authenticate.c

/*
 * Support rsync daemon authentication.
 *
 * Copyright (C) 1998-2000 Andrew Tridgell
 * Copyright (C) 2002-2022 Wayne Davison
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, visit the http://fsf.org website.
 */

#include "rsync.h"
#include "itypes.h"
#include "ifuncs.h"

/* O_CLOEXEC is absent on some still-supported targets.  The random-source fd
 * is read and closed synchronously, so the established zero-value fallback is
 * sufficient without adding a configure dependency. */
#ifndef O_CLOEXEC
#define O_CLOEXEC 0
#endif

extern int read_only;
extern char *password_file;
extern struct name_num_obj valid_auth_checksums;

/***************************************************************************
encode a buffer using base64 - simple and slow algorithm. null terminates
the result.
  ***************************************************************************/
void base64_encode(const char *buf, int len, char *out, int pad)
{
	char *b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
	int bit_offset, byte_offset, idx, i;
	const uchar *d = (const uchar *)buf;
	int bytes = (len*8 + 5)/6;

	for (i = 0; i < bytes; i++) {
		byte_offset = (i*6)/8;
		bit_offset = (i*6)%8;
		if (bit_offset < 3) {
			idx = (d[byte_offset] >> (2-bit_offset)) & 0x3F;
		} else {
			idx = (d[byte_offset] << (bit_offset-2)) & 0x3F;
			if (byte_offset+1 < len) {
				idx |= (d[byte_offset+1] >> (8-(bit_offset-2)));
			}
		}
		out[i] = b64[idx];
	}

	while (pad && (i % 4))
		out[i++] = '=';

	out[i] = '\0';
}

/* Fill buf with len bytes from the kernel CSPRNG.  Returns 1 on success.
 * We read /dev/urandom directly rather than depending on getrandom()/
 * arc4random_buf() availability so this works on every platform rsync
 * targets without new configure probes. */
static int get_random_bytes(char *buf, int len)
{
	int fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
	int got = 0;
	if (fd < 0)
		return 0;
	while (got < len) {
		int n = read(fd, buf + got, len - got);
		if (n <= 0)
			break;
		got += n;
	}
	close(fd);
	return got == len;
}

/* Generate a challenge buffer and return it base64-encoded. */
static void gen_challenge(const char *addr, char *challenge)
{
	char input[32];
	char rnd[32];
	char digest[MAX_DIGEST_LEN];
	struct timeval tv;
	int len;

	memset(input, 0, sizeof input);

	strlcpy(input, addr, 17);
	sys_gettimeofday(&tv);
	SIVAL(input, 16, tv.tv_sec);
	SIVAL(input, 20, tv.tv_usec);
	SIVAL(input, 24, getpid());

	len = sum_init(valid_auth_checksums.negotiated_nni, 0);
	/* The challenge must be unpredictable to a network observer; addr+time
	 * +pid alone is ~35 bits and lets an attacker enumerate the preimage
	 * offline.  Hash 32 bytes from the kernel RNG first so the digest
	 * carries full entropy, keeping the legacy inputs as a mix-in so a
	 * urandom failure degrades to (never below) the old behaviour. */
	if (get_random_bytes(rnd, sizeof rnd))
		sum_update(rnd, sizeof rnd);
	else
		rprintf(FWARNING, "gen_challenge: /dev/urandom unavailable, "
			"falling back to time-based challenge\n");
	sum_update(input, sizeof input);
	sum_end(digest);

	base64_encode(digest, len, challenge, 0);
}

/* Generate an MD4 hash created from the combination of the password
 * and the challenge string and return it base64-encoded. */
static void generate_hash(const char *in, const char *challenge, char *out)
{
	char buf[MAX_DIGEST_LEN];
	int len;

	len = sum_init(valid_auth_checksums.negotiated_nni, 0);
	sum_update(in, strlen(in));
	sum_update(challenge, strlen(challenge));
	sum_end(buf);

	base64_encode(buf, len, out, 0);
}

/* Return the secret for a user from the secret file, null terminated.
 * Maximum length is len (not counting the null). */
static const char *check_secret(int module, const char *user, const char *group,
				const char *challenge, const char *pass)
{
	char line[1024];
	char pass2[MAX_DIGEST_LEN*2];
	const char *fname = lp_secrets_file(module);
	STRUCT_STAT st;
	int ok = 1;
	int user_len = strlen(user);
	int group_len = group ? strlen(group) : 0;
	char *err;
	FILE *fh;

