core: add new RestrictNamespaces= unit file setting

This new setting permits restricting whether namespaces may be created and
managed by processes started by a unit. It installs a seccomp filter blocking
certain invocations of unshare(), clone() and setns().

RestrictNamespaces=no is the default, and does not restrict namespaces in any
way. RestrictNamespaces=yes takes away the ability to create or manage any kind
of namspace. "RestrictNamespaces=mnt ipc" restricts the creation of namespaces
so that only mount and IPC namespaces may be created/managed, but no other
kind of namespaces.

This setting should be improve security quite a bit as in particular user
namespacing was a major source of CVEs in the kernel in the past, and is
accessible to unprivileged processes. With this setting the entire attack
surface may be removed for system services that do not make use of namespaces.

1 files changed, 25 insertions, 0 deletions

diff --git a/src/shared/bus-unit-util.c b/src/shared/bus-unit-util.c
index f639e0e832..35e2c8f18e 100644
--- a/src/shared/bus-unit-util.c
+++ b/src/shared/bus-unit-util.c
@@ -27,6 +27,7 @@
 #include "hashmap.h"
 #include "list.h"
 #include "locale-util.h"
+#include "nsflags.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "process-util.h"
@@ -553,6 +554,30 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
 
                 r = sd_bus_message_close_container(m);
 
+        } else if (streq(field, "RestrictNamespaces")) {
+                bool invert = false;
+                uint64_t flags = 0;
+
+                if (eq[0] == '~') {
+                        invert = true;
+                        eq++;
+                }
+
+                r = parse_boolean(eq);
+                if (r > 0)
+                        flags = 0;
+                else if (r == 0)
+                        flags = NAMESPACE_FLAGS_ALL;
+                else {
+                        r = namespace_flag_from_string_many(eq, &flags);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse %s value %s.", field, eq);
+                }
+
+                if (invert)
+                        flags = (~flags) & NAMESPACE_FLAGS_ALL;
+
+                r = sd_bus_message_append(m, "v", "t", flags);
         } else {
                 log_error("Unknown assignment %s.", assignment);
                 return -EINVAL;