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, 49 insertions, 0 deletions

diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index cbc826809e..e0fa484c1e 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -2907,6 +2907,54 @@ int config_parse_address_families(
 
         return 0;
 }
+
+int config_parse_restrict_namespaces(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        ExecContext *c = data;
+        bool invert = false;
+        int r;
+
+        if (isempty(rvalue)) {
+                /* Reset to the default. */
+                c->restrict_namespaces = NAMESPACE_FLAGS_ALL;
+                return 0;
+        }
+
+        if (rvalue[0] == '~') {
+                invert = true;
+                rvalue++;
+        }
+
+        r = parse_boolean(rvalue);
+        if (r > 0)
+                c->restrict_namespaces = 0;
+        else if (r == 0)
+                c->restrict_namespaces = NAMESPACE_FLAGS_ALL;
+        else {
+                /* Not a boolean argument, in this case it's a list of namespace types. */
+
+                r = namespace_flag_from_string_many(rvalue, &c->restrict_namespaces);
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Failed to parse namespace type string, ignoring: %s", rvalue);
+                        return 0;
+                }
+        }
+
+        if (invert)
+                c->restrict_namespaces = (~c->restrict_namespaces) & NAMESPACE_FLAGS_ALL;
+
+        return 0;
+}
 #endif
 
 int config_parse_unit_slice(
@@ -4327,6 +4375,7 @@ void unit_dump_config_items(FILE *f) {
                 { config_parse_syscall_archs,         "ARCHS" },
                 { config_parse_syscall_errno,         "ERRNO" },
                 { config_parse_address_families,      "FAMILIES" },
+                { config_parse_restrict_namespaces,   "NAMESPACES"  },
 #endif
                 { config_parse_cpu_shares,            "SHARES" },
                 { config_parse_cpu_weight,            "WEIGHT" },