From: Stefan Hajnoczi <stefa...@redhat.com> Many people want to know: what's up with virtiofsd and security? This document provides the answers!
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> --- contrib/virtiofsd/security.rst | 108 +++++++++++++++++++++++++++++++++ 1 file changed, 108 insertions(+) create mode 100644 contrib/virtiofsd/security.rst diff --git a/contrib/virtiofsd/security.rst b/contrib/virtiofsd/security.rst new file mode 100644 index 0000000000..1f3037f4e4 --- /dev/null +++ b/contrib/virtiofsd/security.rst @@ -0,0 +1,108 @@ +======================== +Virtiofsd Security Guide +======================== + +Introduction +============ +This document covers security topics for users of virtiofsd, the daemon that +implements host<->guest file system sharing. Sharing files between one or more +guests and the host raises questions about the trust relationships between +these entities. By understanding these topics users can safely deploy +virtiofsd and control access to their data. + +Architecture +============ +The virtiofsd daemon process acts as a vhost-user device backend, implementing +the virtio-fs device that the corresponding device driver inside the guest +interacts with. + +There is one virtiofsd process per virtio-fs device instance. For example, +when two guests have access to the same shared directory there are still two +virtiofsd processes since there are two virtio-fs device instances. Similarly, +if one guest has access to two shared directories, there are two virtiofsd +processes since there are two virtio-fs device instances. + +Files are created on the host with uid/gid values provided by the guest. +Furthermore, virtiofsd is unable to enforce file permissions since guests have +the ability to access any file within the shared directory. File permissions +are implemented in the guest, just like with traditional local file systems. + +Security Requirements +===================== +Guests have root access to the shared directory. This is necessary for root +file systems on virtio-fs and similar use cases. + +When multiple guests have access to the same shared directory, the guests have +a trust relationship. A broken or malicious guest could delete or corrupt +files. It could exploit symlink or time-of-check-to-time-of-use (TOCTOU) race +conditions against applications in other guests. It could plant device nodes +or setuid executables to gain privileges in other guests. It could perform +denial-of-service (DoS) attacks by consuming available space or making the file +system unavailable to other guests. + +Guests are restricted to the shared directory and cannot access other files on +the host. + +Guests should not be able to gain arbitrary code execution inside the virtiofsd +process. If they do, the process is sandboxed to prevent escaping into other +parts of the host. + +Daemon Sandboxing +================= +The virtiofsd process handles virtio-fs FUSE requests from the untrusted guest. +This attack surface could give the guest access to host resources and must +therefore be protected. Sandboxing mechanisms are integrated into virtiofsd to +reduce the impact in the event that an attacker gains control of the process. + +As a general rule, virtiofsd does not trust inputs from the guest, aside from +uid/gid values. Input validation is performed so that the guest cannot corrupt +memory or otherwise gain arbitrary code execution in the virtiofsd process. + +Sandboxing adds restrictions on the virtiofsd so that even if an attacker is +able to exploit a bug, they will be constrained to the virtiofsd process and +unable to cause damage on the host. + +Seccomp Whitelist +----------------- +Many system calls are not required by virtiofsd to perform its function. For +example, ptrace(2) and execve(2) are not necessary and attackers are likely to +use them to further compromise the system. This is prevented using a seccomp +whitelist in virtiofsd. + +During startup virtiofsd installs a whitelist of allowed system calls. All +other system calls are forbidden for the remaining lifetime of the process. +This list has been built through experience of running virtiofsd on several +flavors of Linux and observing which system calls were encountered. + +It is possible that previously unexplored code paths or newer library versions +will invoke system calls that have not been whitelisted yet. In this case the +process terminates and a seccomp error is captured in the audit log. The log +can typically be viewed using ``journalctl -xe`` and searching for ``SECCOMP``. + +Should it be necessary to extend the whitelist, system call numbers from the +audit log can be translated to names through a CPU architecture-specific +``.tbl`` file in the Linux source tree. They can then be added to the +whitelist in ``seccomp.c`` in the virtiofsd source tree. + +Mount Namespace +--------------- +During startup virtiofsd enters a new mount namespace and releases all mounts +except for the shared directory. This makes the file system root `/` the +shared directory. It is impossible to access files outside the shared +directory since they cannot be looked up by path resolution. + +Several attacks, including `..` traversal and symlink escapes, are prevented by +the mount namespace. + +The current virtiofsd implementation keeps a directory file descriptor to +/proc/self/fd open in order to implement several FUSE requests. This file +descriptor could be used by attackers to access files outside the shared +directory. This limitation will be addressed in a future release of virtiofsd. + +Deployment Best Practices +========================= +If the shared directory is also accessible from a host mount namespace, it is +recommended to keep a parent directory with rwx------ permissions so that other +users on the host are unable to access any setuid executables or device nodes +in the shared directory. The `nosuid` and `nodev` mount options can also be +used to prevent this issue. -- 2.23.0
