/*      $NetBSD: pam_ksu.c,v 1.11 2023/09/07 11:27:57 riastradh Exp $   */

/*-
* Copyright (c) 2002 Jacques A. Vidrine <[email protected]>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
*    notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
*    notice, this list of conditions and the following disclaimer in the
*    documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#include <sys/cdefs.h>
#ifdef __FreeBSD__
__FBSDID("$FreeBSD: src/lib/libpam/modules/pam_ksu/pam_ksu.c,v 1.5 2004/02/10 10:13:21 des Exp $");
#else
__RCSID("$NetBSD: pam_ksu.c,v 1.11 2023/09/07 11:27:57 riastradh Exp $");
#endif

#include <sys/param.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <krb5/krb5.h>

#define PAM_SM_AUTH
#define PAM_SM_CRED
#include <security/pam_appl.h>
#include <security/pam_modules.h>
#include <security/pam_mod_misc.h>

static const char superuser[] = "root";

#define PASSWORD_PROMPT "%s's password:"

static void     log_krb5(krb5_context, krb5_error_code, const char *, ...)
   __printflike(3, 4);
static krb5_error_code  get_su_principal(krb5_context, const char *,
   const char *, char **, krb5_principal *);
static int      auth_krb5(pam_handle_t *, krb5_context, const char *,
                   krb5_principal);

PAM_EXTERN int
pam_sm_authenticate(pam_handle_t *pamh, int flags __unused,
   int argc __unused, const char *argv[] __unused)
{
       krb5_boolean     allow_homedir;
       krb5_context     context;
       krb5_principal   su_principal;
       const char      *user;
       const void      *ruser;
       char            *su_principal_name;
       krb5_error_code  rv;
       int              pamret;

       pamret = pam_get_user(pamh, &user, NULL);
       if (pamret != PAM_SUCCESS)
               return (pamret);
       PAM_LOG("Got user: %s", user);
       pamret = pam_get_item(pamh, PAM_RUSER, &ruser);
       if (pamret != PAM_SUCCESS)
               return (pamret);
       PAM_LOG("Got ruser: %s", (const char *)ruser);
       allow_homedir = krb5_set_home_dir_access(NULL, FALSE);
       rv = krb5_init_context(&context);
       if (rv != 0) {
               log_krb5(context, rv, "krb5_init_context failed");
               pamret = PAM_SERVICE_ERR;
               goto out;
       }
       rv = get_su_principal(context, user, ruser, &su_principal_name, &su_principal);
       if (rv != 0) {
               pamret = PAM_AUTH_ERR;
               goto out;
       }
       PAM_LOG("kuserok: %s -> %s", su_principal_name, user);
       (void)krb5_set_home_dir_access(NULL, TRUE); /* ~user/.k5login */
       rv = krb5_kuserok(context, su_principal, user);
       (void)krb5_set_home_dir_access(NULL, FALSE);
       pamret = rv ? auth_krb5(pamh, context, su_principal_name, su_principal) : PAM_AUTH_ERR;
       free(su_principal_name);
       krb5_free_principal(context, su_principal);
       krb5_free_context(context);
out:    (void)krb5_set_home_dir_access(NULL, allow_homedir);
       return (pamret);
}

PAM_EXTERN int
pam_sm_setcred(pam_handle_t *pamh __unused, int flags __unused,
   int ac __unused, const char *av[] __unused)
{

       return (PAM_SUCCESS);
}

/* Authenticate using Kerberos 5.
*   pamh              -- The PAM handle.
*   context           -- An initialized krb5_context.
*   su_principal_name -- The target principal name, used only for password prompts.
*              If NULL, the password prompts will not include a principal
*              name.
*   su_principal      -- The target krb5_principal.
* Note that a valid keytab in the default location with a host entry
* must be available, and that the PAM application must have sufficient
* privileges to access it.
* Returns PAM_SUCCESS if authentication was successful, or an appropriate
* PAM error code if it was not.
*/
static int
auth_krb5(pam_handle_t *pamh, krb5_context context, const char *su_principal_name,
   krb5_principal su_principal)
{
       krb5_creds       creds;
       krb5_get_init_creds_opt *gic_opt;
       krb5_verify_init_creds_opt vic_opt;
       const char      *pass;
       char             prompt[80];
       krb5_error_code  rv;
       int              pamret;

       rv = krb5_get_init_creds_opt_alloc(context, &gic_opt);
       if (rv != 0) {
               log_krb5(context, rv, "krb5_get_init_creds_opt_alloc");
               return (PAM_SERVICE_ERR);
       }
       krb5_verify_init_creds_opt_init(&vic_opt);
       if (su_principal_name != NULL)
               (void)snprintf(prompt, sizeof(prompt), PASSWORD_PROMPT,
                   su_principal_name);
       else
               (void)snprintf(prompt, sizeof(prompt), "Password:");
       pass = NULL;
       pamret = pam_get_authtok(pamh, PAM_AUTHTOK, &pass, prompt);
       if (pamret != PAM_SUCCESS)
               return (pamret);
       rv = krb5_get_init_creds_password(context, &creds, su_principal,
           pass, NULL, NULL, 0, NULL, gic_opt);
       if (rv != 0) {
               log_krb5(context, rv, "krb5_get_init_creds_password");
               return (PAM_AUTH_ERR);
       }
       krb5_verify_init_creds_opt_set_ap_req_nofail(&vic_opt, 1);
       rv = krb5_verify_init_creds(context, &creds, NULL, NULL, NULL,
           &vic_opt);
       krb5_free_cred_contents(context, &creds);
       if (rv != 0) {
               log_krb5(context, rv, "krb5_verify_init_creds");
               return (PAM_AUTH_ERR);
       }
       return (PAM_SUCCESS);
}

