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.

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 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.