On 5/3/19 11:04 AM, Alex Bennée wrote: > > Stefan Hajnoczi <stefa...@redhat.com> writes: > >> At KVM Forum 2018 I gave a presentation on security in QEMU: >> https://www.youtube.com/watch?v=YAdRf_hwxU8 (video) >> https://vmsplice.net/~stefan/stefanha-kvm-forum-2018.pdf (slides) >> >> This patch adds a security guide to the developer docs. This document >> covers things that developers should know about security in QEMU. It is >> just a starting point that we can expand on later. I hope it will be >> useful as a resource for new contributors and will save code reviewers >> from explaining the same concepts many times. >> >> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> >> --- >> v2: >> * Added mention of passthrough USB and PCI devices [philmd] >> * Reworded resource limits [philmd] >> * Added qemu_log_mask(LOG_GUEST_ERROR) [philmd] >> --- >> docs/devel/index.rst | 1 + >> docs/devel/security.rst | 225 ++++++++++++++++++++++++++++++++++++++++ >> 2 files changed, 226 insertions(+) >> create mode 100644 docs/devel/security.rst >> >> diff --git a/docs/devel/index.rst b/docs/devel/index.rst >> index ebbab636ce..fd0b5fa387 100644 >> --- a/docs/devel/index.rst >> +++ b/docs/devel/index.rst >> @@ -20,3 +20,4 @@ Contents: >> stable-process >> testing >> decodetree >> + security >> diff --git a/docs/devel/security.rst b/docs/devel/security.rst >> new file mode 100644 >> index 0000000000..83c6fb2231 >> --- /dev/null >> +++ b/docs/devel/security.rst >> @@ -0,0 +1,225 @@ >> +============== >> +Security Guide >> +============== >> +Overview >> +-------- >> +This guide covers security topics relevant to developers working on QEMU. >> It >> +includes an explanation of the security requirements that QEMU gives its >> users, >> +the architecture of the code, and secure coding practices. >> + >> +Security Requirements >> +--------------------- >> +QEMU supports many different use cases, some of which have stricter security >> +requirements than others. The community has agreed on the overall security >> +requirements that users may depend on. These requirements define what is >> +considered supported from a security perspective. >> + >> +Virtualization Use Case >> +~~~~~~~~~~~~~~~~~~~~~~~ >> +The virtualization use case covers cloud and virtual private server (VPS) >> +hosting, as well as traditional data center and desktop virtualization. >> These >> +use cases rely on hardware virtualization extensions to execute guest code >> +safely on the physical CPU at close-to-native speed. >> + >> +The following entities are **untrusted**, meaning that they may be buggy or >> +malicious: >> + >> +* Guest >> +* User-facing interfaces (e.g. VNC, SPICE, WebSocket) >> +* Network protocols (e.g. NBD, live migration) >> +* User-supplied files (e.g. disk images, kernels, device trees) >> +* Passthrough devices (e.g. PCI, USB)
Thanks. >> + >> +Bugs affecting these entities are evaluated on whether they can cause >> damage in >> +real-world use cases and treated as security bugs if this is the case. >> + >> +Non-virtualization Use Case >> +~~~~~~~~~~~~~~~~~~~~~~~~~~~ >> +The non-virtualization use case covers emulation using the Tiny Code >> Generator >> +(TCG). In principle the TCG and device emulation code used in conjunction >> with >> +the non-virtualization use case should meet the same security requirements >> as >> +the virtualization use case. However, for historical reasons much of the >> +non-virtualization use case code was not written with these security >> +requirements in mind. >> + >> +Bugs affecting the non-virtualization use case are not considered security >> +bugs at this time. Users with non-virtualization use cases must not rely on >> +QEMU to provide guest isolation or any security guarantees. >> + >> +Architecture >> +------------ >> +This section describes the design principles that ensure the security >> +requirements are met. >> + >> +Guest Isolation >> +~~~~~~~~~~~~~~~ >> +Guest isolation is the confinement of guest code to the virtual machine. >> When >> +guest code gains control of execution on the host this is called escaping >> the >> +virtual machine. Isolation also includes resource limits such as >> throttling of >> +CPU, memory, disk, or network. Guests must be unable to exceed their >> resource >> +limits. >> + >> +QEMU presents an attack surface to the guest in the form of emulated >> devices. >> +The guest must not be able to gain control