diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/devel/index.rst | 1 | ||||
| -rw-r--r-- | docs/devel/secure-coding-practices.rst | 106 |
2 files changed, 107 insertions, 0 deletions
diff --git a/docs/devel/index.rst b/docs/devel/index.rst index ebbab636ce..2a4ddf40ad 100644 --- a/docs/devel/index.rst +++ b/docs/devel/index.rst @@ -20,3 +20,4 @@ Contents: stable-process testing decodetree + secure-coding-practices diff --git a/docs/devel/secure-coding-practices.rst b/docs/devel/secure-coding-practices.rst new file mode 100644 index 0000000000..cbfc8af67e --- /dev/null +++ b/docs/devel/secure-coding-practices.rst @@ -0,0 +1,106 @@ +======================= +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. |