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shadow/libmisc/find_new_uid.c

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C
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/*
* SPDX-FileCopyrightText: 1991 - 1994, Julianne Frances Haugh
* SPDX-FileCopyrightText: 2008 - 2011, Nicolas François
* SPDX-FileCopyrightText: 2014, Red Hat, Inc.
*
* SPDX-License-Identifier: BSD-3-Clause
*/
#include <config.h>
#include <assert.h>
#include <stdio.h>
#include <errno.h>
#include "prototypes.h"
#include "pwio.h"
#include "getdef.h"
/*
* get_ranges - Get the minimum and maximum ID ranges for the search
*
* This function will return the minimum and maximum ranges for IDs
*
* 0: The function completed successfully
* EINVAL: The provided ranges are impossible (such as maximum < minimum)
*
* preferred_min: The special-case minimum value for a specifically-
* requested ID, which may be lower than the standard min_id
*/
static int get_ranges (bool sys_user, uid_t *min_id, uid_t *max_id,
uid_t *preferred_min)
{
uid_t uid_def_max = 0;
if (sys_user) {
/* System users */
/* A requested ID is allowed to be below the autoselect range */
*preferred_min = (uid_t) 1;
/* Get the minimum ID range from login.defs or default to 101 */
*min_id = (uid_t) getdef_ulong ("SYS_UID_MIN", 101UL);
/*
* If SYS_UID_MAX is unspecified, we should assume it to be one
* less than the UID_MIN (which is reserved for non-system accounts)
*/
uid_def_max = (uid_t) getdef_ulong ("UID_MIN", 1000UL) - 1;
*max_id = (uid_t) getdef_ulong ("SYS_UID_MAX",
(unsigned long) uid_def_max);
/* Check that the ranges make sense */
if (*max_id < *min_id) {
(void) fprintf (shadow_logfd,
_("%s: Invalid configuration: SYS_UID_MIN (%lu), "
"UID_MIN (%lu), SYS_UID_MAX (%lu)\n"),
Prog, (unsigned long) *min_id,
getdef_ulong ("UID_MIN", 1000UL),
(unsigned long) *max_id);
return EINVAL;
}
} else {
/* Non-system users */
/* Get the values from login.defs or use reasonable defaults */
*min_id = (uid_t) getdef_ulong ("UID_MIN", 1000UL);
*max_id = (uid_t) getdef_ulong ("UID_MAX", 60000UL);
/*
* The preferred minimum should match the standard ID minimum
* for non-system users.
*/
*preferred_min = *min_id;
/* Check that the ranges make sense */
if (*max_id < *min_id) {
(void) fprintf (shadow_logfd,
_("%s: Invalid configuration: UID_MIN (%lu), "
"UID_MAX (%lu)\n"),
Prog, (unsigned long) *min_id,
(unsigned long) *max_id);
return EINVAL;
}
}
return 0;
}
/*
* check_uid - See if the requested UID is available
*
* On success, return 0
* If the ID is in use, return EEXIST
* If the ID is outside the range, return ERANGE
* In other cases, return errno from getpwuid()
*/
static int check_uid(const uid_t uid,
const uid_t uid_min,
const uid_t uid_max,
bool *used_uids)
{
/* First test that the preferred ID is in the range */
if (uid < uid_min || uid > uid_max) {
return ERANGE;
}
/*
* Check whether we already detected this UID
* using the pw_next() loop
*/
if (used_uids != NULL && used_uids[uid]) {
return EEXIST;
}
/* Check if the UID exists according to NSS */
errno = 0;
if (prefix_getpwuid(uid) != NULL) {
return EEXIST;
} else {
/* getpwuid() was NULL
* we have to ignore errors as temporary
* failures of remote user identity services
* would completely block user/group creation
*/
}
/* If we've made it here, the UID must be available */
return 0;
}
/*
* find_new_uid - Find a new unused UID.
*
* If successful, find_new_uid provides an unused user ID in the
* [UID_MIN:UID_MAX] range.
* This ID should be higher than all the used UID, but if not possible,
* the lowest unused ID in the range will be returned.
*
* Return 0 on success, -1 if no unused UIDs are available.
