secure-coding-practices.rst (4781B)
1 ======================= 2 Secure Coding Practices 3 ======================= 4 This document covers topics that both developers and security researchers must 5 be aware of so that they can develop safe code and audit existing code 6 properly. 7 8 Reporting Security Bugs 9 ----------------------- 10 For details on how to report security bugs or ask questions about potential 11 security bugs, see the `Security Process wiki page 12 <https://wiki.qemu.org/SecurityProcess>`_. 13 14 General Secure C Coding Practices 15 --------------------------------- 16 Most CVEs (security bugs) reported against QEMU are not specific to 17 virtualization or emulation. They are simply C programming bugs. Therefore 18 it's critical to be aware of common classes of security bugs. 19 20 There is a wide selection of resources available covering secure C coding. For 21 example, the `CERT C Coding Standard 22 <https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard>`_ 23 covers the most important classes of security bugs. 24 25 Instead of describing them in detail here, only the names of the most important 26 classes of security bugs are mentioned: 27 28 * Buffer overflows 29 * Use-after-free and double-free 30 * Integer overflows 31 * Format string vulnerabilities 32 33 Some of these classes of bugs can be detected by analyzers. Static analysis is 34 performed regularly by Coverity and the most obvious of these bugs are even 35 reported by compilers. Dynamic analysis is possible with valgrind, tsan, and 36 asan. 37 38 Input Validation 39 ---------------- 40 Inputs from the guest or external sources (e.g. network, files) cannot be 41 trusted and may be invalid. Inputs must be checked before using them in a way 42 that could crash the program, expose host memory to the guest, or otherwise be 43 exploitable by an attacker. 44 45 The most sensitive attack surface is device emulation. All hardware register 46 accesses and data read from guest memory must be validated. A typical example 47 is a device that contains multiple units that are selectable by the guest via 48 an index register:: 49 50 typedef struct { 51 ProcessingUnit unit[2]; 52 ... 53 } MyDeviceState; 54 55 static void mydev_writel(void *opaque, uint32_t addr, uint32_t val) 56 { 57 MyDeviceState *mydev = opaque; 58 ProcessingUnit *unit; 59 60 switch (addr) { 61 case MYDEV_SELECT_UNIT: 62 unit = &mydev->unit[val]; <-- this input wasn't validated! 63 ... 64 } 65 } 66 67 If ``val`` is not in range [0, 1] then an out-of-bounds memory access will take 68 place when ``unit`` is dereferenced. The code must check that ``val`` is 0 or 69 1 and handle the case where it is invalid. 70 71 Unexpected Device Accesses 72 -------------------------- 73 The guest may access device registers in unusual orders or at unexpected 74 moments. Device emulation code must not assume that the guest follows the 75 typical "theory of operation" presented in driver writer manuals. The guest 76 may make nonsense accesses to device registers such as starting operations 77 before the device has been fully initialized. 78 79 A related issue is that device emulation code must be prepared for unexpected 80 device register accesses while asynchronous operations are in progress. A 81 well-behaved guest might wait for a completion interrupt before accessing 82 certain device registers. Device emulation code must handle the case where the 83 guest overwrites registers or submits further requests before an ongoing 84 request completes. Unexpected accesses must not cause memory corruption or 85 leaks in QEMU. 86 87 Invalid device register accesses can be reported with 88 ``qemu_log_mask(LOG_GUEST_ERROR, ...)``. The ``-d guest_errors`` command-line 89 option enables these log messages. 90 91 Live Migration 92 -------------- 93 Device state can be saved to disk image files and shared with other users. 94 Live migration code must validate inputs when loading device state so an 95 attacker cannot gain control by crafting invalid device states. Device state 96 is therefore considered untrusted even though it is typically generated by QEMU 97 itself. 98 99 Guest Memory Access Races 100 ------------------------- 101 Guests with multiple vCPUs may modify guest RAM while device emulation code is 102 running. Device emulation code must copy in descriptors and other guest RAM 103 structures and only process the local copy. This prevents 104 time-of-check-to-time-of-use (TOCTOU) race conditions that could cause QEMU to 105 crash when a vCPU thread modifies guest RAM while device emulation is 106 processing it. 107 108 Use of null-co block drivers 109 ---------------------------- 110 111 The ``null-co`` block driver is designed for performance: its read accesses are 112 not initialized by default. In case this driver has to be used for security 113 research, it must be used with the ``read-zeroes=on`` option which fills read 114 buffers with zeroes. Security issues reported with the default 115 (``read-zeroes=off``) will be discarded.