Peter Maydell <peter.mayd...@linaro.org> writes:
> security.texi is included from qemu-doc.texi but is not used > in the qemu.1 manpage. So we can do a straightforward conversion > of the contents, which go into the system manual. > > Signed-off-by: Peter Maydell <peter.mayd...@linaro.org> > Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> > Message-id: 20200226113034.6741-16-pbonz...@redhat.com Reviewed-by: Alex Bennée <alex.ben...@linaro.org> > --- > docs/system/index.rst | 1 + > docs/system/security.rst | 173 +++++++++++++++++++++++++++++++++++++++ > 2 files changed, 174 insertions(+) > create mode 100644 docs/system/security.rst > > diff --git a/docs/system/index.rst b/docs/system/index.rst > index fc774a18b54..5034f903407 100644 > --- a/docs/system/index.rst > +++ b/docs/system/index.rst > @@ -14,4 +14,5 @@ Contents: > .. toctree:: > :maxdepth: 2 > > + security > vfio-ap > diff --git a/docs/system/security.rst b/docs/system/security.rst > new file mode 100644 > index 00000000000..f2092c8768b > --- /dev/null > +++ b/docs/system/security.rst > @@ -0,0 +1,173 @@ > +Security > +======== > + > +Overview > +-------- > + > +This chapter explains the security requirements that QEMU is designed to meet > +and principles for securely deploying QEMU. > + > +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. > +It is also possible to launch QEMU as a non-root user and configure UNIX > groups > +for access to ``/dev/kvm``, ``/dev/net/tun``, and other device nodes. > +Some Linux distros already ship with UNIX groups for these devices by > default. > + > +- 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. > + > +Sensitive configurations > +------------------------ > + > +There are aspects of QEMU that can have security implications which users & > +management applications must be aware of. > + > +Monitor console (QMP and HMP) > +''''''''''''''''''''''''''''' > + > +The monitor console (whether used with QMP or HMP) provides an interface > +to dynamically control many aspects of QEMU's runtime operation. Many of the > +commands exposed will instruct QEMU to access content on the host file system > +and/or trigger spawning of external processes. > + > +For example, the ``migrate`` command allows for the spawning of arbitrary > +processes for the purpose of tunnelling the migration data stream. The > +``blockdev-add`` command instructs QEMU to open arbitrary files, exposing > +their content to the guest as a virtual disk. > + > +Unless QEMU is otherwise confined using technologies such as SELinux, > AppArmor, > +or Linux namespaces, the monitor console should be considered to have > privileges > +equivalent to those of the user account QEMU is running under. > + > +It is further important to consider the security of the character device > backend > +over which the monitor console is exposed. It needs to have protection > against > +malicious third parties which might try to make unauthorized connections, or > +perform man-in-the-middle attacks. Many of the character device backends do > not > +satisfy this requirement and so must not be used for the monitor console. > + > +The general recommendation is that the monitor console should be exposed over > +a UNIX domain socket backend to the local host only. Use of the TCP based > +character device backend is inappropriate unless configured to use both TLS > +encryption and authorization control policy on client connections. > + > +In summary, the monitor console is considered a privileged control interface > to > +QEMU and as such should only be made accessible to a trusted management > +application or user. -- Alex Bennée
