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Systemd/src/basic/user-util.c
Lennart Poettering 24eccc3414 nss-systemd,user-util: add a way how synthesizing "nobody" can be turned off 
This is quite ugly, but provides us with an avenue for moving
distributions to define the "nobody" user properly without breaking legacy
systems that us the name for other stuff.

The idea is basically, that the distribution adopts the new definition
of "nobody" (and thus recompiles systemd with it) and then touches
/etc/systemd/dont-synthesize-nobody on legacy systems to turn off
possibly conflicting synthesizing of the nobody name by systemd.
2018-01-10 18:26:03 +01:00

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/* SPDX-License-Identifier: LGPL-2.1+ */
/***
This file is part of systemd.
Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd 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
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/
#include <alloca.h>
#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <pwd.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>
#include <utmp.h>
#include "alloc-util.h"
#include "fd-util.h"
#include "fileio.h"
#include "format-util.h"
#include "macro.h"
#include "missing.h"
#include "parse-util.h"
#include "path-util.h"
#include "string-util.h"
#include "strv.h"
#include "user-util.h"
#include "utf8.h"
bool uid_is_valid(uid_t uid) {
/* Also see POSIX IEEE Std 1003.1-2008, 2016 Edition, 3.436. */
/* Some libc APIs use UID_INVALID as special placeholder */
if (uid == (uid_t) UINT32_C(0xFFFFFFFF))
return false;
/* A long time ago UIDs where 16bit, hence explicitly avoid the 16bit -1 too */
if (uid == (uid_t) UINT32_C(0xFFFF))
return false;
return true;
}
int parse_uid(const char *s, uid_t *ret) {
uint32_t uid = 0;
int r;
assert(s);
assert_cc(sizeof(uid_t) == sizeof(uint32_t));
r = safe_atou32(s, &uid);
if (r < 0)
return r;
if (!uid_is_valid(uid))
return -ENXIO; /* we return ENXIO instead of EINVAL
* here, to make it easy to distuingish
* invalid numeric uids from invalid
* strings. */
if (ret)
*ret = uid;
return 0;
}
char* getlogname_malloc(void) {
uid_t uid;
struct stat st;
if (isatty(STDIN_FILENO) && fstat(STDIN_FILENO, &st) >= 0)
uid = st.st_uid;
else
uid = getuid();
return uid_to_name(uid);
}
char *getusername_malloc(void) {
const char *e;
e = getenv("USER");
if (e)
return strdup(e);
return uid_to_name(getuid());
}
int get_user_creds(
const char **username,
uid_t *uid, gid_t *gid,
const char **home,
const char **shell) {
struct passwd *p;
uid_t u;
assert(username);
assert(*username);
/* We enforce some special rules for uid=0 and uid=65534: in order to avoid NSS lookups for root we hardcode
* their user record data. */
if (STR_IN_SET(*username, "root", "0")) {
*username = "root";
if (uid)
*uid = 0;
if (gid)
*gid = 0;
if (home)
*home = "/root";
if (shell)
*shell = "/bin/sh";
return 0;
}
if (synthesize_nobody() &&
