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) > + > +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. > + > +**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. > + > +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> -- Alex Bennée