	/* Daemon 'secrets file = PATH' open.  A planted symlink would be
	 * followed and the strict-modes fstat() check below runs on the target
	 * inode, so a symlink to /etc/shadow (0640 root:shadow) would pass and
	 * the daemon would auth against shadow hashes.  Refuse symlinks not
	 * owned by uid 0 or our euid. */
	if (!fname || !*fname)
		return "no secrets file";
	{
		int fd = open_no_attacker_symlinks(fname, O_RDONLY, 0);
		if (fd < 0)
			return "no secrets file";
		fh = fdopen(fd, "r");
		if (!fh) {
			close(fd);
			return "no secrets file";
		}
	}

	if (do_fstat(fileno(fh), &st) == -1) {
		rsyserr(FLOG, errno, "fstat(%s)", fname);
		ok = 0;
	} else if (lp_strict_modes(module)) {
		if ((st.st_mode & 06) != 0) {
			rprintf(FLOG, "secrets file must not be other-accessible (see strict modes option)\n");
			ok = 0;
		} else if (MY_UID() == ROOT_UID && st.st_uid != ROOT_UID) {
			rprintf(FLOG, "secrets file must be owned by root when running as root (see strict modes)\n");
			ok = 0;
		}
	}
	if (!ok) {
		fclose(fh);
		return "ignoring secrets file";
	}

	if (*user == '#') {
		/* Reject attempt to match a comment. */
		fclose(fh);
		return "invalid username";
	}

	/* Try to find a line that starts with the user (or @group) name and a ':'. */
	err = "secret not found";
	while ((user || group) && fgets(line, sizeof line, fh) != NULL) {
		const char **ptr, *s = strtok(line, "\n\r");
		int len;
		if (!s)
			continue;
		if (*s == '@') {
			ptr = &group;
			len = group_len;
			s++;
		} else {
			ptr = &user;
			len = user_len;
		}
		if (!*ptr || strncmp(s, *ptr, len) != 0 || s[len] != ':')
			continue;
		generate_hash(s+len+1, challenge, pass2);
		if (strcmp(pass, pass2) == 0) {
			err = NULL;
			break;
		}
		err = "password mismatch";
		*ptr = NULL; /* Don't look for name again. */
	}

	fclose(fh);

	force_memzero(line, sizeof line);
	force_memzero(pass2, sizeof pass2);

	return err;
}

static const char *getpassf(const char *filename)
{
	STRUCT_STAT st;
	char buffer[512], *p;
	int n;

	if (!filename)
		return NULL;

	if (strcmp(filename, "-") == 0) {
		n = fgets(buffer, sizeof buffer, stdin) == NULL ? -1 : (int)strlen(buffer);
	} else {
		int fd;

		/* --password-file=PATH client open.  Its first line is sent as the
		 * auth response, so a planted symlink leaks the target's content
		 * (e.g. shadow hashes) to a malicious daemon; the do_stat()
		 * other-access check runs on the target mode and passes 0640
		 * root:shadow.  Refuse symlinks not owned by uid 0 or our euid. */
		if ((fd = open_no_attacker_symlinks(filename, O_RDONLY, 0)) < 0) {
			rsyserr(FERROR, errno, "could not open password file %s", filename);
			exit_cleanup(RERR_SYNTAX);
		}