STR_IN_SET(*username, NOBODY_USER_NAME, "65534")) {
*username = NOBODY_USER_NAME;
if (uid)
*uid = UID_NOBODY;
if (gid)
*gid = GID_NOBODY;
if (home)
*home = "/";
if (shell)
*shell = "/sbin/nologin";
return 0;
}
if (parse_uid(*username, &u) >= 0) {
errno = 0;
p = getpwuid(u);
/* If there are multiple users with the same id, make
* sure to leave $USER to the configured value instead
* of the first occurrence in the database. However if
* the uid was configured by a numeric uid, then let's
* pick the real username from /etc/passwd. */
if (p)
*username = p->pw_name;
} else {
errno = 0;
p = getpwnam(*username);
}
if (!p)
return errno > 0 ? -errno : -ESRCH;
if (uid) {
if (!uid_is_valid(p->pw_uid))
return -EBADMSG;
*uid = p->pw_uid;
}
if (gid) {
if (!gid_is_valid(p->pw_gid))
return -EBADMSG;
*gid = p->pw_gid;
}
if (home)
*home = p->pw_dir;
if (shell)
*shell = p->pw_shell;
return 0;
}
int get_user_creds_clean(
const char **username,
uid_t *uid, gid_t *gid,
const char **home,
const char **shell) {
int r;
/* Like get_user_creds(), but resets home/shell to NULL if they don't contain anything relevant. */
r = get_user_creds(username, uid, gid, home, shell);
if (r < 0)
return r;
if (shell &&
(isempty(*shell) || PATH_IN_SET(*shell,
"/bin/nologin",
"/sbin/nologin",
"/usr/bin/nologin",
"/usr/sbin/nologin")))
*shell = NULL;
if (home &&
(isempty(*home) || path_equal(*home, "/")))
*home = NULL;
return 0;
}
int get_group_creds(const char **groupname, gid_t *gid) {
struct group *g;
gid_t id;
assert(groupname);
/* We enforce some special rules for gid=0: in order to avoid
* NSS lookups for root we hardcode its data. */
if (STR_IN_SET(*groupname, "root", "0")) {
*groupname = "root";
if (gid)
*gid = 0;
return 0;
}
if (synthesize_nobody() &&
STR_IN_SET(*groupname, NOBODY_GROUP_NAME, "65534")) {
*groupname = NOBODY_GROUP_NAME;
if (gid)
*gid = GID_NOBODY;
return 0;
}
if (parse_gid(*groupname, &id) >= 0) {
errno = 0;
g = getgrgid(id);
if (g)
*groupname = g->gr_name;
} else {
errno = 0;
g = getgrnam(*groupname);
}
if (!g)
return errno > 0 ? -errno : -ESRCH;
if (gid) {
if (!gid_is_valid(g->gr_gid))
return -EBADMSG;
*gid = g->gr_gid;
}
return 0;
}
char* uid_to_name(uid_t uid) {
char *ret;
int r;
/* Shortcut things to avoid NSS lookups */
if (uid == 0)
return strdup("root");
if (synthesize_nobody() &&
uid == UID_NOBODY)
return strdup(NOBODY_USER_NAME);
if (uid_is_valid(uid)) {
long bufsize;
bufsize = sysconf(_SC_GETPW_R_SIZE_MAX);
if (bufsize <= 0)
bufsize = 4096;
for (;;) {
struct passwd pwbuf, *pw = NULL;
_cleanup_free_ char *buf = NULL;
buf = malloc(bufsize);
if (!buf)
return NULL;
r = getpwuid_r(uid, &pwbuf, buf, (size_t) bufsize, &pw);
if (r == 0 && pw)
return strdup(pw->pw_name);
if (r != ERANGE)
break;
bufsize *= 2;
}
}
if (asprintf(&ret, UID_FMT, uid) < 0)
return NULL;
return ret;
}
char* gid_to_name(gid_t gid) {
char *ret;
int r;
if (gid == 0)
return strdup("root");
if (synthesize_nobody() &&
gid == GID_NOBODY)