*/
int find_new_uid(bool sys_user,
uid_t *uid,
/*@null@*/uid_t const *preferred_uid)
{
bool *used_uids;
const struct passwd *pwd;
uid_t uid_min, uid_max, preferred_min;
uid_t id;
uid_t lowest_found, highest_found;
int result;
int nospam = 0;
assert (uid != NULL);
/*
* First, figure out what ID range is appropriate for
* automatic assignment
*/
result = get_ranges (sys_user, &uid_min, &uid_max, &preferred_min);
if (result == EINVAL) {
return -1;
}
/* Check if the preferred UID is available */
if (preferred_uid) {
result = check_uid (*preferred_uid, preferred_min, uid_max, NULL);
if (result == 0) {
/*
* Make sure the UID isn't queued for use already
*/
if (pw_locate_uid (*preferred_uid) == NULL) {
*uid = *preferred_uid;
return 0;
}
/*
* pw_locate_uid() found the UID in an as-yet uncommitted
* entry. We'll proceed below and auto-set an UID.
*/
} else if (result == EEXIST || result == ERANGE) {
/*
* Continue on below. At this time, we won't
* treat these two cases differently.
*/
} else {
/*
* An unexpected error occurred. We should report
* this and fail the user creation.
* This differs from the automatic creation
* behavior below, since if a specific UID was
* requested and generated an error, the user is
* more likely to want to stop and address the
* issue.
*/
fprintf (shadow_logfd,
_("%s: Encountered error attempting to use "
"preferred UID: %s\n"),
Prog, strerror (result));
return -1;
}
}
/*
* Search the entire passwd file,
* looking for the next unused value.
*
* We first check the local database with pw_rewind/pw_next to find
* all local values that are in use.
*
* We then compare the next free value to all databases (local and
* remote) and iterate until we find a free one. If there are free
* values beyond the lowest (system users) or highest (non-system
* users), we will prefer those and avoid potentially reclaiming a
* deleted user (which can be a security issue, since it may grant
* access to files belonging to that former user).
*
* If there are no UIDs available at the end of the search, we will
* have no choice but to iterate through the range looking for gaps.
*
*/
/* Create an array to hold all of the discovered UIDs */
used_uids = malloc (sizeof (bool) * (uid_max +1));
if (NULL == used_uids) {
fprintf (shadow_logfd,
_("%s: failed to allocate memory: %s\n"),
Prog, strerror (errno));
return -1;
}
memset (used_uids, false, sizeof (bool) * (uid_max + 1));
/* First look for the lowest and highest value in the local database */
(void) pw_rewind ();
highest_found = uid_min;
lowest_found = uid_max;
while ((pwd = pw_next ()) != NULL) {
/*
* Does this entry have a lower UID than the lowest we've found
* so far?
*/
if ((pwd->pw_uid <= lowest_found) && (pwd->pw_uid >= uid_min)) {
lowest_found = pwd->pw_uid - 1;
}
/*
* Does this entry have a higher UID than the highest we've found
* so far?
*/
if ((pwd->pw_uid >= highest_found) && (pwd->pw_uid <= uid_max)) {
highest_found = pwd->pw_uid + 1;
}
/* create index of used UIDs */
if (pwd->pw_uid >= uid_min
&& pwd->pw_uid <= uid_max) {
used_uids[pwd->pw_uid] = true;
}
}
if (sys_user) {
/*
* For system users, we want to start from the
* top of the range and work downwards.
*/
/*
* At the conclusion of the pw_next() search, we will either
* have a presumed-free UID or we will be at UID_MIN - 1.
*/
if (lowest_found < uid_min) {
/*
* In this case, an UID is in use at UID_MIN.
*
* We will reset the search to UID_MAX and proceed down
* through all the UIDs (skipping those we detected with
* used_uids) for a free one. It is a known issue that
* this may result in reusing a previously-deleted UID,
* so administrators should be instructed to use this
* auto-detection with care (and prefer to assign UIDs
* explicitly).
*/
lowest_found = uid_max;
}
/* Search through all of the IDs in the range */
for (id = lowest_found; id >= uid_min; id--) {
result = check_uid (id, uid_min, uid_max, used_uids);
if (result == 0) {
/* This UID is available. Return it. */
*uid = id;
free (used_uids);
return 0;
} else if (result == EEXIST) {
/* This UID is in use, we'll continue to the next */
} else {
/*
* An unexpected error occurred.
*
* Only report it the first time to avoid spamming
* the logs
*
*/
if (!nospam) {
fprintf (shadow_logfd,
_("%s: Can't get unique system UID (%s). "
"Suppressing additional messages.\n"),
Prog, strerror (result));
SYSLOG ((LOG_ERR,
"Error checking available UIDs: %s",
strerror (result)));
nospam = 1;
}
/*
* We will continue anyway. Hopefully a later UID
* will work properly.
