src/lib/restrict-access.c

/* Copyright (c) 2002-2018 Dovecot authors, see the included COPYING file */

#define _GNU_SOURCE /* setresgid() */
#include <stdio.h> /* for AIX */
#include <sys/types.h>
#include <unistd.h>

#include "lib.h"
#include "str.h"
#include "restrict-access.h"
#include "env-util.h"
#include "ipwd.h"

#include <time.h>
#ifdef HAVE_PR_SET_DUMPABLE
#  include <sys/prctl.h>
#endif

static gid_t process_primary_gid = (gid_t)-1;
static gid_t process_privileged_gid = (gid_t)-1;
static bool process_using_priv_gid = FALSE;
static char *chroot_dir = NULL;

void restrict_access_init(struct restrict_access_settings *set)
{
    i_zero(set);

    set->uid = (uid_t)-1;
    set->gid = (gid_t)-1;
    set->privileged_gid = (gid_t)-1;
}

static const char *get_uid_str(uid_t uid)
{
    struct passwd pw;
    const char *ret;
    int old_errno = errno;

    if (i_getpwuid(uid, &pw) <= 0)
        ret = dec2str(uid);
    else
        ret = t_strdup_printf("%s(%s)", dec2str(uid), pw.pw_name);
    errno = old_errno;
    return ret;
}

static const char *get_gid_str(gid_t gid)
{
    struct group group;
    const char *ret;
    int old_errno = errno;

    if (i_getgrgid(gid, &group) <= 0)
        ret = dec2str(gid);
    else
        ret = t_strdup_printf("%s(%s)", dec2str(gid), group.gr_name);
    errno = old_errno;
    return ret;
}

static void restrict_init_groups(gid_t primary_gid, gid_t privileged_gid,
                 const char *gid_source)
{
    string_t *str;

    if (privileged_gid == (gid_t)-1) {
        if (primary_gid == getgid() && primary_gid == getegid()) {
            /* everything is already set */
            return;
        }

        if (setgid(primary_gid) == 0)
            return;

        str = t_str_new(128);
        str_printfa(str, "setgid(%s", get_gid_str(primary_gid));
        if (gid_source != NULL)
            str_printfa(str, " from %s", gid_source);
        str_printfa(str, ") failed with euid=%s, gid=%s, egid=%s: %m "
                "(This binary should probably be called with "
                "process group set to %s instead of %s)",
                get_uid_str(geteuid()),
                get_gid_str(getgid()), get_gid_str(getegid()),
                get_gid_str(primary_gid), get_gid_str(getegid()));
        i_fatal("%s", str_c(str));
    }

    if (getegid() != 0 && primary_gid == getgid() &&
        primary_gid == getegid()) {
        /* privileged_gid is hopefully in saved ID. if not,
           there's nothing we can do about it. */
        return;
    }

#ifdef HAVE_SETRESGID
    if (setresgid(primary_gid, primary_gid, privileged_gid) != 0) {
        i_fatal("setresgid(%s,%s,%s) failed with euid=%s: %m",
            get_gid_str(primary_gid), get_gid_str(primary_gid),
            get_gid_str(privileged_gid), get_uid_str(geteuid()));
    }
#else
    if (geteuid() == 0) {
        /* real, effective, saved -> privileged_gid */
        if (setgid(privileged_gid) < 0) {
            i_fatal("setgid(%s) failed: %m",
                get_gid_str(privileged_gid));
        }
    }
    /* real, effective -> primary_gid
       saved -> keep */
    if (setregid(primary_gid, primary_gid) != 0) {
        i_fatal("setregid(%s,%s) failed with euid=%s: %m",
            get_gid_str(primary_gid), get_gid_str(privileged_gid),
            get_uid_str(geteuid()));
    }
#endif
}

gid_t *restrict_get_groups_list(unsigned int *gid_count_r)
{
    gid_t *gid_list;
    int ret, gid_count;

    if ((gid_count = getgroups(0, NULL)) < 0)
        i_fatal("getgroups() failed: %m");

    /* @UNSAFE */
    gid_list = t_new(gid_t, gid_count+1); /* +1 in case gid_count=0 */
    if ((ret = getgroups(gid_count, gid_list)) < 0)
        i_fatal("getgroups() failed: %m");

