/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 2003 Networks Associates Technology, Inc.
 * Copyright (c) 2004-2011 Dag-Erling Smørgrav
 * All rights reserved.
 *
 * This software was developed for the FreeBSD Project by ThinkSec AS and
 * NAI Labs, the Security Research Division of Network Associates, Inc.
 * under DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
 * DARPA CHATS research program.
 *
 * 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.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior written
 *    permission.
 *
 * 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/param.h>
#include <sys/wait.h>

#include <errno.h>
#include <fcntl.h>
#include <paths.h>
#include <pwd.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#define PAM_SM_AUTH
#define PAM_SM_SESSION

#include <security/pam_appl.h>
#include <security/pam_modules.h>
#include <security/openpam.h>

#include <openssl/evp.h>

#define __bounded__(x, y, z)
#include "authfd.h"
#include "authfile.h"
#include "sshkey.h"

#define ssh_add_identity(auth, key, comment) \
        ssh_add_identity_constrained(auth, key, comment, 0, 0, 0, NULL, NULL, 0)

extern char **environ;

struct pam_ssh_key {
        struct sshkey   *key;
        char            *comment;
};

static const char *pam_ssh_prompt = "SSH passphrase: ";
static const char *pam_ssh_have_keys = "pam_ssh_have_keys";

static const char *pam_ssh_keyfiles[] = {
        ".ssh/id_rsa",          /* SSH2 RSA key */
        ".ssh/id_dsa",          /* SSH2 DSA key */
        ".ssh/id_ecdsa",        /* SSH2 ECDSA key */
        ".ssh/id_ed25519",      /* SSH2 Ed25519 key */
        NULL
};

static const char *pam_ssh_agent = "/usr/bin/ssh-agent";
static char str_ssh_agent[] = "ssh-agent";
static char str_dash_s[] = "-s";
static char *const pam_ssh_agent_argv[] = { str_ssh_agent, str_dash_s, NULL };
static char *const pam_ssh_agent_envp[] = { NULL };

/*
 * Attempts to load a private key from the specified file in the specified
 * directory, using the specified passphrase.  If successful, returns a
 * struct pam_ssh_key containing the key and its comment.
 */
static struct pam_ssh_key *
pam_ssh_load_key(const char *dir, const char *kfn, const char *passphrase,
    int nullok)
{
        char fn[PATH_MAX];
        struct pam_ssh_key *psk;
        struct sshkey *key;
        char *comment;
        int ret;

        if (snprintf(fn, sizeof(fn), "%s/%s", dir, kfn) > (int)sizeof(fn))
                return (NULL);
        /*
         * If the key is unencrypted, OpenSSL ignores the passphrase, so
         * it will seem like the user typed in the right one.  This allows
         * a user to circumvent nullok by providing a dummy passphrase.
         * Verify that the key really *is* encrypted by trying to load it
         * with an empty passphrase, and if the key is not encrypted,
         * accept only an empty passphrase.
         */
        ret = sshkey_load_private(fn, "", &key, &comment);
        if (ret == 0 && !(*passphrase == '\0' && nullok)) {
                sshkey_free(key);
                return (NULL);
        }
        if (ret != 0)
                ret = sshkey_load_private(fn, passphrase, &key, &comment);
        if (ret != 0) {
                openpam_log(PAM_LOG_DEBUG, "failed to load key from %s", fn);
                return (NULL);
        }

        openpam_log(PAM_LOG_DEBUG, "loaded '%s' from %s", comment, fn);
        if ((psk = malloc(sizeof(*psk))) == NULL) {
                sshkey_free(key);
                free(comment);
                return (NULL);
        }
        psk->key = key;
        psk->comment = comment;
        return (psk);
}

/*
 * Wipes a private key and frees the associated resources.
 */
static void
pam_ssh_free_key(pam_handle_t *pamh __unused,
    void *data, int pam_err __unused)
{
        struct pam_ssh_key *psk;

        psk = data;
        sshkey_free(psk->key);
        free(psk->comment);
        free(psk);
}

PAM_EXTERN int
pam_sm_authenticate(pam_handle_t *pamh, int flags __unused,
    int argc __unused, const char *argv[] __unused)
{
        const char **kfn, *passphrase, *user;
        const void *item;
        struct passwd *pwd;
        struct pam_ssh_key *psk;
        int nkeys, nullok, pam_err, pass;

        nullok = (openpam_get_option(pamh, "nullok") != NULL);

        /* PEM is not loaded by default */
        OpenSSL_add_all_algorithms();

