security.rst (7924B)
1 Security 2 ======== 3 4 Overview 5 -------- 6 7 This chapter explains the security requirements that QEMU is designed to meet 8 and principles for securely deploying QEMU. 9 10 Security Requirements 11 --------------------- 12 13 QEMU supports many different use cases, some of which have stricter security 14 requirements than others. The community has agreed on the overall security 15 requirements that users may depend on. These requirements define what is 16 considered supported from a security perspective. 17 18 Virtualization Use Case 19 ''''''''''''''''''''''' 20 21 The virtualization use case covers cloud and virtual private server (VPS) 22 hosting, as well as traditional data center and desktop virtualization. These 23 use cases rely on hardware virtualization extensions to execute guest code 24 safely on the physical CPU at close-to-native speed. 25 26 The following entities are untrusted, meaning that they may be buggy or 27 malicious: 28 29 - Guest 30 - User-facing interfaces (e.g. VNC, SPICE, WebSocket) 31 - Network protocols (e.g. NBD, live migration) 32 - User-supplied files (e.g. disk images, kernels, device trees) 33 - Passthrough devices (e.g. PCI, USB) 34 35 Bugs affecting these entities are evaluated on whether they can cause damage in 36 real-world use cases and treated as security bugs if this is the case. 37 38 Non-virtualization Use Case 39 ''''''''''''''''''''''''''' 40 41 The non-virtualization use case covers emulation using the Tiny Code Generator 42 (TCG). In principle the TCG and device emulation code used in conjunction with 43 the non-virtualization use case should meet the same security requirements as 44 the virtualization use case. However, for historical reasons much of the 45 non-virtualization use case code was not written with these security 46 requirements in mind. 47 48 Bugs affecting the non-virtualization use case are not considered security 49 bugs at this time. Users with non-virtualization use cases must not rely on 50 QEMU to provide guest isolation or any security guarantees. 51 52 Architecture 53 ------------ 54 55 This section describes the design principles that ensure the security 56 requirements are met. 57 58 Guest Isolation 59 ''''''''''''''' 60 61 Guest isolation is the confinement of guest code to the virtual machine. When 62 guest code gains control of execution on the host this is called escaping the 63 virtual machine. Isolation also includes resource limits such as throttling of 64 CPU, memory, disk, or network. Guests must be unable to exceed their resource 65 limits. 66 67 QEMU presents an attack surface to the guest in the form of emulated devices. 68 The guest must not be able to gain control of QEMU. Bugs in emulated devices 69 could allow malicious guests to gain code execution in QEMU. At this point the 70 guest has escaped the virtual machine and is able to act in the context of the 71 QEMU process on the host. 72 73 Guests often interact with other guests and share resources with them. A 74 malicious guest must not gain control of other guests or access their data. 75 Disk image files and network traffic must be protected from other guests unless 76 explicitly shared between them by the user. 77 78 Principle of Least Privilege 79 '''''''''''''''''''''''''''' 80 81 The principle of least privilege states that each component only has access to 82 the privileges necessary for its function. In the case of QEMU this means that 83 each process only has access to resources belonging to the guest. 84 85 The QEMU process should not have access to any resources that are inaccessible 86 to the guest. This way the guest does not gain anything by escaping into the 87 QEMU process since it already has access to those same resources from within 88 the guest. 89 90 Following the principle of least privilege immediately fulfills guest isolation 91 requirements. For example, guest A only has access to its own disk image file 92 ``a.img`` and not guest B's disk image file ``b.img``. 93 94 In reality certain resources are inaccessible to the guest but must be 95 available to QEMU to perform its function. For example, host system calls are 96 necessary for QEMU but are not exposed to guests. A guest that escapes into 97 the QEMU process can then begin invoking host system calls. 98 99 New features must be designed to follow the principle of least privilege. 100 Should this not be possible for technical reasons, the security risk must be 101 clearly documented so users are aware of the trade-off of enabling the feature. 102 103 Isolation mechanisms 104 '''''''''''''''''''' 105 106 Several isolation mechanisms are available to realize this architecture of 107 guest isolation and the principle of least privilege. With the exception of 108 Linux seccomp, these mechanisms are all deployed by management tools that 109 launch QEMU, such as libvirt. They are also platform-specific so they are only 110 described briefly for Linux here. 111 112 The fundamental isolation mechanism is that QEMU processes must run as 113 unprivileged users. Sometimes it seems more convenient to launch QEMU as 114 root to give it access to host devices (e.g. ``/dev/net/tun``) but this poses a 115 huge security risk. File descriptor passing can be used to give an otherwise 116 unprivileged QEMU process access to host devices without running QEMU as root. 117 It is also possible to launch QEMU as a non-root user and configure UNIX groups 118 for access to ``/dev/kvm``, ``/dev/net/tun``, and other device nodes. 119 Some Linux distros already ship with UNIX groups for these devices by default. 120 121 - SELinux and AppArmor make it possible to confine processes beyond the 122 traditional UNIX process and file permissions model. They restrict the QEMU 123 process from accessing processes and files on the host system that are not 124 needed by QEMU. 125 126 - Resource limits and cgroup controllers provide throughput and utilization 127 limits on key resources such as CPU time, memory, and I/O bandwidth. 128 129 - Linux namespaces can be used to make process, file system, and other system 130 resources unavailable to QEMU. A namespaced QEMU process is restricted to only 131 those resources that were granted to it. 132 133 - Linux seccomp is available via the QEMU ``--sandbox`` option. It disables 134 system calls that are not needed by QEMU, thereby reducing the host kernel 135 attack surface. 136 137 Sensitive configurations 138 ------------------------ 139 140 There are aspects of QEMU that can have security implications which users & 141 management applications must be aware of. 142 143 Monitor console (QMP and HMP) 144 ''''''''''''''''''''''''''''' 145 146 The monitor console (whether used with QMP or HMP) provides an interface 147 to dynamically control many aspects of QEMU's runtime operation. Many of the 148 commands exposed will instruct QEMU to access content on the host file system 149 and/or trigger spawning of external processes. 150 151 For example, the ``migrate`` command allows for the spawning of arbitrary 152 processes for the purpose of tunnelling the migration data stream. The 153 ``blockdev-add`` command instructs QEMU to open arbitrary files, exposing 154 their content to the guest as a virtual disk. 155 156 Unless QEMU is otherwise confined using technologies such as SELinux, AppArmor, 157 or Linux namespaces, the monitor console should be considered to have privileges 158 equivalent to those of the user account QEMU is running under. 159 160 It is further important to consider the security of the character device backend 161 over which the monitor console is exposed. It needs to have protection against 162 malicious third parties which might try to make unauthorized connections, or 163 perform man-in-the-middle attacks. Many of the character device backends do not 164 satisfy this requirement and so must not be used for the monitor console. 165 166 The general recommendation is that the monitor console should be exposed over 167 a UNIX domain socket backend to the local host only. Use of the TCP based 168 character device backend is inappropriate unless configured to use both TLS 169 encryption and authorization control policy on client connections. 170 171 In summary, the monitor console is considered a privileged control interface to 172 QEMU and as such should only be made accessible to a trusted management 173 application or user.