key2root.c

/* See LICENSE file for copyright and license details. */
#include <sys/mman.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#include <errno.h>
#include <fcntl.h>
#include <pwd.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "arg.h"
#include "crypt.h"


#define EXIT_AUTH   124
#define EXIT_ERROR  125
#define EXIT_EXEC   126
#define EXIT_NOENT  127


char *argv0;

/* Keep list in sync with asroot(8)'s list */
static const char *env_whitelist[] = {
	"DISPLAY=",
	"WAYLAND_DISPLAY=",
	"PATH=",
	"TERM=",
	"COLORTERM=",
	"XAUTHORITY=",
	"LANG=",
	"LANGUAGE=",
	"LOCALE=",
	"LC_CTYPE=",
	"LC_NUMERIC=",
	"LC_TIME=",
	"LC_COLLATE=",
	"LC_MONETARY=",
	"LC_MESSAGES=",
	"LC_PAPER=",
	"LC_NAME=",
	"LC_ADDRESS=",
	"LC_TELEPHONE=",
	"LC_MEASUREMENT=",
	"LC_IDENTIFICATION=",
	"LC_ALL=",
	"LOCPATH=",
	"NLSPATH=",
	"TZ=",
	"TZDIR=",
	"SDL_VIDEO_FULLSCREEN_DISPLAY=",
	"EDITOR=",
	"VISUAL=",
	"BROWSER=",
	"DESKTOP_SESSION=",
	"LS_COLORS=",
	"GTK_THEME=",
	"QT_STYLE_OVERRIDE=",
	"PWD=",
	"OLDPWD=",
	"JAVA_HOME=",
	"_JAVA_AWT_WM_NONREPARENTING=",
	"_JAVA_OPTIONS=",
	"MAIN_ALSA_MIXER=",
	"MAIN_ALSA_CARD=",
	"XDG_SEAT=",
	"XDG_SESSION_TYPE=",
	"XDG_SESSION_CLASS=",
	"XDG_VTNR=",
	"XDG_SESSION_ID=",
	"XDG_DATA_DIRS=",
	"XDG_CONFIG_DIRS=",
	"MANPATH=",
	"INFODIR=",
	"PAGER=",
	"ftp_proxy=",
	"http_proxy=",
	NULL
};


static void
usage(void)
{
	fprintf(stderr, "usage: %s [-k key-name] [-e] command [argument] ...\n", argv0);
	exit(EXIT_ERROR);
}


static int
forward(char *data, size_t len)
{
	int fds[2];
	size_t off;
	ssize_t r;

	/* We are using sockets because they cannot be hijacked via /proc/<pid>/fd/ */

	if (socketpair(PF_LOCAL, SOCK_STREAM, 0, fds)) {
		fprintf(stderr, "%s: socketpair PF_LOCAL SOCK_STREAM 0: %s\n", argv0, strerror(errno));
		return -1;
	}
	if (shutdown(fds[0], SHUT_WR)) {
		fprintf(stderr, "%s: shutdown <socket> SHUT_WR: %s\n", argv0, strerror(errno));
		close(fds[0]);
		close(fds[1]);
		return -1;
	}
	if (shutdown(fds[1], SHUT_RD)) {
		fprintf(stderr, "%s: shutdown <socket> SHUT_RD: %s\n", argv0, strerror(errno));
		close(fds[0]);
		close(fds[1]);
		return -1;
	}

	switch (fork()) {
	case -1:
		fprintf(stderr, "%s: fork: %s\n", argv0, strerror(errno));
		close(fds[0]);
		close(fds[1]);
		return -1;
	case 0:
		if (mlockall(MCL_CURRENT))
			fprintf(stderr, "%s: mlockall MCL_CURRENT: %s\n", argv0, strerror(errno));
		close(fds[0]);
		break;
	default:
		close(fds[1]);
		return fds[0];
	}

	for (off = 0; off < len; off += (size_t)r) {
		r = write(fds[1], &data[off], len - off);
		if (r < 0) {
			fprintf(stderr, "%s: write <socket>: %s\n", argv0, strerror(errno));
			close(fds[1]);
			_exit(1);
		}
		explicit_bzero(&data[off], (size_t)r);
	}

	close(fds[1]);
	_exit(0);
}


static char **
set_environ(void)
{
	char **new_environ;
	size_t i, j, n, len;
	struct passwd *pw;

	new_environ = calloc(sizeof(env_whitelist) / sizeof(*env_whitelist) + 5, sizeof(*env_whitelist));
	if (!new_environ) {
		fprintf(stderr, "%s: calloc %zu %zu: %s\n",
			argv0, sizeof(env_whitelist) / sizeof(*env_whitelist) + 5, sizeof(*env_whitelist), strerror(errno));
		exit(EXIT_ERROR);
	}
	for (i = 0, n = 0; env_whitelist[i]; i++) {
		len = strlen(env_whitelist[i]);
		for (j = 0; environ[j]; j++) {
			if (!strncmp(environ[j], env_whitelist[i], len)) {
				new_environ[n++] = environ[j];
				break;
			}
		}
	}

