Re: [PATCH v3] 9pfs: deprecate 'proxy' backend

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Saturday, June 10, 2023 3:39:44 PM CEST Christian Schoenebeck wrote:
> As recent CVE-2023-2861 once again showed, the 9p 'proxy' fs driver is in
> bad shape. Using the 'proxy' backend was already discouraged for safety
> reasons before and we recommended to use the 'local' backend instead,
> but now it is time to officially deprecate the 'proxy' backend.
> 
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  v2 -> v3:
>  * Fix copy wasted typo (-> 'backend').
> 
>  MAINTAINERS                        |  7 +++++++
>  docs/about/deprecated.rst          | 17 +++++++++++++++++
>  docs/tools/virtfs-proxy-helper.rst |  3 +++
>  fsdev/qemu-fsdev.c                 |  5 +++++
>  fsdev/virtfs-proxy-helper.c        |  5 +++++
>  hw/9pfs/9p-proxy.c                 |  5 +++++
>  hw/9pfs/9p-proxy.h                 |  5 +++++
>  meson.build                        |  2 +-
>  qemu-options.hx                    |  6 +++++-
>  softmmu/vl.c                       |  5 +++++
>  10 files changed, 58 insertions(+), 2 deletions(-)

Or would it be better to split this up, e.g. into 3 separate patches (runtime
messages, docs, MAINTAINERS)?

> diff --git a/MAINTAINERS b/MAINTAINERS
> index 436b3f0afe..185d694b2e 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -2118,13 +2118,20 @@ S: Odd Fixes
>  W: https://wiki.qemu.org/Documentation/9p
>  F: hw/9pfs/
>  X: hw/9pfs/xen-9p*
> +X: hw/9pfs/9p-proxy*
>  F: fsdev/
> +X: fsdev/virtfs-proxy-helper.c
>  F: docs/tools/virtfs-proxy-helper.rst

I missed virtfs-proxy-helper.rst here. That should be moved to the new 'proxy'
section below as well.