*/
}
}
/*
* If we get all the way through the loop, try again from UID_MAX,
* unless that was where we previously started. (NOTE: the worst-case
* scenario here is that we will run through (UID_MAX - UID_MIN - 1)
* cycles *again* if we fall into this case with lowest_found as
* UID_MAX - 1, all users in the range in use and maintained by
* network services such as LDAP.)
*/
if (lowest_found != uid_max) {
for (id = uid_max; id >= uid_min; id--) {
result = check_uid (id, uid_min, uid_max, used_uids);
if (result == 0) {
/* This UID is available. Return it. */
*uid = id;
free (used_uids);
return 0;
} else if (result == EEXIST) {
/* This UID is in use, we'll continue to the next */
} else {
/*
* An unexpected error occurred.
*
* Only report it the first time to avoid spamming
* the logs
*
*/
if (!nospam) {
fprintf (shadow_logfd,
_("%s: Can't get unique system UID (%s). "
"Suppressing additional messages.\n"),
Prog, strerror (result));
SYSLOG((LOG_ERR,
"Error checking available UIDs: %s",
strerror (result)));
nospam = 1;
}
/*
* We will continue anyway. Hopefully a later UID
* will work properly.
*/
}
}
}
} else { /* !sys_user */
/*
* For non-system users, we want to start from the
* bottom of the range and work upwards.
*/
/*
* At the conclusion of the pw_next() search, we will either
* have a presumed-free UID or we will be at UID_MAX + 1.
*/
if (highest_found > uid_max) {
/*
* In this case, a UID is in use at UID_MAX.
*
* We will reset the search to UID_MIN and proceed up
* through all the UIDs (skipping those we detected with
* used_uids) for a free one. It is a known issue that
* this may result in reusing a previously-deleted UID,
* so administrators should be instructed to use this
* auto-detection with care (and prefer to assign UIDs
* explicitly).
*/
highest_found = uid_min;
}
/* Search through all of the IDs in the range */
for (id = highest_found; id <= uid_max; id++) {
result = check_uid (id, uid_min, uid_max, used_uids);
if (result == 0) {
/* This UID is available. Return it. */
*uid = id;
free (used_uids);
return 0;
} else if (result == EEXIST) {
/* This UID is in use, we'll continue to the next */
} else {
/*
* An unexpected error occurred.
*
* Only report it the first time to avoid spamming
* the logs
*
*/
if (!nospam) {
fprintf (shadow_logfd,
_("%s: Can't get unique UID (%s). "
"Suppressing additional messages.\n"),
Prog, strerror (result));
SYSLOG ((LOG_ERR,
"Error checking available UIDs: %s",
strerror (result)));
nospam = 1;
}
/*
* We will continue anyway. Hopefully a later UID
* will work properly.
*/
}
}
/*
* If we get all the way through the loop, try again from UID_MIN,
* unless that was where we previously started. (NOTE: the worst-case
* scenario here is that we will run through (UID_MAX - UID_MIN - 1)
* cycles *again* if we fall into this case with highest_found as
* UID_MIN + 1, all users in the range in use and maintained by
* network services such as LDAP.)
*/
if (highest_found != uid_min) {
for (id = uid_min; id <= uid_max; id++) {
result = check_uid (id, uid_min, uid_max, used_uids);
if (result == 0) {
/* This UID is available. Return it. */
*uid = id;
free (used_uids);
return 0;
} else if (result == EEXIST) {
/* This UID is in use, we'll continue to the next */
} else {
/*
* An unexpected error occurred.
*
* Only report it the first time to avoid spamming
* the logs
*
*/
if (!nospam) {
fprintf (shadow_logfd,
_("%s: Can't get unique UID (%s). "
"Suppressing additional messages.\n"),
Prog, strerror (result));
SYSLOG ((LOG_ERR,
"Error checking available UIDs: %s",
strerror (result)));
nospam = 1;
}
/*
* We will continue anyway. Hopefully a later UID
* will work properly.
*/
}
}
}
}
/* The code reached here and found no available IDs in the range */
fprintf (shadow_logfd,
_("%s: Can't get unique UID (no more available UIDs)\n"),
Prog);
SYSLOG ((LOG_WARN, "no more available UIDs on the system"));
free (used_uids);
return -1;
}
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