	errno = 0;
	pw = getpwuid(0);
	if (!pw) {
		if (errno)
			fprintf(stderr, "%s: getpwuid 0: %s\n", argv0, strerror(errno));
		else
			fprintf(stderr, "%s: cannot find root user\n", argv0);
		exit(EXIT_ERROR);
	}

	if (pw->pw_dir && *pw->pw_dir) {
		len = strlen(pw->pw_dir);
		len += sizeof("HOME=");
		new_environ[n] = malloc(len);
		if (!new_environ[n])
			fprintf(stderr, "%s: malloc %zu: %s\n", argv0, len, strerror(errno));
		stpcpy(stpcpy(new_environ[n++], "HOME="), pw->pw_dir);
	}
	if (pw->pw_name && *pw->pw_name) {
		len = strlen(pw->pw_name);
		len += sizeof("LOGNAME=");
		new_environ[n] = malloc(len);
		if (!new_environ[n])
			fprintf(stderr, "%s: malloc %zu: %s\n", argv0, len, strerror(errno));
		stpcpy(stpcpy(new_environ[n++], "LOGNAME="), pw->pw_name);

		len -= sizeof("LOGNAME=");
		len += sizeof("USER=");
		new_environ[n] = malloc(len);
		if (!new_environ[n])
			fprintf(stderr, "%s: malloc %zu: %s\n", argv0, len, strerror(errno));
		stpcpy(stpcpy(new_environ[n++], "USER="), pw->pw_name);

		len -= sizeof("USER=");
		len += sizeof("MAIL=/var/spool/mail/");
		new_environ[n] = malloc(len);
		if (!new_environ[n])
			fprintf(stderr, "%s: malloc %zu: %s\n", argv0, len, strerror(errno));
		stpcpy(stpcpy(new_environ[n++], "MAIL=/var/spool/mail/"), pw->pw_name);
	}
	if (pw->pw_shell && *pw->pw_shell) {
		len = strlen(pw->pw_shell);
		len += sizeof("SHELL=");
		new_environ[n] = malloc(len);
		if (!new_environ[n])
			fprintf(stderr, "%s: malloc %zu: %s\n", argv0, len, strerror(errno));
		stpcpy(stpcpy(new_environ[n++], "SHELL="), pw->pw_shell);
	}
	new_environ[n] = NULL;

	return new_environ;
}


static int
hashequal(const char *a, const char *b)
{
	size_t an = strlen(a) + 1;
	size_t bn = strlen(b) + 1;
	size_t n = an < bn ? an : bn;
	size_t i;
	int diff = 0;
	for (i = 0; i < n; i++)
		diff |= a[i] ^ b[i];
	return !diff;
}


static int
checkauth(char *data, size_t whead, size_t *rheadp, size_t *rhead2p, size_t *linenop, const char *path,
          const char *keyname, size_t keyname_len, char *key, size_t key_len, int *key_foundp)
{
	int failed = 0, match;
	char *hash, *sp;
	size_t len;

	while (*rhead2p < whead && data[*rhead2p] != '\n')
		++*rhead2p;

	if (data[*rhead2p] != '\n')
		return 0;

	len = *rhead2p - *rheadp;
	*linenop += 1;

	if (memchr(&data[*rheadp], '\0', len)) {
		fprintf(stderr, "%s: NUL byte found in %s on line %zu\n", argv0, path, *linenop);
		failed = 1;
	}
	sp = memchr(&data[*rheadp], ' ', len);
	if (!sp) {
		fprintf(stderr, "%s: no SP byte found in %s on line %zu\n", argv0, path, *linenop);
		failed = 1;
	}

	if (!failed && !keyname) {
		keyname_len = (size_t)(sp - &data[*rheadp]);
		goto check;
	} else if (failed || keyname_len >= len || data[*rheadp + keyname_len] != ' ' ||
	           memcmp(&data[*rheadp], keyname, keyname_len)) {
		*rheadp = ++*rhead2p;
		return 0;
	} else {
	check:
		*rheadp += keyname_len + 1;
		*key_foundp = 1;
		data[(*rhead2p)++] = '\0';
		hash = key2root_crypt(key, key_len, &data[*rheadp], 0);
		match = hash && hashequal(hash, &data[*rheadp]);
		free(hash);
		*rheadp = *rhead2p;
		return match;
	}
}


static int
authenticate(const char *path, const char *keyname, char *key, size_t key_len, int *key_foundp)
{
	int fd;
	char *data = NULL;
	size_t size = 0;
	size_t whead = 0;
	size_t rhead = 0;
	size_t rhead2 = 0;
	size_t lineno = 0;
	ssize_t r = 1;
	size_t keyname_len = keyname ? strlen(keyname) : 0;

	fd = open(path, O_RDONLY);
	if (fd < 0) {
		if (errno != ENOENT)
			fprintf(stderr, "%s: open %s O_RDONLY: %s\n", argv0, path, strerror(errno));
		return 0;
	}