		/* fstat the opened fd, not the pathname: a same-object check
		 * (matching check_secret() above) so an attacker who swaps the
		 * path between open and check can't make the owner/mode test
		 * validate a different inode than the one we read the password
		 * from. */
		if (do_fstat(fd, &st) == -1) {
			rsyserr(FERROR, errno, "fstat(%s)", filename);
			exit_cleanup(RERR_SYNTAX);
		}
		if ((st.st_mode & 06) != 0) {
			rprintf(FERROR, "ERROR: password file must not be other-accessible\n");
			exit_cleanup(RERR_SYNTAX);
		}
		if (MY_UID() == ROOT_UID && st.st_uid != ROOT_UID) {
			rprintf(FERROR, "ERROR: password file must be owned by root when running as root\n");
			exit_cleanup(RERR_SYNTAX);
		}

		n = read(fd, buffer, sizeof buffer - 1);
		close(fd);
	}

	if (n > 0) {
		buffer[n] = '\0';
		if ((p = strtok(buffer, "\n\r")) != NULL)
			return strdup(p);
	}

	rprintf(FERROR, "ERROR: failed to read a password from %s\n", filename);
	exit_cleanup(RERR_SYNTAX);
}

/* Possibly negotiate authentication with the client.  Use "leader" to
 * start off the auth if necessary.
 *
 * Return NULL if authentication failed.  Return "" if anonymous access.
 * Otherwise return username.
 */
char *auth_server(int f_in, int f_out, int module, const char *host,
		  const char *addr, const char *leader)
{
	char *users = lp_auth_users(module);
	char challenge[MAX_DIGEST_LEN*2];
	char line[BIGPATHBUFLEN];
	const char **auth_uid_groups = NULL;
	int auth_uid_groups_cnt = -1;
	const char *err = NULL;
	int group_match = -1;
	char *tok, *pass;
	char opt_ch = '\0';

	/* if no auth list then allow anyone in! */
	if (!users || !*users)
		return "";

	negotiate_daemon_auth(f_out, 0);

	/* Enforce a configured minimum auth digest (default: none).  This refuses
	 * a peer that negotiated -- or, via an omitted digest list / old protocol,
	 * fell back to -- a digest weaker than the operator-required floor, e.g. a
	 * client downgraded to md5/md4.  Lower rank == stronger (the auth list is
	 * ordered strongest-first), so a higher rank than the floor is too weak. */
	{
		const char *min_digest = lp_auth_digest(module);
		if (min_digest && *min_digest) {
			int floor_rank = auth_digest_rank(min_digest);
			int got_rank = auth_digest_rank(valid_auth_checksums.negotiated_nni->name);
			if (floor_rank < 0) {
				rprintf(FLOG, "auth failed on module %s from %s (%s): the "
					"configured 'auth digest = %s' is not a supported digest "
					"on this build\n",
					lp_name(module), host, addr, min_digest);
				return NULL;
			}
			if (got_rank < 0 || got_rank > floor_rank) {
				rprintf(FLOG, "auth failed on module %s from %s (%s): negotiated "
					"auth digest %s is weaker than the required "
					"'auth digest = %s'\n",
					lp_name(module), host, addr,
					valid_auth_checksums.negotiated_nni->name, min_digest);
				return NULL;
			}
		}
	}

	gen_challenge(addr, challenge);

	io_printf(f_out, "%s%s\n", leader, challenge);

	if (!read_line_old(f_in, line, sizeof line, 0)
	 || (pass = strchr(line, ' ')) == NULL) {
		rprintf(FLOG, "auth failed on module %s from %s (%s): "
			"invalid challenge response\n",
			lp_name(module), host, addr);
		return NULL;
	}
	*pass++ = '\0';

	users = strdup(users);

