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

[Greg Kurz]

Hi Christian,

On Sat, 10 Jun 2023 15:39:44 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> 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.
> 

For the records :

The 'proxy' backend is an old thing that predates vhost-user. It
really turns QEMU into a proxy : all requests go to QEMU, which
forwads them to the helper and the other way around for responses.
Data is copied both ways. All of this severely damages latency.

If someone really wants to offload 9p stuff to a separate process,
they should come up with a vhost-user-9p implementation. The whole
server would be offloaded, possibly sharing most of the code with
the in-QEMU server, QEMU wouldn't be involved anymore in the I/Os.
No more copies.

> 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(-)
> 
> 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
>  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!
> + */
> +

What about having the proxy to print a deprecation warning as well ?

>  #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"),

Since an `-fsdev` is generated internally for each `-virtfs` we end up with
two warnings, which seems a bit weird :

$ qemu-system-x86_64-mbuild -virtfs proxy,id=fsdev0,mount_tag=9p
qemu-system-x86_64: -virtfs proxy,id=fsdev0,mount_tag=9p: warning: '-virtfs 
proxy' is deprecated, use '-virtfs local' instead
qemu-system-x86_64: -virtfs proxy,id=fsdev0,mount_tag=9p: warning: '-fsdev 
proxy' is deprecated, use '-fsdev local' instead
...

Since it might be quite complex to guess in qemu_fsdev_add() that
we're dealing with such an internally generated opt, I'd adapt
the text so that it works for both cases.

e.g. '-fsdev proxy' and '-virtfs proxy' are deprecated, use 'local' instead of 
'proxy'

Cheers,

-- 
Greg