qemu/fsdev/virtfs-proxy-helper.texi

@example
@c man begin SYNOPSIS
@command{virtfs-proxy-helper} @var{options}
@c man end
@end example

@c man begin DESCRIPTION
@table @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

1) TOCTTOU vulnerability: Following symbolic links in the server could
provide access to files beyond 9p export path.

2) 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.

@end table
@c man end

@c man begin OPTIONS
The following options are supported:
@table @option
@item -h
@findex -h
Display help and exit
@item -p|--path path
Path to export for proxy filesystem driver
@item -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 -f|--fd
@item -s|--socket socket-file
Creates named socket file for communicating with qemu proxy fs driver
@item -u|--uid uid -g|--gid gid
uid:gid combination to give access to named socket file
@item -n|--nodaemon
Run as a normal program. By default program will run in daemon mode
@end table
@c man end

@setfilename virtfs-proxy-helper
@settitle QEMU 9p virtfs proxy filesystem helper

@c man begin AUTHOR
M. Mohan Kumar
@c man end