return strdup(NOBODY_GROUP_NAME);
if (gid_is_valid(gid)) {
long bufsize;
bufsize = sysconf(_SC_GETGR_R_SIZE_MAX);
if (bufsize <= 0)
bufsize = 4096;
for (;;) {
struct group grbuf, *gr = NULL;
_cleanup_free_ char *buf = NULL;
buf = malloc(bufsize);
if (!buf)
return NULL;
r = getgrgid_r(gid, &grbuf, buf, (size_t) bufsize, &gr);
if (r == 0 && gr)
return strdup(gr->gr_name);
if (r != ERANGE)
break;
bufsize *= 2;
}
}
if (asprintf(&ret, GID_FMT, gid) < 0)
return NULL;
return ret;
}
int in_gid(gid_t gid) {
long ngroups_max;
gid_t *gids;
int r, i;
if (getgid() == gid)
return 1;
if (getegid() == gid)
return 1;
if (!gid_is_valid(gid))
return -EINVAL;
ngroups_max = sysconf(_SC_NGROUPS_MAX);
assert(ngroups_max > 0);
gids = newa(gid_t, ngroups_max);
r = getgroups(ngroups_max, gids);
if (r < 0)
return -errno;
for (i = 0; i < r; i++)
if (gids[i] == gid)
return 1;
return 0;
}
int in_group(const char *name) {
int r;
gid_t gid;
r = get_group_creds(&name, &gid);
if (r < 0)
return r;
return in_gid(gid);
}
int get_home_dir(char **_h) {
struct passwd *p;
const char *e;
char *h;
uid_t u;
assert(_h);
/* Take the user specified one */
e = secure_getenv("HOME");
if (e && path_is_absolute(e)) {
h = strdup(e);
if (!h)
return -ENOMEM;
*_h = h;
return 0;
}
/* Hardcode home directory for root and nobody to avoid NSS */
u = getuid();
if (u == 0) {
h = strdup("/root");
if (!h)
return -ENOMEM;
*_h = h;
return 0;
}
if (synthesize_nobody() &&
u == UID_NOBODY) {
h = strdup("/");
if (!h)
return -ENOMEM;
*_h = h;
return 0;
}
/* Check the database... */
errno = 0;
p = getpwuid(u);
if (!p)
return errno > 0 ? -errno : -ESRCH;
if (!path_is_absolute(p->pw_dir))
return -EINVAL;
h = strdup(p->pw_dir);
if (!h)
return -ENOMEM;
*_h = h;
return 0;
}
int get_shell(char **_s) {
struct passwd *p;
const char *e;
char *s;
uid_t u;
assert(_s);
/* Take the user specified one */
e = getenv("SHELL");
if (e) {
s = strdup(e);
if (!s)
return -ENOMEM;
*_s = s;
return 0;
}
/* Hardcode shell for root and nobody to avoid NSS */
u = getuid();
if (u == 0) {
s = strdup("/bin/sh");
if (!s)
return -ENOMEM;
*_s = s;
return 0;
}
if (synthesize_nobody() &&
u == UID_NOBODY) {
s = strdup("/sbin/nologin");
if (!s)
return -ENOMEM;
*_s = s;
return 0;
}
/* Check the database... */
errno = 0;
p = getpwuid(u);
if (!p)
return errno > 0 ? -errno : -ESRCH;
if (!path_is_absolute(p->pw_shell))
return -EINVAL;
s = strdup(p->pw_shell);
if (!s)
return -ENOMEM;
*_s = s;
return 0;
}
int reset_uid_gid(void) {
int r;
r = maybe_setgroups(0, NULL);
if (r < 0)
return r;
if (setresgid(0, 0, 0) < 0)
return -errno;
if (setresuid(0, 0, 0) < 0)
return -errno;
return 0;
}
int take_etc_passwd_lock(const char *root) {
struct flock flock = {
.l_type = F_WRLCK,
.l_whence = SEEK_SET,
.l_start = 0,
.l_len = 0,
};
const char *path;
int fd, r;
/* This is roughly the same as lckpwdf(), but not as awful. We
* don't want to use alarm() and signals, hence we implement
* our own trivial version of this.
