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107 lines
4.3 KiB
ReStructuredText
107 lines
4.3 KiB
ReStructuredText
=======================
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Secure Coding Practices
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=======================
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This document covers topics that both developers and security researchers must
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be aware of so that they can develop safe code and audit existing code
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properly.
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Reporting Security Bugs
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-----------------------
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For details on how to report security bugs or ask questions about potential
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security bugs, see the `Security Process wiki page
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<https://wiki.qemu.org/SecurityProcess>`_.
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General Secure C Coding Practices
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---------------------------------
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Most CVEs (security bugs) reported against QEMU are not specific to
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virtualization or emulation. They are simply C programming bugs. Therefore
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it's critical to be aware of common classes of security bugs.
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There is a wide selection of resources available covering secure C coding. For
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example, the `CERT C Coding Standard
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<https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard>`_
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covers the most important classes of security bugs.
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Instead of describing them in detail here, only the names of the most important
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classes of security bugs are mentioned:
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* Buffer overflows
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* Use-after-free and double-free
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* Integer overflows
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* Format string vulnerabilities
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Some of these classes of bugs can be detected by analyzers. Static analysis is
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performed regularly by Coverity and the most obvious of these bugs are even
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reported by compilers. Dynamic analysis is possible with valgrind, tsan, and
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asan.
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Input Validation
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----------------
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Inputs from the guest or external sources (e.g. network, files) cannot be
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trusted and may be invalid. Inputs must be checked before using them in a way
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that could crash the program, expose host memory to the guest, or otherwise be
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exploitable by an attacker.
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The most sensitive attack surface is device emulation. All hardware register
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accesses and data read from guest memory must be validated. A typical example
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is a device that contains multiple units that are selectable by the guest via
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an index register::
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typedef struct {
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ProcessingUnit unit[2];
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...
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} MyDeviceState;
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static void mydev_writel(void *opaque, uint32_t addr, uint32_t val)
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{
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MyDeviceState *mydev = opaque;
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ProcessingUnit *unit;
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switch (addr) {
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case MYDEV_SELECT_UNIT:
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unit = &mydev->unit[val]; <-- this input wasn't validated!
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...
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}
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}
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If ``val`` is not in range [0, 1] then an out-of-bounds memory access will take
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place when ``unit`` is dereferenced. The code must check that ``val`` is 0 or
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1 and handle the case where it is invalid.
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Unexpected Device Accesses
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--------------------------
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The guest may access device registers in unusual orders or at unexpected
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moments. Device emulation code must not assume that the guest follows the
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typical "theory of operation" presented in driver writer manuals. The guest
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may make nonsense accesses to device registers such as starting operations
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before the device has been fully initialized.
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A related issue is that device emulation code must be prepared for unexpected
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device register accesses while asynchronous operations are in progress. A
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well-behaved guest might wait for a completion interrupt before accessing
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certain device registers. Device emulation code must handle the case where the
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guest overwrites registers or submits further requests before an ongoing
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request completes. Unexpected accesses must not cause memory corruption or
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leaks in QEMU.
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Invalid device register accesses can be reported with
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``qemu_log_mask(LOG_GUEST_ERROR, ...)``. The ``-d guest_errors`` command-line
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option enables these log messages.
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Live Migration
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--------------
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Device state can be saved to disk image files and shared with other users.
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Live migration code must validate inputs when loading device state so an
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attacker cannot gain control by crafting invalid device states. Device state
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is therefore considered untrusted even though it is typically generated by QEMU
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itself.
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Guest Memory Access Races
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-------------------------
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Guests with multiple vCPUs may modify guest RAM while device emulation code is
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running. Device emulation code must copy in descriptors and other guest RAM
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structures and only process the local copy. This prevents
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time-of-check-to-time-of-use (TOCTOU) race conditions that could cause QEMU to
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crash when a vCPU thread modifies guest RAM while device emulation is
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processing it.
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