docs/devel/secure-coding-practices.rst

=======================
Secure Coding Practices
=======================
This document covers topics that both developers and security researchers must
be aware of so that they can develop safe code and audit existing code
properly.

Reporting Security Bugs
-----------------------
For details on how to report security bugs or ask questions about potential
security bugs, see the `Security Process wiki page
<https://wiki.qemu.org/SecurityProcess>`_.

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.

Invalid device register accesses can be reported with
``qemu_log_mask(LOG_GUEST_ERROR, ...)``.  The ``-d guest_errors`` command-line
option enables these log messages.

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.

Use of null-co block drivers
----------------------------

The ``null-co`` block driver is designed for performance: its read accesses are
not initialized by default. In case this driver has to be used for security
research, it must be used with the ``read-zeroes=on`` option which fills read
buffers with zeroes. Security issues reported with the default
(``read-zeroes=off``) will be discarded.