virtfs-proxy-helper: Convert documentation to rST

The virtfs-proxy-helper documentation is currently in
fsdev/qemu-trace-stap.texi in Texinfo format, which we
present to the user as:
 * a virtfs-proxy-helper manpage
 * but not (unusually for QEMU) part of the HTML docs

Convert the documentation to rST format that lives in
the docs/ subdirectory, and present it to the user as:
 * a virtfs-proxy-helper manpage
 * part of the interop/ Sphinx manual

There are minor formatting changes to suit Sphinx, but no
content changes. In particular I've split the -u and -g
options into each having their own description text.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Greg Kurz <groug@kaod.org>
Message-id: 20200124162606.8787-9-peter.maydell@linaro.org

docs/interop/conf.py

@@ -24,5 +24,8 @@ man_pages = [
     ('qemu-nbd', 'qemu-nbd', u'QEMU Disk Network Block Device Server',
      ['Anthony Liguori <anthony@codemonkey.ws>'], 8),
     ('qemu-trace-stap', 'qemu-trace-stap', u'QEMU SystemTap trace tool',
-     [], 1)
+     [], 1),
+    ('virtfs-proxy-helper', 'virtfs-proxy-helper',
+     u'QEMU 9p virtfs proxy filesystem helper',
+     ['M. Mohan Kumar'], 1)
 ]

docs/interop/index.rst

@@ -23,3 +23,4 @@ Contents:
    qemu-trace-stap
    vhost-user
    vhost-user-gpu
+   virtfs-proxy-helper

docs/interop/virtfs-proxy-helper.rst

@@ -0,0 +1,72 @@
+QEMU 9p virtfs proxy filesystem helper
+======================================
+
+Synopsis
+--------
+
+**virtfs-proxy-helper** [*OPTIONS*]
+
+Description
+-----------
+
+Pass-through security model in QEMU 9p server needs root privilege to do
+few file operations (like chown, chmod to any mode/uid:gid).  There are two
+issues in pass-through security model:
+
+- TOCTTOU vulnerability: Following symbolic links in the server could
+  provide access to files beyond 9p export path.
+
+- Running QEMU with root privilege could be a security issue.
+
+To overcome above issues, following approach is used: A new filesystem
+type 'proxy' is introduced. Proxy FS uses chroot + socket combination
+for securing the vulnerability known with following symbolic links.
+Intention of adding a new filesystem type is to allow qemu to run
+in non-root mode, but doing privileged operations using socket IO.
+
+Proxy helper (a stand alone binary part of qemu) is invoked with
+root privileges. Proxy helper chroots into 9p export path and creates
+a socket pair or a named socket based on the command line parameter.
+QEMU and proxy helper communicate using this socket. QEMU proxy fs
+driver sends filesystem request to proxy helper and receives the
+response from it.
+
+The proxy helper is designed so that it can drop root privileges except
+for the capabilities needed for doing filesystem operations.
+
+Options
+-------
+
+The following options are supported:
+
+.. program:: virtfs-proxy-helper
+
+.. option:: -h
+
+  Display help and exit
+
+.. option:: -p, --path PATH
+
+  Path to export for proxy filesystem driver
+
+.. option:: -f, --fd SOCKET_ID
+
+  Use given file descriptor as socket descriptor for communicating with
+  qemu proxy fs drier. Usually a helper like libvirt will create
+  socketpair and pass one of the fds as parameter to this option.
+
+.. option:: -s, --socket SOCKET_FILE
+
+  Creates named socket file for communicating with qemu proxy fs driver
+
+.. option:: -u, --uid UID
+
+  uid to give access to named socket file; used in combination with -g.
+
+.. option:: -g, --gid GID
+
+  gid to give access to named socket file; used in combination with -u.
+
+.. option:: -n, --nodaemon
+
+  Run as a normal program. By default program will run in daemon mode