validate.c

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/*
 *  OpenVPN -- An application to securely tunnel IP networks
 *             over a single TCP/UDP port, with support for SSL/TLS-based
 *             session authentication and key exchange,
 *             packet encryption, packet authentication, and
 *             packet compression.
 *
 *  Copyright (C) 2016-2026 Selva Nair <selva.nair@gmail.com>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License version 2
 *  as published by the Free Software Foundation.
 *
 *  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, see <https://www.gnu.org/licenses/>.
 */
 
#include "validate.h"
 
#include <lmaccess.h>
#include <shlwapi.h>
#include <pathcch.h>
#include <lm.h>
 
static const WCHAR *white_list[] = {
    L"auth-retry",
    L"config",
    L"log",
    L"log-append",
    L"management",
    L"management-forget-disconnect",
    L"management-hold",
    L"management-query-passwords",
    L"management-query-proxy",
    L"management-signal",
    L"management-up-down",
    L"mute",
    L"setenv",
    L"service",
    L"verb",
    L"pull-filter",
    L"script-security",
 
    NULL /* last value */
};
 
static BOOL IsUserInGroup(PSID sid, const PTOKEN_GROUPS groups, const WCHAR *group_name);
 
static PTOKEN_GROUPS GetTokenGroups(const HANDLE token);
 
/*
 * Check that config path is inside config_dir
 * The logic here is simple: if the path isn't prefixed with config_dir it's rejected
 */
static BOOL
CheckConfigPath(const WCHAR *workdir, const WCHAR *fname, const settings_t *s)
{
    HRESULT res;
    WCHAR config_path[MAX_PATH];
    const size_t config_dir_len = wcslen(s->config_dir);
 
    /* config_dir must end with a '\' or the prefix check below could be satisfied by a sibling directory */
    if (config_dir_len == 0 || s->config_dir[config_dir_len - 1] != L'\\')
    {
        return FALSE;
    }
    /* fname = stdin is special: do not treat it as a relative path */
    if (wcscmp(fname, L"stdin") == 0)
    {
        return FALSE;
    }
    /* do not accept forward slashes in paths, as PathCch* functions do not handle these */
    if (wcschr(workdir, L'/') || wcschr(fname, L'/'))
    {
        return FALSE;
    }
    /* convert fname to full canonical path */
    if (PathIsRelativeW(fname))
    {
        res = PathCchCombine(config_path, _countof(config_path), workdir, fname);
    }
    else
    {
        res = PathCchCanonicalize(config_path, _countof(config_path), fname);
    }
 
    return res == S_OK && wcsnicmp(config_path, s->config_dir, config_dir_len) == 0;
}
 
 
/*
 * A simple linear search meant for a small wchar_t *array.
 * Returns index to the item if found, -1 otherwise.
 */
static int
OptionLookup(const WCHAR *name, const WCHAR *white_list[])
{
    int i;
 
    for (i = 0; white_list[i]; i++)
    {
        if (wcscmp(white_list[i], name) == 0)
        {
            return i;
        }
    }
 
    return -1;
}
 
/*
 * The Administrators group may be localized or renamed by admins.
 * Get the local name of the group using the SID.
 */
static BOOL
GetBuiltinAdminGroupName(WCHAR *name, DWORD nlen)
{
    BOOL b = FALSE;
    PSID admin_sid = NULL;
    DWORD sid_size = SECURITY_MAX_SID_SIZE;
    SID_NAME_USE snu;
 
    WCHAR domain[MAX_NAME];
    DWORD dlen = _countof(domain);
 
    admin_sid = malloc(sid_size);
    if (!admin_sid)
    {
        return FALSE;
    }
 
    b = CreateWellKnownSid(WinBuiltinAdministratorsSid, NULL, admin_sid, &sid_size);
    if (b)
    {
        b = LookupAccountSidW(NULL, admin_sid, name, &nlen, domain, &dlen, &snu);
    }
 
    free(admin_sid);
 
    return b;
}
 
BOOL
IsAuthorizedUser(PSID sid, const HANDLE token, const WCHAR *ovpn_admin_group,
                 const WCHAR *ovpn_service_user)
{
    const WCHAR *admin_group[2];
    WCHAR username[MAX_NAME];
    WCHAR domain[MAX_NAME];
    WCHAR sysadmin_group[MAX_NAME];
    DWORD len = MAX_NAME;
    BOOL ret = FALSE;
    SID_NAME_USE sid_type;
 
    /* Get username */
    if (!LookupAccountSidW(NULL, sid, username, &len, domain, &len, &sid_type))
    {
        MsgToEventLog(M_SYSERR, L"LookupAccountSid");
        /* not fatal as this is now used only for logging */
        username[0] = '\0';
        domain[0] = '\0';
    }
 
    /* is this service account? */
    if ((wcscmp(username, ovpn_service_user) == 0) && (wcscmp(domain, L"NT SERVICE") == 0))
    {
        return TRUE;
    }
 
    if (GetBuiltinAdminGroupName(sysadmin_group, _countof(sysadmin_group)))
    {
        admin_group[0] = sysadmin_group;
    }
    else
    {
        MsgToEventLog(M_SYSERR,
                      L"Failed to get the name of Administrators group. Using the default.");
        /* use the default value */
        admin_group[0] = SYSTEM_ADMIN_GROUP;
    }
    admin_group[1] = ovpn_admin_group;
 