*
* Note that shadow-utils also takes per-database locks in
* addition to lckpwdf(). However, we don't given that they
* are redundant as they invoke lckpwdf() first and keep
* it during everything they do. The per-database locks are
* awfully racy, and thus we just won't do them. */
if (root)
path = prefix_roota(root, "/etc/.pwd.lock");
else
path = "/etc/.pwd.lock";
fd = open(path, O_WRONLY|O_CREAT|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW, 0600);
if (fd < 0)
return -errno;
r = fcntl(fd, F_SETLKW, &flock);
if (r < 0) {
safe_close(fd);
return -errno;
}
return fd;
}
bool valid_user_group_name(const char *u) {
const char *i;
long sz;
/* Checks if the specified name is a valid user/group name. Also see POSIX IEEE Std 1003.1-2008, 2016 Edition,
* 3.437. We are a bit stricter here however. Specifically we deviate from POSIX rules:
*
* - We don't allow any dots (this would break chown syntax which permits dots as user/group name separator)
* - We require that names fit into the appropriate utmp field
* - We don't allow empty user names
*
* Note that other systems are even more restrictive, and don't permit underscores or uppercase characters.
*/
if (isempty(u))
return false;
if (!(u[0] >= 'a' && u[0] <= 'z') &&
!(u[0] >= 'A' && u[0] <= 'Z') &&
u[0] != '_')
return false;
for (i = u+1; *i; i++) {
if (!(*i >= 'a' && *i <= 'z') &&
!(*i >= 'A' && *i <= 'Z') &&
!(*i >= '0' && *i <= '9') &&
!IN_SET(*i, '_', '-'))
return false;
}
sz = sysconf(_SC_LOGIN_NAME_MAX);
assert_se(sz > 0);
if ((size_t) (i-u) > (size_t) sz)
return false;
if ((size_t) (i-u) > UT_NAMESIZE - 1)
return false;
return true;
}
bool valid_user_group_name_or_id(const char *u) {
/* Similar as above, but is also fine with numeric UID/GID specifications, as long as they are in the right
* range, and not the invalid user ids. */
if (isempty(u))
return false;
if (valid_user_group_name(u))
return true;
return parse_uid(u, NULL) >= 0;
}
bool valid_gecos(const char *d) {
if (!d)
return false;
if (!utf8_is_valid(d))
return false;
if (string_has_cc(d, NULL))
return false;
/* Colons are used as field separators, and hence not OK */
if (strchr(d, ':'))
return false;
return true;
}
bool valid_home(const char *p) {
if (isempty(p))
return false;
if (!utf8_is_valid(p))
return false;
if (string_has_cc(p, NULL))
return false;
if (!path_is_absolute(p))
return false;
if (!path_is_normalized(p))
return false;
/* Colons are used as field separators, and hence not OK */
if (strchr(p, ':'))
return false;
return true;
}
int maybe_setgroups(size_t size, const gid_t *list) {
int r;
/* Check if setgroups is allowed before we try to drop all the auxiliary groups */
if (size == 0) { /* Dropping all aux groups? */
_cleanup_free_ char *setgroups_content = NULL;
bool can_setgroups;
r = read_one_line_file("/proc/self/setgroups", &setgroups_content);
if (r == -ENOENT)
/* Old kernels don't have /proc/self/setgroups, so assume we can use setgroups */
can_setgroups = true;
else if (r < 0)
return r;
else
can_setgroups = streq(setgroups_content, "allow");
if (!can_setgroups) {
log_debug("Skipping setgroups(), /proc/self/setgroups is set to 'deny'");
return 0;
}
}
if (setgroups(size, list) < 0)
return -errno;
return 0;
}
bool synthesize_nobody(void) {
#ifdef NOLEGACY
return true;
#else
/* Returns true when we shall synthesize the "nobody" user (which we do by default). This can be turned off by
* touching /etc/systemd/dont-synthesize-nobody in order to provide upgrade compatibility with legacy systems
* that used the "nobody" user name and group name for other UIDs/GIDs than 65534.
*
* Note that we do not employ any kind of synchronization on the following caching variable. If the variable is
* accessed in multi-threaded programs in the worst case it might happen that we initialize twice, but that
* shouldn't matter as each initialization should come to the same result. */
static int cache = -1;
if (cache < 0)
cache = access("/etc/systemd/dont-synthesize-nobody", F_OK) < 0;
return cache;
#endif
}
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