	/* conf_strtok() honours the documented leading-comma form: a value that
	 * starts with a comma splits on commas ALONE, so an entry may contain
	 * spaces -- which is how a group name with a space is written.  Splitting
	 * on whitespace here tore such an entry apart, so the rule the admin wrote
	 * never matched and a rule they never wrote appeared from its tail.  The
	 * daemon's gid field already uses this parser (clientserver.c). */
	for (tok = conf_strtok(users); tok; tok = conf_strtok(NULL)) {
		char *opts;
		/* See if the user appended :deny, :ro, or :rw. */
		if ((opts = strchr(tok, ':')) != NULL) {
			*opts++ = '\0';
			opt_ch = isUpper(opts) ? toLower(opts) : *opts;
			if (opt_ch == 'r') { /* handle ro and rw */
				opt_ch = isUpper(opts+1) ? toLower(opts+1) : opts[1];
				if (opt_ch == 'o')
					opt_ch = 'r';
				else if (opt_ch != 'w')
					opt_ch = '\0';
			} else if (opt_ch != 'd') /* if it's not deny, ignore it */
				opt_ch = '\0';
		} else
			opt_ch = '\0';
		if (*tok != '@') {
			/* Match the username */
			if (wildmatch(tok, line))
				break;
		} else {
#ifdef HAVE_GETGROUPLIST
			int j;
			/* See if authorizing user is a real user, and if so, see
			 * if it is in a group that matches tok+1 wildmat. */
			if (auth_uid_groups_cnt < 0) {
				item_list gid_list = EMPTY_ITEM_LIST;
				uid_t auth_uid;
				if (!user_to_uid(line, &auth_uid, False)
				 || getallgroups(auth_uid, &gid_list) != NULL)
					auth_uid_groups_cnt = 0;
				else {
					gid_t *gid_array = gid_list.items;
					auth_uid_groups_cnt = gid_list.count;
					auth_uid_groups = new_array(const char *, auth_uid_groups_cnt);
					for (j = 0; j < auth_uid_groups_cnt; j++)
						auth_uid_groups[j] = gid_to_group(gid_array[j]);
				}
			}
			for (j = 0; j < auth_uid_groups_cnt; j++) {
				if (auth_uid_groups[j] && wildmatch(tok+1, auth_uid_groups[j])) {
					group_match = j;
					break;
				}
			}
			if (group_match >= 0)
				break;
#else
			rprintf(FLOG, "your computer doesn't support getgrouplist(), so no @group authorization is possible.\n");
#endif
		}
	}

	free(users);

	if (!tok)
		err = "no matching rule";
	else if (opt_ch == 'd')
		err = "denied by rule";
	else {
		const char *group = group_match >= 0 ? auth_uid_groups[group_match] : NULL;
		err = check_secret(module, line, group, challenge, pass);
	}

	force_memzero(challenge, sizeof challenge);
	force_memzero(pass, strlen(pass));

	if (auth_uid_groups) {
		int j;
		for (j = 0; j < auth_uid_groups_cnt; j++) {
			if (auth_uid_groups[j])
				free((char*)auth_uid_groups[j]);
		}
		free(auth_uid_groups);
	}

	if (err) {
		rprintf(FLOG, "auth failed on module %s from %s (%s) for %s: %s\n",
			lp_name(module), host, addr, line, err);
		return NULL;
	}

	if (opt_ch == 'r')
		read_only = 1;
	else if (opt_ch == 'w')
		read_only = 0;

	return strdup(line);
}

void auth_client(int fd, const char *user, const char *challenge)
{
	const char *pass;
	char pass2[MAX_DIGEST_LEN*2];

	if (!user || !*user)
		user = "nobody";
	negotiate_daemon_auth(-1, 1);

	if (!(pass = getpassf(password_file))
	 && !(pass = getenv("RSYNC_PASSWORD"))) {
		/* XXX: cyeoh says that getpass is deprecated, because
		 * it may return a truncated password on some systems,
		 * and it is not in the LSB.
		 *
		 * Andrew Klein says that getpassphrase() is present
		 * on Solaris and reads up to 256 characters.
		 *
		 * OpenBSD has a readpassphrase() that might be more suitable.
		 */
		pass = getpass("Password: ");
	}

	if (!pass)
		pass = "";

	generate_hash(pass, challenge, pass2);
	io_printf(fd, "%s %s\n", user, pass2);
}