    *gid_count_r = ret;
    return gid_list;
}

static void drop_restricted_groups(const struct restrict_access_settings *set,
                   gid_t *gid_list, unsigned int *gid_count,
                   bool *have_root_group)
{
    /* @UNSAFE */
    unsigned int i, used;

    for (i = 0, used = 0; i < *gid_count; i++) {
        if (gid_list[i] >= set->first_valid_gid &&
            (set->last_valid_gid == 0 ||
             gid_list[i] <= set->last_valid_gid)) {
            if (gid_list[i] == 0)
                *have_root_group = TRUE;
            gid_list[used++] = gid_list[i];
        }
    }
    *gid_count = used;
}

static gid_t get_group_id(const char *name)
{
    struct group group;
    gid_t gid;

    if (str_to_gid(name, &gid) == 0)
        return gid;

    switch (i_getgrnam(name, &group)) {
    case -1:
        i_fatal("getgrnam(%s) failed: %m", name);
    case 0:
        i_fatal("unknown group name in extra_groups: %s", name);
    default:
        return group.gr_gid;
    }
}

static void fix_groups_list(const struct restrict_access_settings *set,
                bool preserve_existing, bool *have_root_group)
{
    gid_t gid, *gid_list, *gid_list2;
    const char *const *tmp, *empty = NULL;
    unsigned int i, gid_count;
    bool add_primary_gid;

    /* if we're using a privileged GID, we can temporarily drop our
       effective GID. we still want to be able to use its privileges,
       so add it to supplementary groups. */
    add_primary_gid = process_privileged_gid != (gid_t)-1;

    tmp = set->extra_groups == NULL ? &empty :
        t_strsplit_spaces(set->extra_groups, ", ");

    if (preserve_existing) {
        gid_list = restrict_get_groups_list(&gid_count);
        drop_restricted_groups(set, gid_list, &gid_count,
                       have_root_group);
        /* see if the list already contains the primary GID */
        for (i = 0; i < gid_count; i++) {
            if (gid_list[i] == process_primary_gid) {
                add_primary_gid = FALSE;
                break;
            }
        }
    } else {
        gid_list = NULL;
        gid_count = 0;
    }
    if (gid_count == 0) {
        /* Some OSes don't like an empty groups list,
           so use the primary GID as the only one. */
        gid_list = t_new(gid_t, 2);
        gid_list[0] = process_primary_gid;
        gid_count = 1;
        add_primary_gid = FALSE;
    }

    if (*tmp != NULL || add_primary_gid) {
        /* @UNSAFE: add extra groups and/or primary GID to gids list */
        gid_list2 = t_new(gid_t, gid_count + str_array_length(tmp) + 1);
        memcpy(gid_list2, gid_list, gid_count * sizeof(gid_t));
        for (; *tmp != NULL; tmp++) {
            gid = get_group_id(*tmp);
            if (gid != process_primary_gid)
                gid_list2[gid_count++] = gid;
        }
        if (add_primary_gid)
            gid_list2[gid_count++] = process_primary_gid;
        gid_list = gid_list2;
    }

    if (setgroups(gid_count, gid_list) < 0) {
        if (errno == EINVAL) {
            i_fatal("setgroups(%s) failed: Too many extra groups",
                set->extra_groups == NULL ? "" :
                set->extra_groups);
        } else {
            i_fatal("setgroups() failed: %m");
        }
    }
}

static const char *
get_setuid_error_str(const struct restrict_access_settings *set, uid_t target_uid)
{
    string_t *str = t_str_new(128);

    str_printfa(str, "setuid(%s", get_uid_str(target_uid));
    if (set->uid_source != NULL)
        str_printfa(str, " from %s", set->uid_source);
    str_printfa(str, ") failed with euid=%s: %m ",
            get_uid_str(geteuid()));
    if (errno == EAGAIN) {
        str_append(str, "(ulimit -u reached)");
    } else {
        str_printfa(str, "(This binary should probably be called with "
                "process user set to %s instead of %s)",
                get_uid_str(target_uid), get_uid_str(geteuid()));
    }
    return str_c(str);
}

void restrict_access(const struct restrict_access_settings *set,
             enum restrict_access_flags flags, const char *home)
{
    bool is_root, have_root_group, preserve_groups = FALSE;
    bool allow_root_gid;
    bool allow_root = (flags & RESTRICT_ACCESS_FLAG_ALLOW_ROOT) != 0;
    uid_t target_uid = set->uid;

    is_root = geteuid() == 0;

    if (!is_root &&
        !set->allow_setuid_root &&
        getuid() == 0) {
        /* recover current effective UID */
        if (target_uid == (uid_t)-1)
            target_uid = geteuid();
        else
            i_assert(target_uid > 0);
        /* try to elevate to root */
        if (seteuid(0) < 0)
            i_fatal("seteuid(0) failed: %m");
        is_root = TRUE;
    }

