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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.