Add pivot_root() and unpivot_root() to switch the root dir and restore it.

This will be used to more accurately handling command resolution and
path matching when a new root directory is specified.

plugins/sudoers/pivot.c

@@ -0,0 +1,90 @@
+/*
+ * SPDX-License-Identifier: ISC
+ *
+ * Copyright (c) 2023 Todd C. Miller <Todd.Miller@sudo.ws>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * This is an open source non-commercial project. Dear PVS-Studio, please check it.
+ * PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
+ */
+
+#include <config.h>
+
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "sudoers.h"
+
+#define OLD_ROOT	0
+#define OLD_CWD		1
+
+/*
+ * Pivot to a new root directory, storing the old root and old cwd
+ * in fds[2].  Changes current working directory to the new root.
+ * Returns true on success, else false.
+ */
+bool
+pivot_root(const char *new_root, int fds[2])
+{
+    debug_decl(pivot_root, SUDOERS_DEBUG_UTIL);
+
+    fds[OLD_ROOT] = open("/", O_RDONLY);
+    fds[OLD_CWD] = open(".", O_RDONLY);
+    if (fds[OLD_ROOT] == -1 || fds[OLD_CWD] == -1 || chroot(new_root) == -1) {
+	if (fds[OLD_ROOT] != -1) {
+	    close(fds[OLD_ROOT]);
+	    fds[OLD_ROOT] = -1;
+	}
+	if (fds[OLD_CWD] != -1) {
+	    close(fds[OLD_CWD]);
+	    fds[OLD_CWD] = -1;
+	}
+	debug_return_bool(false);
+    }
+    debug_return_bool(chdir("/") == 0);
+}
+
+/*
+ * Pivot back to the stored root directory and restore the old cwd.
+ * Returns true on success, else false.
+ */
+bool
+unpivot_root(int fds[2])
+{
+    bool ret = true;
+    debug_decl(unpivot_root, SUDOERS_DEBUG_UTIL);
+
+    /* Order is imporant: restore old root, *then* change cwd. */
+    if (fds[OLD_ROOT] != -1) {
+	if (fchdir(fds[OLD_ROOT]) == -1 || chroot(".") == -1) {
+	    sudo_warn("%s", U_("unable to restore root directory"));
+	    ret = false;
+	}
+	close(fds[OLD_ROOT]);
+	fds[OLD_ROOT] = -1;
+    }
+    if (fds[OLD_CWD] != -1) {
+	if (fchdir(fds[OLD_CWD]) == -1) {
+	    sudo_warn("%s", U_("unable to restore current working directory"));
+	    ret = false;
+	}
+	close(fds[OLD_CWD]);
+	fds[OLD_CWD] = -1;
+    }
+
+    debug_return_bool(ret);
+}