Overview
+--------
+
+This chapter explains the security requirements that QEMU is designed to meet
+and principles for securely deploying QEMU.
+
+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.
+
+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:
+
+- Guest
+- User-facing interfaces (e.g. VNC, SPICE, WebSocket)
+- Network protocols (e.g. NBD, live migration)
+- User-supplied files (e.g. disk images, kernels, device trees)
+- Passthrough devices (e.g. PCI, USB)
+
+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.
+
+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.
+
+Architecture
+------------
+
+This section describes the design principles that ensure the security
+requirements are met.
+
+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.
+
+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
+``a.img`` and not guest B's disk image file ``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.
+
+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. ``/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 ``/dev/kvm``, ``/dev/net/tun``, and other device nodes.
+Some Linux distros already ship with UNIX groups for these devices by default.
+
+- 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.
+
+- Resource limits and cgroup controllers provide throughput and utilization
+  limits on key resources such as CPU time, memory, and I/O bandwidth.
+
+- 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.
+
+- Linux seccomp is available via the QEMU ``--sandbox`` option.  It disables
+  system calls that are not needed by QEMU, thereby reducing the host kernel
+  attack surface.
+
+Sensitive configurations
+------------------------
+
+There are aspects of QEMU that can have security implications which users &
+management applications must be aware of.
+
+Monitor console (QMP and HMP)
+'''''''''''''''''''''''''''''
+
+The monitor console (whether used with QMP or HMP) provides an interface
+to dynamically control many aspects of QEMU's runtime operation. Many of the
+commands exposed will instruct QEMU to access content on the host file system
+and/or trigger spawning of external processes.
+
+For example, the ``migrate`` command allows for the spawning of arbitrary
+processes for the purpose of tunnelling the migration data stream. The
+``blockdev-add`` command instructs QEMU to open arbitrary files, exposing
+their content to the guest as a virtual disk.
+
+Unless QEMU is otherwise confined using technologies such as SELinux, AppArmor,
+or Linux namespaces, the monitor console should be considered to have privileges
+equivalent to those of the user account QEMU is running under.
+
+It is further important to consider the security of the character device backend
+over which the monitor console is exposed. It needs to have protection against
+malicious third parties which might try to make unauthorized connections, or
+perform man-in-the-middle attacks. Many of the character device backends do not
+satisfy this requirement and so must not be used for the monitor console.
+
+The general recommendation is that the monitor console should be exposed over
+a UNIX domain socket backend to the local host only. Use of the TCP based
+character device backend is inappropriate unless configured to use both TLS
+encryption and authorization control policy on client connections.
+
+In summary, the monitor console is considered a privileged control interface to
+QEMU and as such should only be made accessible to a trusted management
+application or user.