[Qemu-devel] [PATCH] security.rst: add Security Guide to developer docs

[Stefan Hajnoczi]

At KVM Forum 2018 I gave a presentation on security in QEMU:
https://www.youtube.com/watch?v=YAdRf_hwxU8 (video)
https://vmsplice.net/~stefan/stefanha-kvm-forum-2018.pdf (slides)

This patch adds a security guide to the developer docs.  This document
covers things that developers should know about security in QEMU.  It is
just a starting point that we can expand on later.  I hope it will be
useful as a resource for new contributors and will save code reviewers
from explaining the same concepts many times.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 docs/devel/index.rst    |   1 +
 docs/devel/security.rst | 220 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 221 insertions(+)
 create mode 100644 docs/devel/security.rst

diff --git a/docs/devel/index.rst b/docs/devel/index.rst
index ebbab636ce..fd0b5fa387 100644
--- a/docs/devel/index.rst
+++ b/docs/devel/index.rst
@@ -20,3 +20,4 @@ Contents:
    stable-process
    testing
    decodetree
+   security
diff --git a/docs/devel/security.rst b/docs/devel/security.rst
new file mode 100644
index 0000000000..c6a6c9973d
--- /dev/null
+++ b/docs/devel/security.rst
@@ -0,0 +1,220 @@
+==============
+Security Guide
+==============
+Overview
+--------
+This guide covers security topics relevant to developers working on QEMU.  It
+includes an explanation of the security requirements that QEMU gives its users,
+the architecture of the code, and secure coding practices.
+
+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)
+
+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 CPU, memory,
+disk, or network throttling.  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.
+
+**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.
+
+Secure coding practices
+-----------------------
+At the source code level there are several points to keep in mind.  Both
+developers and security researchers must be aware of them so that they can
+develop safe code and audit existing code properly.
+
+General Secure C Coding Practices
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Most CVEs (security bugs) reported against QEMU are not specific to
+virtualization or emulation.  They are simply C programming bugs.  Therefore
+it's critical to be aware of common classes of security bugs.
+
+There is a wide selection of resources available covering secure C coding.  For
+example, the `CERT C Coding Standard
+<https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard>`_
+covers the most important classes of security bugs.
+
+Instead of describing them in detail here, only the names of the most important
+classes of security bugs are mentioned:
+
+* Buffer overflows
+* Use-after-free and double-free
+* Integer overflows
+* Format string vulnerabilities
+
+Some of these classes of bugs can be detected by analyzers.  Static analysis is
+performed regularly by Coverity and the most obvious of these bugs are even
+reported by compilers.  Dynamic analysis is possible with valgrind, tsan, and
+asan.
+
+Input Validation
+~~~~~~~~~~~~~~~~
+Inputs from the guest or external sources (e.g. network, files) cannot be
+trusted and may be invalid.  Inputs must be checked before using them in a way
+that could crash the program, expose host memory to the guest, or otherwise be
+exploitable by an attacker.
+
+The most sensitive attack surface is device emulation.  All hardware register
+accesses and data read from guest memory must be validated.  A typical example
+is a device that contains multiple units that are selectable by the guest via
+an index register::
+
+  typedef struct {
+      ProcessingUnit unit[2];
+      ...
+  } MyDeviceState;
+
+  static void mydev_writel(void *opaque, uint32_t addr, uint32_t val)
+  {
+      MyDeviceState *mydev = opaque;
+      ProcessingUnit *unit;
+
+      switch (addr) {
+      case MYDEV_SELECT_UNIT:
+          unit = &mydev->unit[val];   <-- this input wasn't validated!
+          ...
+      }
+  }
+
+If ``val`` is not in range [0, 1] then an out-of-bounds memory access will take
+place when ``unit`` is dereferenced.  The code must check that ``val`` is 0 or
+1 and handle the case where it is invalid.
+
+Unexpected Device Accesses
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+The guest may access device registers in unusual orders or at unexpected
+moments.  Device emulation code must not assume that the guest follows the
+typical "theory of operation" presented in driver writer manuals.  The guest
+may make nonsense accesses to device registers such as starting operations
+before the device has been fully initialized.
+
+A related issue is that device emulation code must be prepared for unexpected
+device register accesses while asynchronous operations are in progress.  A
+well-behaved guest might wait for a completion interrupt before accessing
+certain device registers.  Device emulation code must handle the case where the
+guest overwrites registers or submits further requests before an ongoing
+request completes.  Unexpected accesses must not cause memory corruption or
+leaks in QEMU.
+
+Live migration
+~~~~~~~~~~~~~~
+Device state can be saved to disk image files and shared with other users.
+Live migration code must validate inputs when loading device state so an
+attacker cannot gain control by crafting invalid device states.  Device state
+is therefore considered untrusted even though it is typically generated by QEMU
+itself.
+
+Guest Memory Access Races
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Guests with multiple vCPUs may modify guest RAM while device emulation code is
+running.  Device emulation code must copy in descriptors and other guest RAM
+structures and only process the local copy.  This prevents
+time-of-check-to-time-of-use (TOCTOU) race conditions that could cause QEMU to
+crash when a vCPU thread modifies guest RAM while device emulation is
+processing it.
-- 
2.20.1