Add --assert-userns-disabled option

We can't combine --disable-userns with entering an existing user
namespace via --userns if the existing user namespace was created with
--disable-userns, because its ability to create nested user namespaces
has already been disabled. However, the next best thing is to verify
that we are already in the desired state.

Signed-off-by: Simon McVittie <smcv@collabora.com>

bubblewrap.c

@@ -73,6 +73,7 @@ static const char *opt_file_label = NULL;
 static bool opt_as_pid_1;
 
 const char *opt_chdir_path = NULL;
+bool opt_assert_userns_disabled = FALSE;
 bool opt_disable_userns = FALSE;
 bool opt_unshare_user = FALSE;
 bool opt_unshare_user_try = FALSE;
@@ -313,6 +314,7 @@ usage (int ecode, FILE *out)
            "    --userns FD                  Use this user namespace (cannot combine with --unshare-user)\n"
            "    --userns2 FD                 After setup switch to this user namespace, only useful with --userns\n"
            "    --disable-userns             Disable further use of user namespaces inside sandbox\n"
+           "    --assert-userns-disabled     Fail unless further use of user namespace inside sandbox is disabled\n"
            "    --pidns FD                   Use this pid namespace (as parent namespace if using --unshare-pid)\n"
            "    --uid UID                    Custom uid in the sandbox (requires --unshare-user or --userns)\n"
            "    --gid GID                    Custom gid in the sandbox (requires --unshare-user or --userns)\n"
@@ -1783,6 +1785,10 @@ parse_args_recurse (int          *argcp,
         {
           opt_disable_userns = TRUE;
         }
+      else if (strcmp (arg, "--assert-userns-disabled") == 0)
+        {
+          opt_assert_userns_disabled = TRUE;
+        }
       else if (strcmp (arg, "--remount-ro") == 0)
         {
           if (argc < 2)
@@ -3202,18 +3208,18 @@ main (int    argc,
       /* We're in a new user namespace, we got back the bounding set, clear it again */
       drop_cap_bounding_set (FALSE);
 
-      if (opt_disable_userns)
+      write_uid_gid_map (opt_sandbox_uid, ns_uid,
+                         opt_sandbox_gid, ns_gid,
+                         -1, FALSE, FALSE);
+    }
+
+  if (opt_disable_userns || opt_assert_userns_disabled)
     {
       /* Verify that we can't make a new userns again */
       res = unshare (CLONE_NEWUSER);
 
       if (res == 0)
-            die ("unable to disable creation of new user namespaces");
+        die ("creation of new user namespaces was not disabled as requested");
-        }
-
-      write_uid_gid_map (opt_sandbox_uid, ns_uid,
-                         opt_sandbox_gid, ns_gid,
-                         -1, FALSE, FALSE);
     }
 
   /* All privileged ops are done now, so drop caps we don't need */

bwrap.xml

@@ -158,6 +158,17 @@
         in the setuid version of bubblewrap.
       </para></listitem>
     </varlistentry>
+    <varlistentry>
+      <term><option>--assert-userns-disabled</option></term>
+      <listitem><para>
+        Confirm that the process in the sandbox has been prevented from
+        creating further user namespaces, but without taking any particular
+        action to prevent that. For example, this can be combined with
+        <option>--userns</option> to check that the given user namespace
+        has already been set up to prevent the creation of further user
+        namespaces.
+      </para></listitem>
+    </varlistentry>
     <varlistentry>
       <term><option>--pidns <arg choice="plain">FD</arg></option></term>
       <listitem><para>Use an existing pid namespace instead of creating one. This is often used with --userns, because the pid namespace must be owned by the same user namespace that bwrap uses. </para>

completions/bash/bwrap

@@ -10,6 +10,7 @@ _bwrap() {
 	# Please keep sorted in LC_ALL=C order
 	local boolean_options="
 		--as-pid-1
+		--assert-userns-disabled
 		--clearenv
 		--disable-userns
 		--help

completions/zsh/_bwrap

@@ -27,6 +27,7 @@ _bwrap_args=(
 
     # Please sort alphabetically (in LC_ALL=C order) by option name
     '--add-seccomp-fd[Load and use seccomp rules from FD]: :_guard "[0-9]#" "file descriptor to read seccomp rules from"'
+    '--assert-userns-disabled[Fail unless further use of user namespace inside sandbox is disabled]'
     '--args[Parse NUL-separated args from FD]: :_guard "[0-9]#" "file descriptor with NUL-separated arguments"'
     '--as-pid-1[Do not install a reaper process with PID=1]'
     '--bind-try[Equal to --bind but ignores non-existent SRC]:source:_files:destination:_files'

tests/test-run.sh

@@ -126,10 +126,12 @@ else
     echo "ok - can pivot to new rootfs recursively"
 
     $BWRAP --dev-bind / / -- true
+    ! $BWRAP --assert-userns-disabled --dev-bind / / -- true
     $BWRAP --unshare-user --disable-userns --dev-bind / / -- true
     ! $BWRAP --unshare-user --disable-userns --dev-bind / / -- $BWRAP --dev-bind / / -- true
     $BWRAP --unshare-user --disable-userns --dev-bind / / -- sh -c "echo 2 > /proc/sys/user/max_user_namespaces || true; ! $BWRAP --dev-bind / / -- true"
     $BWRAP --unshare-user --disable-userns --dev-bind / / -- sh -c "echo 100 > /proc/sys/user/max_user_namespaces || true; ! $BWRAP --dev-bind / / -- true"
+    $BWRAP --unshare-user --disable-userns --dev-bind / / -- sh -c "! $BWRAP --dev-bind / / --assert-userns-disabled -- true"
     echo "ok - can disable nested userns"
 fi