    /* set the primary/privileged group */
    process_primary_gid = set->gid;
    process_privileged_gid = set->privileged_gid;
    if (process_privileged_gid == process_primary_gid) {
        /* a pointless configuration, ignore it */
        process_privileged_gid = (gid_t)-1;
    }

    have_root_group = process_primary_gid == 0;
    if (process_primary_gid != (gid_t)-1 ||
        process_privileged_gid != (gid_t)-1) {
        if (process_primary_gid == (gid_t)-1)
            process_primary_gid = getegid();
        restrict_init_groups(process_primary_gid,
                     process_privileged_gid, set->gid_source);
    } else {
        if (process_primary_gid == (gid_t)-1)
            process_primary_gid = getegid();
    }

    /* set system user's groups */
    if (set->system_groups_user != NULL && is_root) {
        if (initgroups(set->system_groups_user,
                   process_primary_gid) < 0) {
            i_fatal("initgroups(%s, %s) failed: %m",
                set->system_groups_user,
                get_gid_str(process_primary_gid));
        }
        preserve_groups = TRUE;
    }

    /* add extra groups. if we set system user's groups, drop the
       restricted groups at the same time. */
    if (is_root) T_BEGIN {
        fix_groups_list(set, preserve_groups,
                &have_root_group);
    } T_END;

    /* chrooting */
    if (set->chroot_dir != NULL) {
        /* kludge: localtime() must be called before chroot(),
           or the timezone isn't known */
        time_t t = 0;
        (void)localtime(&t);

        if (chroot(set->chroot_dir) != 0)
            i_fatal("chroot(%s) failed: %m", set->chroot_dir);
        /* makes static analyzers happy, and is more secure */
        if (chdir("/") != 0)
            i_fatal("chdir(/) failed: %m");

        chroot_dir = i_strdup(set->chroot_dir);

        if (home != NULL) {
            if (chdir(home) < 0) {
                i_error("chdir(%s) failed: %m", home);
            }
        }
    }

    /* uid last */
    if (target_uid != (uid_t)-1) {
        if (setuid(target_uid) != 0)
            i_fatal("%s", get_setuid_error_str(set, target_uid));
    }

    /* verify that we actually dropped the privileges */
    if ((target_uid != (uid_t)-1 && target_uid != 0) || !allow_root) {
        if (setuid(0) == 0) {
            if (!allow_root &&
                (target_uid == 0 || target_uid == (uid_t)-1))
                i_fatal("This process must not be run as root");

            i_fatal("We couldn't drop root privileges");
        }
    }

    if (set->first_valid_gid != 0)
        allow_root_gid = FALSE;
    else if (process_primary_gid == 0 || process_privileged_gid == 0)
        allow_root_gid = TRUE;
    else
        allow_root_gid = FALSE;

    if (!allow_root_gid && target_uid != 0 &&
        (target_uid != (uid_t)-1 || !is_root)) {
        if (getgid() == 0 || getegid() == 0 || setgid(0) == 0) {
            if (process_primary_gid == 0)
                i_fatal("GID 0 isn't permitted");
            i_fatal("We couldn't drop root group privileges "
                "(wanted=%s, gid=%s, egid=%s)",
                get_gid_str(process_primary_gid),
                get_gid_str(getgid()), get_gid_str(getegid()));
        }
    }
}

void restrict_access_set_env(const struct restrict_access_settings *set)
{
    if (set->system_groups_user != NULL &&
        *set->system_groups_user != '\0') {
        env_put(t_strconcat("RESTRICT_USER=",
                    set->system_groups_user, NULL));
    }
    if (set->chroot_dir != NULL && *set->chroot_dir != '\0')
        env_put(t_strconcat("RESTRICT_CHROOT=", set->chroot_dir, NULL));

    if (set->uid != (uid_t)-1) {
        env_put(t_strdup_printf("RESTRICT_SETUID=%s",
                    dec2str(set->uid)));
    }
    if (set->gid != (gid_t)-1) {
        env_put(t_strdup_printf("RESTRICT_SETGID=%s",
                    dec2str(set->gid)));
    }
    if (set->privileged_gid != (gid_t)-1) {
        env_put(t_strdup_printf("RESTRICT_SETGID_PRIV=%s",
                    dec2str(set->privileged_gid)));
    }
    if (set->extra_groups != NULL && *set->extra_groups != '\0') {
        env_put(t_strconcat("RESTRICT_SETEXTRAGROUPS=",
                    set->extra_groups, NULL));
    }

    if (set->first_valid_gid != 0) {
        env_put(t_strdup_printf("RESTRICT_GID_FIRST=%s",
                    dec2str(set->first_valid_gid)));
    }
    if (set->last_valid_gid != 0) {
        env_put(t_strdup_printf("RESTRICT_GID_LAST=%s",
                    dec2str(set->last_valid_gid)));
    }
}