    PTOKEN_GROUPS token_groups = GetTokenGroups(token);
    for (int i = 0; i < 2; ++i)
    {
        ret = IsUserInGroup(sid, token_groups, admin_group[i]);
        if (ret)
        {
            MsgToEventLog(M_INFO,
                          L"Authorizing user '%ls@%ls' by virtue of membership in group '%ls'",
                          username, domain, admin_group[i]);
            goto out;
        }
    }
 
out:
    free(token_groups);
    return ret;
}
 
static PTOKEN_GROUPS
GetTokenGroups(const HANDLE token)
{
    PTOKEN_GROUPS groups = NULL;
    DWORD buf_size = 0;
 
    if (!GetTokenInformation(token, TokenGroups, groups, buf_size, &buf_size)
        && GetLastError() == ERROR_INSUFFICIENT_BUFFER)
    {
        groups = malloc(buf_size);
    }
    if (!groups)
    {
        MsgToEventLog(M_SYSERR, L"GetTokenGroups");
    }
    else if (!GetTokenInformation(token, TokenGroups, groups, buf_size, &buf_size))
    {
        MsgToEventLog(M_SYSERR, L"GetTokenInformation");
        free(groups);
    }
    return groups;
}
 
/*
 * Find SID from name
 *
 * On input sid buffer should have space for at least sid_size bytes.
 * Returns true on success, false on failure.
 * Suggest: in caller allocate sid to hold SECURITY_MAX_SID_SIZE bytes
 */
static BOOL
LookupSID(const WCHAR *name, PSID sid, DWORD sid_size)
{
    SID_NAME_USE su;
    WCHAR domain[MAX_NAME];
    DWORD dlen = _countof(domain);
 
    if (!LookupAccountName(NULL, name, sid, &sid_size, domain, &dlen, &su))
    {
        return FALSE; /* not fatal as the group may not exist */
    }
    return TRUE;
}
 
static BOOL
IsUserInGroup(PSID sid, const PTOKEN_GROUPS token_groups, const WCHAR *group_name)
{
    BOOL ret = FALSE;
    DWORD_PTR resume = 0;
    DWORD err;
    BYTE grp_sid[SECURITY_MAX_SID_SIZE];
    int nloop = 0; /* a counter used to not get stuck in the do .. while() */
 
    /* first check in the token groups */
    if (token_groups && LookupSID(group_name, (PSID)grp_sid, _countof(grp_sid)))
    {
        for (DWORD i = 0; i < token_groups->GroupCount; ++i)
        {
            if (EqualSid((PSID)grp_sid, token_groups->Groups[i].Sid))
            {
                return TRUE;
            }
        }
    }
 
    /* check user's SID is a member of the group */
    if (!sid)
    {
        return FALSE;
    }
    do
    {
        DWORD nread, nmax;
        LOCALGROUP_MEMBERS_INFO_0 *members = NULL;
        err = NetLocalGroupGetMembers(NULL, group_name, 0, (LPBYTE *)&members, MAX_PREFERRED_LENGTH,
                                      &nread, &nmax, &resume);
        if ((err != NERR_Success && err != ERROR_MORE_DATA))
        {
            break;
        }
        /* If a match is already found, ret == TRUE and the loop is skipped */
        for (DWORD i = 0; i < nread && !ret; ++i)
        {
            ret = EqualSid(members[i].lgrmi0_sid, sid);
        }
        NetApiBufferFree(members);
        /* MSDN says the lookup should always iterate until err != ERROR_MORE_DATA */
    } while (err == ERROR_MORE_DATA && nloop++ < 100);
 
    if (err != NERR_Success && err != NERR_GroupNotFound)
    {
        SetLastError(err);
        MsgToEventLog(M_SYSERR, L"In NetLocalGroupGetMembers for group '%ls'", group_name);
    }
 
    return ret;
}
 
/*
 * Check whether option argv[0] is white-listed. If argv[0] == "--config", also
 * check that argv[1], if present, passes CheckConfigPath(). If argv[0] is "--setenv",
 * check that we do not allow random options to be passed via "--setenv opt ...".
 * The caller should set argc to the number of valid elements in argv[] array.
 */
BOOL
CheckOption(const WCHAR *workdir, int argc, WCHAR *argv[], const settings_t *s)
{
    /* Do not modify argv or *argv -- ideally it should be const WCHAR *const *, but alas...*/
 
    if (wcscmp(argv[0], L"--config") == 0 && argc > 1 && !CheckConfigPath(workdir, argv[1], s))
    {
        return FALSE;
    }
 
    /* option name starts at 2 characters from argv[i] */
    if (OptionLookup(argv[0] + 2, white_list) == -1) /* not found */
    {
        return FALSE;
    }
 
    /* Do not allow "--setenv opt ..." */
    if (wcscmp(argv[0], L"--setenv") == 0 && argc > 1 && wcscmp(argv[1], L"opt") == 0)
    {
        return FALSE;
    }
 
    return TRUE;
}