[PATCH] 9pfs: deprecate 'proxy' backend

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

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>
---
 MAINTAINERS               |  7 +++++++
 docs/about/deprecated.rst | 17 +++++++++++++++++
 fsdev/qemu-fsdev.c        |  5 +++++
 meson.build               |  2 +-
 softmmu/vl.c              |  5 +++++
 5 files changed, 35 insertions(+), 1 deletion(-)

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/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/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/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"),
-- 
2.30.2