	while (r) {
		if (whead == size) {
			memmove(data, &data[rhead], whead -= rhead);
			rhead2 -= rhead;
			rhead = 0;
			if (whead == size) {
				data = realloc(data, size += 1024);
				if (!data) {
					fprintf(stderr, "%s: realloc: %s\n", argv0, strerror(errno));
					close(fd);
					return 0;
				}
			}
		}
		r = read(fd, &data[whead], size - whead);
		if (r < 0) {
			fprintf(stderr, "%s: read %s: %s\n", argv0, path, strerror(errno));
			close(fd);
			return 0;
		}
		whead += (size_t)r;

		while (rhead2 < whead) {
			if (checkauth(data, whead, &rhead, &rhead2, &lineno, path,
			              keyname, keyname_len, key, key_len, key_foundp)) {
				close(fd);
				return 1;
			}
		}
	}

	if (rhead != whead) {
		fprintf(stderr, "%s: file truncated: %s\n", argv0, path);
		if (memchr(&data[rhead], '\0', whead - rhead))
			fprintf(stderr, "%s: NUL byte found in %s on line %zu\n", argv0, path, lineno + 1);
	}

	close(fd);
	return 0;
}


int
main(int argc, char *argv[])
{
	int keep_env = 0;
	char **new_environ = NULL;
	const char *key_name = NULL;
	char *key = NULL, *key_new;
	size_t key_len = 0;
	size_t key_size = 0;
	ssize_t r;
	int fd, key_found;
	char path_user_id[sizeof(KEYPATH"/") + 3 * sizeof(uintmax_t)];
	char *path_user_name;
	struct passwd *pwd;

	ARGBEGIN {
	case 'e':
		keep_env = 1;
		break;
	case 'k':
		if (key_name)
			usage();
		key_name = EARGF(usage());
		break;
	default:
		usage();
	} ARGEND;

	if (!argc)
		usage();

	sprintf(path_user_id, "%s/%ju", KEYPATH, (uintmax_t)getuid());
	errno = 0;
	pwd = getpwuid(getuid());
	if (!pwd || !pwd->pw_name || !*pwd->pw_name) {
		if (errno)
			fprintf(stderr, "%s: getpwuid: %s\n", argv0, strerror(errno));
		else
			fprintf(stderr, "%s: your user does not exist\n", argv0);
		exit(EXIT_ERROR);
	}
	path_user_name = malloc(sizeof(KEYPATH"/") + strlen(pwd->pw_name));
	if (!path_user_name) {
		fprintf(stderr, "%s: malloc: %s\n", argv0, strerror(errno));
		exit(EXIT_ERROR);
	}
	stpcpy(stpcpy(path_user_name, KEYPATH"/"), pwd->pw_name);

	for (;;) {
		if (key_len == key_size) {
			key_new = malloc(1 + (key_size += 1024));
			if (!key_new) {
				explicit_bzero(key, key_len);
				fprintf(stderr, "%s: malloc: %s\n", argv0, strerror(errno));
				exit(EXIT_ERROR);
			}
			memcpy(key_new, key, key_len);
			explicit_bzero(key, key_len);
			free(key);
			key = key_new;
		}
		r = read(STDIN_FILENO, &key[key_len], key_size - key_len);
		if (r <= 0) {
			if (!r)
				break;
			explicit_bzero(key, key_len);
			fprintf(stderr, "%s: read <stdin>: %s\n", argv0, strerror(errno));
			exit(EXIT_ERROR);
		}
		key_len += (size_t)r;
	}

	key_found = 0;
	if (!authenticate(path_user_id, key_name, key, key_len, &key_found) &&
	    !authenticate(path_user_name, key_name, key, key_len, &key_found)) {
		fprintf(stderr, "%s: authentication failed: %s\n", argv0,
		        key_name ? (key_found ? "key mismatch" : "key not found")
		                 : (key_found ? "no matching key found" : "no key found"));
		explicit_bzero(key, key_len);
		exit(EXIT_AUTH);
	}
	free(path_user_name);

	fd = forward(key, key_len);
	if (fd < 0) {
		explicit_bzero(key, key_len);
		exit(EXIT_ERROR);
	}

	explicit_bzero(key, key_len);

	if (!keep_env)
		new_environ = set_environ();

	if (setgid(0)) {
		fprintf(stderr, "%s: setgid 0: %s\n", argv0, strerror(errno));
		exit(EXIT_ERROR);
	}
	if (setuid(0)) {
		fprintf(stderr, "%s: setuid 0: %s\n", argv0, strerror(errno));
		exit(EXIT_ERROR);
	}

	if (fd != STDIN_FILENO) {
		if (dup2(fd, STDIN_FILENO) != STDIN_FILENO) {
			fprintf(stderr, "%s: dup2 <socket> <stdin>: %s\n", argv0, strerror(errno));
			exit(EXIT_ERROR);
		}
		close(fd);
	}

	if (new_environ)
		environ = new_environ;
	execvp(argv[0], argv);
	fprintf(stderr, "%s: execvpe %s: %s\n", argv0, argv[0], strerror(errno));
	return errno == ENOENT ? EXIT_NOENT : EXIT_EXEC;
}