of QEMU. Bugs in emulated >> devices >> +could allow malicious guests to gain code execution in QEMU. At this point >> the >> +guest has escaped the virtual machine and is able to act in the context of >> the >> +QEMU process on the host. >> + >> +Guests often interact with other guests and share resources with them. A >> +malicious guest must not gain control of other guests or access their data. >> +Disk image files and network traffic must be protected from other guests >> unless >> +explicitly shared between them by the user. >> + >> +Principle of Least Privilege >> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >> +The principle of least privilege states that each component only has access >> to >> +the privileges necessary for its function. In the case of QEMU this means >> that >> +each process only has access to resources belonging to the guest. >> + >> +The QEMU process should not have access to any resources that are >> inaccessible >> +to the guest. This way the guest does not gain anything by escaping into >> the >> +QEMU process since it already has access to those same resources from within >> +the guest. >> + >> +Following the principle of least privilege immediately fulfills guest >> isolation >> +requirements. For example, guest A only has access to its own disk image >> file >> +``a.img`` and not guest B's disk image file ``b.img``. >> + >> +In reality certain resources are inaccessible to the guest but must be >> +available to QEMU to perform its function. For example, host system calls >> are >> +necessary for QEMU but are not exposed to guests. A guest that escapes into >> +the QEMU process can then begin invoking host system calls. >> + >> +New features must be designed to follow the principle of least privilege. >> +Should this not be possible for technical reasons, the security risk must be >> +clearly documented so users are aware of the trade-off of enabling the >> feature. >> + >> +Isolation mechanisms >> +~~~~~~~~~~~~~~~~~~~~ >> +Several isolation mechanisms are available to realize this architecture of >> +guest isolation and the principle of least privilege. With the exception of >> +Linux seccomp, these mechanisms are all deployed by management tools that >> +launch QEMU, such as libvirt. They are also platform-specific so they are >> only >> +described briefly for Linux here. >> + >> +The fundamental isolation mechanism is that QEMU processes must run as >> +**unprivileged users**. Sometimes it seems more convenient to launch QEMU >> as >> +root to give it access to host devices (e.g. ``/dev/net/tun``) but this >> poses a >> +huge security risk. File descriptor passing can be used to give an >> otherwise >> +unprivileged QEMU process access to host devices without running QEMU >> as root. > > Should we mention that you can still maintain running as a user and just > make the devices you need available to the user/group rather than > becoming root? For example I generally make /dev/kvm group accessible to > my user account. Good suggestion. >> + >> +**SELinux** and **AppArmor** make it possible to confine processes beyond >> the >> +traditional UNIX process and file permissions model. They restrict the QEMU >> +process from accessing processes and files on the host system that are not >> +needed by QEMU. >> + >> +**Resource limits** and **cgroup controllers** provide throughput and >> utilization >> +limits on key resources such as CPU time, memory, and I/O bandwidth. >> + >> +**Linux namespaces** can be used to make process, file system, and other >> system >> +resources unavailable to QEMU. A namespaced QEMU process is restricted to >> only >> +those resources that were granted to it. >> + >> +**Linux seccomp** is available via the QEMU ``--sandbox`` option. It >> disables >> +system calls that are not needed by QEMU, thereby reducing the host kernel >> +attack surface. >> + >> +Secure coding practices >> +----------------------- >> +At the source code level there are several points to keep in mind. Both >> +developers and security researchers must be aware of them so that they can >> +develop safe code and audit existing code properly. >> + >> +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. Thanks for adding this section! >> + >> +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. > > Anyway: > > Reviewed-by: Alex Bennée <alex.ben...@linaro.org> Reviewed-by: Philippe Mathieu-Daudé <phi...@redhat.com>