static const char *null_if_empty(const char *str)
{
    return str == NULL || *str == '\0' ? NULL : str;
}

void restrict_access_get_env(struct restrict_access_settings *set_r)
{
    const char *value;

    restrict_access_init(set_r);
    if ((value = getenv("RESTRICT_SETUID")) != NULL) {
        if (str_to_uid(value, &set_r->uid) < 0)
            i_fatal("Invalid uid: %s", value);
    }
    if ((value = getenv("RESTRICT_SETGID")) != NULL) {
        if (str_to_gid(value, &set_r->gid) < 0)
            i_fatal("Invalid gid: %s", value);
    }
    if ((value = getenv("RESTRICT_SETGID_PRIV")) != NULL) {
        if (str_to_gid(value, &set_r->privileged_gid) < 0)
            i_fatal("Invalid privileged_gid: %s", value);
    }
    if ((value = getenv("RESTRICT_GID_FIRST")) != NULL) {
        if (str_to_gid(value, &set_r->first_valid_gid) < 0)
            i_fatal("Invalid first_valid_gid: %s", value);
    }
    if ((value = getenv("RESTRICT_GID_LAST")) != NULL) {
        if (str_to_gid(value, &set_r->last_valid_gid) < 0)
            i_fatal("Invalid last_value_gid: %s", value);
    }

    set_r->extra_groups = null_if_empty(getenv("RESTRICT_SETEXTRAGROUPS"));
    set_r->system_groups_user = null_if_empty(getenv("RESTRICT_USER"));
    set_r->chroot_dir = null_if_empty(getenv("RESTRICT_CHROOT"));
}

void restrict_access_by_env(enum restrict_access_flags flags, const char *home)
{
    struct restrict_access_settings set;

    restrict_access_get_env(&set);
    restrict_access(&set, flags, home);

    /* clear the environment, so we don't fail if we get back here */
    env_remove("RESTRICT_SETUID");
    if (process_privileged_gid == (gid_t)-1) {
        /* if we're dropping privileges before executing and
           a privileged group is set, the groups must be fixed
           after exec */
        env_remove("RESTRICT_SETGID");
        env_remove("RESTRICT_SETGID_PRIV");
    }
    env_remove("RESTRICT_GID_FIRST");
    env_remove("RESTRICT_GID_LAST");
    if (getuid() != 0)
        env_remove("RESTRICT_SETEXTRAGROUPS");
    else {
        /* Preserve RESTRICT_SETEXTRAGROUPS, so if we're again dropping
           more privileges we'll still preserve the extra groups. This
           mainly means preserving service { extra_groups } for lmtp
           and doveadm accesses. */
    }
    env_remove("RESTRICT_USER");
    env_remove("RESTRICT_CHROOT");
}

const char *restrict_access_get_current_chroot(void)
{
    return chroot_dir;
}

void restrict_access_set_dumpable(bool allow ATTR_UNUSED)
{
#ifdef HAVE_PR_SET_DUMPABLE
    if (prctl(PR_SET_DUMPABLE, allow ? 1 : 0, 0, 0, 0) < 0)
        i_error("prctl(PR_SET_DUMPABLE) failed: %m");
#endif
}

bool restrict_access_get_dumpable(void)
{
#ifdef HAVE_PR_SET_DUMPABLE
    bool allow = FALSE;
    if (prctl(PR_GET_DUMPABLE, &allow, 0, 0, 0) < 0)
        i_error("prctl(PR_GET_DUMPABLE) failed: %m");
    return allow;
#endif
    return TRUE;
}

void restrict_access_allow_coredumps(bool allow)
{
    if (getenv("PR_SET_DUMPABLE") != NULL) {
        restrict_access_set_dumpable(allow);
    }
}

int restrict_access_use_priv_gid(void)
{
    i_assert(!process_using_priv_gid);

    if (process_privileged_gid == (gid_t)-1)
        return 0;
    if (setegid(process_privileged_gid) < 0) {
        i_error("setegid(privileged) failed: %m");
        return -1;
    }
    process_using_priv_gid = TRUE;
    return 0;
}

void restrict_access_drop_priv_gid(void)
{
    if (!process_using_priv_gid)
        return;

    if (setegid(process_primary_gid) < 0)
        i_fatal("setegid(primary) failed: %m");
    process_using_priv_gid = FALSE;
}

bool restrict_access_have_priv_gid(void)
{
    return process_privileged_gid != (gid_t)-1;
}