[PATCH 23/25] virtiofsd: add security guide document

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: Stefan Hajnoczi <stefanha@redhat.com>

Many people want to know: what's up with virtiofsd and security?  This
document provides the answers!

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 contrib/virtiofsd/security.rst | 108 +++++++++++++++++++++++++++++++++
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 create mode 100644 contrib/virtiofsd/security.rst

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+========================
+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