>  F: tests/qtest/virtio-9p-test.c
>  F: tests/qtest/libqos/virtio-9p*
>  T: git https://gitlab.com/gkurz/qemu.git 9p-next
>  T: git https://github.com/cschoenebeck/qemu.git 9p.next
>  
> +virtio-9p-proxy
> +F: hw/9pfs/9p-proxy*
> +F: fsdev/virtfs-proxy-helper.c
> +S: Obsolete
> +
>  virtio-blk
>  M: Stefan Hajnoczi <stefanha@redhat.com>
>  L: qemu-block@nongnu.org
> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> index 0743459862..9b2c780365 100644
> --- a/docs/about/deprecated.rst
> +++ b/docs/about/deprecated.rst
> @@ -343,6 +343,23 @@ the addition of volatile memory support, it is now necessary to distinguish
>  between persistent and volatile memory backends.  As such, memdev is deprecated
>  in favor of persistent-memdev.
>  
> +``-fsdev proxy`` and ``-virtfs proxy`` (since 8.1)
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The 9p ``proxy`` filesystem backend driver has been deprecated and will be
> +removed in a future version of QEMU. Please use ``-fsdev local`` or
> +``-virtfs local`` for using the ``local`` 9p filesystem backend instead.
> +
> +The 9p ``proxy`` backend was originally developed as an alternative to the 9p
> +``local`` backend. The idea was to enhance security by dispatching actual low
> +level filesystem operations from 9p server (QEMU process) over to a separate
> +process (the virtfs-proxy-helper binary). However this alternative never gained
> +momentum. The proxy backend is much slower than the local backend, hasn't seen
> +any development in years, and showed to be less secure, especially due to the
> +fact that its helper daemon must be run as root, whereas with the local backend
> +QEMU is typically run as unprivileged user and allows to tighten behaviour by
> +mapping permissions et al.
> +
>  
>  Block device options
>  ''''''''''''''''''''
> diff --git a/docs/tools/virtfs-proxy-helper.rst b/docs/tools/virtfs-proxy-helper.rst
> index 6cdeedf8e9..bd310ebb07 100644
> --- a/docs/tools/virtfs-proxy-helper.rst
> +++ b/docs/tools/virtfs-proxy-helper.rst
> @@ -9,6 +9,9 @@ Synopsis
>  Description
>  -----------
>  
> +NOTE: The 9p 'proxy' backend is deprecated (since QEMU 8.1) and will be
> +removed, along with this daemon, in a future version of QEMU!
> +
>  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:
> diff --git a/fsdev/qemu-fsdev.c b/fsdev/qemu-fsdev.c
> index 3da64e9f72..242f54ab49 100644
> --- a/fsdev/qemu-fsdev.c
> +++ b/fsdev/qemu-fsdev.c
> @@ -133,6 +133,11 @@ int qemu_fsdev_add(QemuOpts *opts, Error **errp)
>      }
>  
>      if (fsdriver) {
> +        if (strncmp(fsdriver, "proxy", 5) == 0) {
> +            warn_report("'-fsdev proxy' is deprecated, use '-fsdev local' "
> +                        "instead");
> +        }
> +
>          for (i = 0; i < ARRAY_SIZE(FsDrivers); i++) {
>              if (strcmp(FsDrivers[i].name, fsdriver) == 0) {
>                  break;
> diff --git a/fsdev/virtfs-proxy-helper.c b/fsdev/virtfs-proxy-helper.c
> index d9511f429c..5dd5d99284 100644
> --- a/fsdev/virtfs-proxy-helper.c
> +++ b/fsdev/virtfs-proxy-helper.c
> @@ -9,6 +9,11 @@
>   * the COPYING file in the top-level directory.
>   */
>  
> +/*
> + * NOTE: The 9p 'proxy' backend is deprecated (since QEMU 8.1) and will be
> + * removed in a future version of QEMU!
> + */
> +
>  #include "qemu/osdep.h"
>  #include <glib/gstdio.h>
>  #include <sys/resource.h>
> diff --git a/hw/9pfs/9p-proxy.c b/hw/9pfs/9p-proxy.c
> index 99d115ff0d..905cae6992 100644
> --- a/hw/9pfs/9p-proxy.c
> +++ b/hw/9pfs/9p-proxy.c
> @@ -15,6 +15,11 @@
>   * https://wiki.qemu.org/Documentation/9p
>   */
>  
> +/*
> + * NOTE: The 9p 'proxy' backend is deprecated (since QEMU 8.1) and will be
> + * removed in a future version of QEMU!
> + */
> +
>  #include "qemu/osdep.h"
>  #include <sys/socket.h>
>  #include <sys/un.h>
> diff --git a/hw/9pfs/9p-proxy.h b/hw/9pfs/9p-proxy.h
> index b84301d001..9be4718d3e 100644
> --- a/hw/9pfs/9p-proxy.h
> +++ b/hw/9pfs/9p-proxy.h
> @@ -10,6 +10,11 @@
>   * the COPYING file in the top-level directory.
>   */
>  
> +/*
> + * NOTE: The 9p 'proxy' backend is deprecated (since QEMU 8.1) and will be
> + * removed in a future version of QEMU!
> + */
> +
>  #ifndef QEMU_9P_PROXY_H
>  #define QEMU_9P_PROXY_H
>  
> diff --git a/meson.build b/meson.build
> index 34306a6205..05c01b72bb 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -4170,7 +4170,7 @@ if have_block
>    summary_info += {'Block whitelist (ro)': get_option('block_drv_ro_whitelist')}
>    summary_info += {'Use block whitelist in tools': get_option('block_drv_whitelist_in_tools')}
>    summary_info += {'VirtFS (9P) support':    have_virtfs}
> -  summary_info += {'VirtFS (9P) Proxy Helper support': have_virtfs_proxy_helper}
> +  summary_info += {'VirtFS (9P) Proxy Helper support (deprecated)': have_virtfs_proxy_helper}
>    summary_info += {'Live block migration': config_host_data.get('CONFIG_LIVE_BLOCK_MIGRATION')}
>    summary_info += {'replication support': config_host_data.get('CONFIG_REPLICATION')}
>    summary_info += {'bochs support':     get_option('bochs').allowed()}
> diff --git a/qemu-options.hx b/qemu-options.hx
> index b57489d7ca..3a6c7d3ef9 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -1735,7 +1735,9 @@ SRST
>          Accesses to the filesystem are done by QEMU.
>  
>      ``proxy``
> -        Accesses to the filesystem are done by virtfs-proxy-helper(1).
> +        Accesses to the filesystem are done by virtfs-proxy-helper(1). This
> +        option is deprecated (since QEMU 8.1) and will be removed in a future
> +        version of QEMU. Use ``local`` instead.
>  
>      ``synth``
>          Synthetic filesystem, only used by QTests.
> @@ -1867,6 +1869,8 @@ SRST
>  
>      ``proxy``
>          Accesses to the filesystem are done by virtfs-proxy-helper(1).
> +        This option is deprecated (since QEMU 8.1) and will be removed in a
> +        future version of QEMU. Use ``local`` instead.
>  
>      ``synth``
>          Synthetic filesystem, only used by QTests.
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index b0b96f67fa..e60648b591 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -3102,6 +3102,11 @@ void qemu_init(int argc, char **argv)
>                      error_report("Usage: -virtfs fsdriver,mount_tag=tag");
>                      exit(1);
>                  }
> +                if (strncmp(qemu_opt_get(opts, "fsdriver"), "proxy", 5) == 0) {
> +                    warn_report("'-virtfs proxy' is deprecated, use "
> +                                "'-virtfs local' instead");
> +                }
> +
>                  fsdev = qemu_opts_create(qemu_find_opts("fsdev"),
>                                           qemu_opts_id(opts) ?:
>                                           qemu_opt_get(opts, "mount_tag"),
>