Add a new plugin to enable Linux-specific namespace functionality

A plugin is a convenient place to hide Linux-specific functionality.
Implemented in this initial version are:

- Optional private mounts during scriptlet execution, useful for
  protecting the system from scriptlets (eg /home) and the scriptlets
  from themselves (eg insecure /tmp usage)
- Optionally disable network access during scriptlet execution

Note that at this time, scriplets executed with the embedded Lua
interpreter are not covered by this because they run inside the main rpm
process instead of forking (#2635).

Suggested-by: Johannes Segitz <jsegitz@suse.de>

Fixes: #2632
Fixes: #2665

docs/man/CMakeLists.txt

@@ -28,6 +28,9 @@ if (ENABLE_PLUGINS)
 	if (WITH_SELINUX)
 		list(APPEND manuals rpm-plugin-selinux.8)
 	endif()
+	if (HAVE_UNSHARE)
+		list(APPEND manuals rpm-plugin-unshare.8)
+	endif()
 endif()
 
 foreach(man ${manuals})

docs/man/rpm-plugin-unshare.8.md

@@ -0,0 +1,40 @@
+---
+date: 15 Sep 2023
+section: 8
+title: 'RPM-UNSHARE'
+---
+
+NAME
+====
+
+rpm-plugin-unshare - Unshare plugin for the RPM Package Manager
+
+Description
+===========
+
+This plugin allows using various Linux-specific namespace-related
+technologies inside transactions, such as to harden and limit
+scriptlet access to resources.
+
+Configuration
+=============
+
+This plugin implements the following configurables:
+
+`%__transaction_unshare_paths`
+
+:   A colon-separated list of paths to privately mount during scriptlet
+    execution. Typical examples would be `/tmp` to protect against
+    insecure temporary file usage inside scriptlets, and `/home` to
+    prevent scriptlets from accessing user home directories.
+
+`%__transaction_unshare_nonet`
+
+:   Non-zero value disables network access during scriptlet execution.
+
+See **rpm-plugins**(8) on how to control plugins in general.
+
+SEE ALSO
+========
+
+*dbus-monitor*(1) *rpm-plugins*(8)

macros.in

@@ -1194,6 +1194,11 @@ Supplements:   (%{name} = %{version}-%{release} and langpacks-%{1})\
 %__transaction_prioreset	%{__plugindir}/prioreset.so
 %__transaction_audit		%{__plugindir}/audit.so
 %__transaction_dbus_announce	%{__plugindir}/dbus_announce.so
+%__transaction_unshare		%{__plugindir}/unshare.so
+
+# Unshare specific configuration
+%__transaction_unshare_paths /tmp:/home
+%__transaction_unshare_nonet 1
 
 #------------------------------------------------------------------------------
 # Macros for further automated spec %setup and patch application

plugins/CMakeLists.txt

@@ -38,6 +38,10 @@ if(WITH_FSVERITY)
 	target_include_directories(fsverity PRIVATE ${CMAKE_SOURCE_DIR}/sign)
 endif()
 
+if (HAVE_UNSHARE)
+	add_library(unshare MODULE unshare.c)
+endif()
+
 set(plugindir ${CMAKE_INSTALL_FULL_LIBDIR}/rpm-plugins)
 
 get_property(plugins DIRECTORY PROPERTY BUILDSYSTEM_TARGETS)

plugins/unshare.c

@@ -0,0 +1,74 @@
+#include "system.h"
+
+#include <sched.h>
+#include <sys/mount.h>
+#include <errno.h>
+#include <string.h>
+
+#include <rpm/rpmts.h>
+#include <rpm/rpmplugin.h>
+#include <rpm/rpmlog.h>
+#include <rpm/rpmmacro.h>
+
+#include "debug.h"
+
+static ARGV_t private_mounts = NULL;
+static int unshare_flags = 0;
+
+static rpmRC unshare_init(rpmPlugin plugin, rpmts ts)
+{
+    char *paths = rpmExpand("%{?__transaction_unshare_paths}", NULL);
+    private_mounts = argvSplitString(paths, ":", ARGV_SKIPEMPTY);
+    if (private_mounts)
+	unshare_flags |= CLONE_NEWNS;
+    free(paths);
+
+    if (rpmExpandNumeric("%{?__transaction_unshare_nonet}"))
+	unshare_flags |= CLONE_NEWNET;
+
+    return RPMRC_OK;
+}
+
+static void unshare_cleanup(rpmPlugin plugin)
+{
+    /* ensure clean state for possible next transaction */
+    private_mounts = argvFree(private_mounts);
+    unshare_flags = 0;
+}
+
+static rpmRC unshare_scriptlet_fork_post(rpmPlugin plugin,
+						 const char *path, int type)
+{
+    rpmRC rc = RPMRC_FAIL;
+
+    if (unshare_flags && (unshare(unshare_flags) == -1)) {
+	rpmlog(RPMLOG_ERR, _("unshare with flags x%x failed: %s\n"),
+		unshare_flags, strerror(errno));
+	goto exit;
+    }
+
+    if (private_mounts) {
+	if (mount("/", "/", NULL, MS_REC | MS_PRIVATE, NULL) == -1) {
+	    rpmlog(RPMLOG_ERR, _("failed to mount private %s: %s\n"),
+		    "/", strerror(errno));
+	    goto exit;
+	}
+	for (ARGV_t mnt = private_mounts; mnt && *mnt; mnt++) {
+	    if (mount("none", *mnt, "tmpfs", 0, NULL) == -1) {
+		rpmlog(RPMLOG_ERR, _("failed to mount private %s: %s\n"),
+			*mnt, strerror(errno));
+		goto exit;
+	    }
+	}
+    }
+    rc = RPMRC_OK;
+
+exit:
+    return rc;
+}
+
+struct rpmPluginHooks_s unshare_hooks = {
+    .init = unshare_init,
+    .cleanup = unshare_cleanup,
+    .scriptlet_fork_post = unshare_scriptlet_fork_post,
+};