Provided by: qemu-system-common_4.0+dfsg-0ubuntu9_amd64 bug


       virtfs-proxy-helper - QEMU 9p virtfs proxy filesystem helper


       virtfs-proxy-helper options


           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.


       The following options are supported:

       -h  Display help and exit

       -p|--path path
           Path to export for proxy filesystem driver

       -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

       -s|--socket socket-file
           Creates named socket file for communicating with qemu proxy fs driver

       -u|--uid uid -g|--gid gid
           uid:gid combination to give access to named socket file

           Run as a normal program. By default program will run in daemon mode


       M. Mohan Kumar

                                            2019-09-24                   VIRTFS-PROXY-HELPER.1(1)