util/mkhtemp: extremely hardened mkhtemp

This will also be used in lbmk itself at some point,
which currently just uses regular mktemp, for tmpdir
handling during the build process.

Renamed util/nvmutil to util/libreboot-utils, which
now contains two tools. The new tool, mkhtemp, is a
hardened implementation of mktemp, which nvmutil
also uses now. Still experimental, but good enough
for nvmutil.

Mkhtemp attempts to provide TOCTOU resistance on
Linux, by using modern features in Linux such as
Openat2 (syscall) with O_EXCL and O_TMPFILE,
and many various security checks e.g.
inode/dev during creation. Checks are done constantly,
to try to detect race conditions. The code is very
strict about things like sticky bits in world writeable
directories, also ownership (it can be made to bar even
root access on files and directories it doesn't own).

It's a security-first implementation of mktemp, likely
even more secure than the OpenBSD mkstemp, but more
auditing and testing is needed - more features are
also planned, including a compatibility mode to make
it also work like traditional mktemp/mkstemp. The
intention, once this becomes stable, is that it will
become a modern drop-in replacement for mkstemp on
Linux and BSD systems.

Some legacy code has been removed, and in general
cleaned up. I wrote mkhtemp for nvmutil, as part of
its atomic write behaviour, but mktemp was the last
remaining liability, so I rewrote that too!

Docs/manpage/website will be made for mkhtemp once
the code is mature.

Other changes have also been made. This is from another
experimental branch of Libreboot, that I'm pushing
early. For example, nvmutil's state machine has been
tidied up, moving more logic back into main.

Mktemp is historically prone to race conditions,
e.g. symlink attacks, directory replacement, remounting
during operation, all sorts of things. Mkhtemp has
been written to solve, or otherwise mitigate, that
problem. Mkhtemp is currently experimental and will
require a major cleanup at some point, but it
already works well enough, and you can in fact use
it; at this time, the -d, -p and -q flags are
supported, and you can add a custom template at
the end, e.g.

mkhtemp -p test -d

Eventually, I will make this have complete parity
with the GNU and BSD implementations, so that it is
fully useable on existing setups, while optionally
providing the hardening as well.

A lot of code has also been tidied up. I didn't
track the changes I made with this one, because
it was a major re-write of nvmutil; it is now
libreboot-utils, and I will continue to write
more programs in here over time. It's basically
now a bunch of hardened wrappers around various
libc functions, e.g. there is also a secure I/O
wrapper for read/write.

There is a custom randomisation function, rlong,
which simply uses arc4random or getrandom, on
BSD and Linux respectively. Efforts are made to
make it as reliable as possible, to the extent
that it never returns with failure; in the unlikely
event that it fails, it aborts. It also sleeps
between failure, to mitigate certain DoS attacks.

You can just go in util/libreboot-utils and
type make, then you will have the nvmutil and
mkhtemp binaries, which you can just use. It
all works. Everything was massively rewritten.

Signed-off-by: Leah Rowe <leah@libreboot.org>

1 files changed, 114 insertions, 0 deletions

diff --git a/util/libreboot-utils/lib/rand.c b/util/libreboot-utils/lib/rand.c
new file mode 100644
index 00000000..c4cf008c
--- /dev/null
+++ b/util/libreboot-utils/lib/rand.c
@@ -0,0 +1,114 @@
+/* SPDX-License-Identifier: MIT
+ * Copyright (c) 2026 Leah Rowe <leah@libreboot.org>
+ *
+ * Random number generation
+ */
+
+#ifndef RAND_H
+#define RAND_H
+
+#ifdef __OpenBSD__
+#include <sys/param.h>
+#endif
+#include <sys/types.h>
+
+#include <errno.h>
+#if !((defined(__OpenBSD__) && (OpenBSD) >= 201) || \
+    defined(__FreeBSD__) || \
+    defined(__NetBSD__) || defined(__APPLE__))
+#include <fcntl.h> /* if not arc4random: /dev/urandom */
+#endif
+#include <limits.h>
+#include <stddef.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+#include "../include/common.h"
+
+/* Random numbers
+ */
+
+/* when calling this: save errno
+ * first, then set errno to zero.
+ * on error, this function will
+ * set errno and possibly return
+ *
+ * rlong also preserves errno
+ * and leaves it unchanged on
+ * success, so if you do it
+ * right, you can detect error.
+ * this is because it uses
+ * /dev/urandom which can err.
+ * ditto getrandom (EINTR),
+ * theoretically.
+ */
+
+/* for the linux version: we use only the
+ * syscall, because we cannot trust /dev/urandom
+ * to be as robust, and some libc implementations
+ * may default to /dev/urandom under fault conditions.
+ *
+ * for general high reliability, we must abort on
+ * failure. in practise, it will likely never fail.
+ * the arc4random call on bsd never returns error.
+ */
+
+size_t
+rlong(void)
+{
+	size_t rval;
+	int saved_errno = errno;
+	errno = 0;
+
+#if (defined(__OpenBSD__) || defined(__FreeBSD__) || \
+    defined(__NetBSD__) || defined(__APPLE__) || \
+    defined(__DragonFly__))
+
+	arc4random_buf(&rval, sizeof(size_t));
+	goto out;
+
+#elif defined(__linux__)
+
+	size_t off = 0;
+	size_t len = sizeof(rval);
+	ssize_t rc;
+
+retry_rand:
+	rc = (ssize_t)syscall(SYS_getrandom,
+	    (char *)&rval + off, len - off, 0);
+
+	if (rc < 0) {
+		if (errno == EINTR || errno == EAGAIN) {
+			usleep(100);
+			goto retry_rand;
+		}
+
+		goto err; /* possibly unsupported by kernel */
+	}
+
+	if ((off += (size_t)rc) < len)
+		goto retry_rand;
+
+	goto out;
+err:
+	/*
+	 * getrandom can return with error, but arc4random
+	 * doesn't. generally, getrandom will be reliable,
+	 * but we of course have to maintain parity with
+	 * BSD. So a rand failure is to be interpreted as
+	 * a major systems failure, and we act accordingly.
+	 */
+	err_no_cleanup(1, ECANCELED,
+	    "Randomisation failure, possibly unsupported in your kernel.");
+	exit(EXIT_FAILURE);
+
+#else
+#error Unsupported operating system (possibly unsecure randomisation)
+#endif
+
+out:
+	errno = saved_errno;
+	return rval;
+}
+#endif