static void
log_krb5(krb5_context ctx, krb5_error_code err, const char *fmt, ...)
{
       char b1[1024], b2[1024];
       const char *errtxt;
       va_list ap;

       va_start(ap, fmt);
       vsnprintf(b1, sizeof(b1), fmt, ap);
       va_end(ap);
       if (ctx)
               errtxt = krb5_get_error_message(ctx, err);
       else
               errtxt = NULL;
       if (errtxt != NULL) {
               snprintf(b2, sizeof(b2), "%s", errtxt);
               krb5_free_error_message(ctx, errtxt);
       } else {
               snprintf(b2, sizeof(b2), "unknown %d", (int)err);
       }
       PAM_LOG("%s (%s)", b1, b2);
}

/* Determine the target principal given the current user and the target user.
*   context           -- An initialized krb5_context.
*   target_user       -- The target username.
*   current_user      -- The current username.
*   su_principal_name -- (out) The target principal name.
*   su_principal      -- (out) The target krb5_principal.
* When the target user is `root', the target principal will be a `root
* instance', e.g. `luser/[email protected]'.  Otherwise, the target principal
* will simply be the current user's default principal name.  Note that
* in any case, if KRB5CCNAME is set and a credentials cache exists, the
* principal name found there will be the `starting point', rather than
* the ruser parameter.
*
* Returns 0 for success, or a com_err error code on failure.
*/
static krb5_error_code
get_su_principal(krb5_context context, const char *target_user, const char *current_user,
   char **su_principal_name, krb5_principal *su_principal)
{
       krb5_principal   default_principal;
       krb5_ccache      ccache;
       char            *principal_name, *ccname, *p;
       krb5_error_code  rv;
       uid_t            euid, ruid;

       *su_principal = NULL;
       default_principal = NULL;
       /* Unless KRB5CCNAME was explicitly set, we won't really be able
        * to look at the credentials cache since krb5_cc_default will
        * look at getuid().
        */
       ruid = getuid();
       euid = geteuid();
       rv = seteuid(ruid);
       if (rv != 0)
               return (errno);
       p = getenv("KRB5CCNAME");
       if (p != NULL)
               ccname = strdup(p);
       else
               (void)asprintf(&ccname, "%s%lu", KRB5_DEFAULT_CCROOT, (unsigned long)ruid);
       if (ccname == NULL)
               return (errno);
       rv = krb5_cc_resolve(context, ccname, &ccache);
       free(ccname);
       if (rv == 0) {
               rv = krb5_cc_get_principal(context, ccache, &default_principal);
               krb5_cc_close(context, ccache);
               if (rv != 0)
                       default_principal = NULL; /* just to be safe */
       }
       rv = seteuid(euid);
       if (rv != 0)
               return (errno);
       if (default_principal == NULL) {
               rv = krb5_make_principal(context, &default_principal, NULL, current_user, NULL);
               if (rv != 0) {
                       PAM_LOG("Could not determine default principal name.");
                       return (rv);
               }
       }
       /* Now that we have some principal, if the target account is
        * `root', then transform it into a `root' instance, e.g.
        * `[email protected]' -> `user/[email protected]'.
        */
       rv = krb5_unparse_name(context, default_principal, &principal_name);
       krb5_free_principal(context, default_principal);
       if (rv != 0) {
               log_krb5(context, rv, "krb5_unparse_name");
               return (rv);
       }
       PAM_LOG("Default principal name: %s", principal_name);
       if (strcmp(target_user, superuser) == 0) {
               p = strrchr(principal_name, '@');
               if (p == NULL) {
                       PAM_LOG("malformed principal name `%s'", principal_name);
                       free(principal_name);
                       return (rv);
               }
               *p++ = '\0';
               *su_principal_name = NULL;
               (void)asprintf(su_principal_name, "%s/%s@%s", principal_name, superuser, p);
               free(principal_name);
       } else
               *su_principal_name = principal_name;

       if (*su_principal_name == NULL)
               return (errno);
       rv = krb5_parse_name(context, *su_principal_name, &default_principal);
       if (rv != 0) {
               log_krb5(context, rv, "krb5_parse_name `%s'",
                   *su_principal_name);
               return (rv);
       }
       PAM_LOG("Target principal name: %s", *su_principal_name);
       *su_principal = default_principal;
       return (0);
}

PAM_MODULE_ENTRY("pam_ksu");

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