        /* get user name and home directory */
        pam_err = pam_get_user(pamh, &user, NULL);
        if (pam_err != PAM_SUCCESS)
                return (pam_err);
        pwd = getpwnam(user);
        if (pwd == NULL)
                return (PAM_USER_UNKNOWN);
        if (pwd->pw_dir == NULL)
                return (PAM_AUTH_ERR);

        nkeys = 0;
        pass = (pam_get_item(pamh, PAM_AUTHTOK, &item) == PAM_SUCCESS &&
            item != NULL);
 load_keys:
        /* get passphrase */
        pam_err = pam_get_authtok(pamh, PAM_AUTHTOK,
            &passphrase, pam_ssh_prompt);
        if (pam_err != PAM_SUCCESS)
                return (pam_err);

        /* switch to user credentials */
        pam_err = openpam_borrow_cred(pamh, pwd);
        if (pam_err != PAM_SUCCESS)
                return (pam_err);

        /* try to load keys from all keyfiles we know of */
        for (kfn = pam_ssh_keyfiles; *kfn != NULL; ++kfn) {
                psk = pam_ssh_load_key(pwd->pw_dir, *kfn, passphrase, nullok);
                if (psk != NULL) {
                        pam_set_data(pamh, *kfn, psk, pam_ssh_free_key);
                        ++nkeys;
                }
        }

        /* switch back to arbitrator credentials */
        openpam_restore_cred(pamh);

        /*
         * If we tried an old token and didn't get anything, and
         * try_first_pass was specified, try again after prompting the
         * user for a new passphrase.
         */
        if (nkeys == 0 && pass == 1 &&
            openpam_get_option(pamh, "try_first_pass") != NULL) {
                pam_set_item(pamh, PAM_AUTHTOK, NULL);
                pass = 0;
                goto load_keys;
        }

        /* no keys? */
        if (nkeys == 0)
                return (PAM_AUTH_ERR);

        pam_set_data(pamh, pam_ssh_have_keys, NULL, NULL);
        return (PAM_SUCCESS);
}

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

        return (PAM_SUCCESS);
}

/*
 * Parses a line from ssh-agent's output.
 */
static void
pam_ssh_process_agent_output(pam_handle_t *pamh, FILE *f)
{
        char *line, *p, *key, *val;
        size_t len;

        while ((line = fgetln(f, &len)) != NULL) {
                if (len < 4 || strncmp(line, "SSH_", 4) != 0)
                        continue;

                /* find equal sign at end of key */
                for (p = key = line; p < line + len; ++p)
                        if (*p == '=')
                                break;
                if (p == line + len || *p != '=')
                        continue;
                *p = '\0';

                /* find semicolon at end of value */
                for (val = ++p; p < line + len; ++p)
                        if (*p == ';')
                                break;
                if (p == line + len || *p != ';')
                        continue;
                *p = '\0';

                /* store key-value pair in environment */
                openpam_log(PAM_LOG_DEBUG, "got %s: %s", key, val);
                pam_setenv(pamh, key, val, 1);
        }
}

/*
 * Starts an ssh agent and stores the environment variables derived from
 * its output.
 */
static int
pam_ssh_start_agent(pam_handle_t *pamh)
{
        int agent_pipe[2];
        pid_t pid;
        FILE *f;

        /* get a pipe which we will use to read the agent's output */
        if (pipe(agent_pipe) == -1)
                return (PAM_SYSTEM_ERR);

        /* start the agent */
        openpam_log(PAM_LOG_DEBUG, "starting an ssh agent");
        pid = fork();
        if (pid == (pid_t)-1) {
                /* failed */
                close(agent_pipe[0]);
                close(agent_pipe[1]);
                return (PAM_SYSTEM_ERR);
        }
        if (pid == 0) {
                int fd;

                /* child: drop privs, close fds and start agent */
                setgid(getegid());
                setuid(geteuid());
                close(STDIN_FILENO);
                open(_PATH_DEVNULL, O_RDONLY);
                dup2(agent_pipe[1], STDOUT_FILENO);
                dup2(agent_pipe[1], STDERR_FILENO);
                for (fd = 3; fd < getdtablesize(); ++fd)
                        close(fd);
                execve(pam_ssh_agent, pam_ssh_agent_argv, pam_ssh_agent_envp);
                _exit(127);
        }

        /* parent */
        close(agent_pipe[1]);
        if ((f = fdopen(agent_pipe[0], "r")) == NULL)
                return (PAM_SYSTEM_ERR);
        pam_ssh_process_agent_output(pamh, f);
        fclose(f);

        return (PAM_SUCCESS);
}

/*
 * Adds previously stored keys to a running agent.
 */
static int
pam_ssh_add_keys_to_agent(pam_handle_t *pamh)
{
        const struct pam_ssh_key *psk;
        const char **kfn;
        const void *item;
        char **envlist, **env;
        int fd, pam_err;

        /* switch to PAM environment */
        envlist = environ;
        if ((environ = pam_getenvlist(pamh)) == NULL) {
                environ = envlist;
                return (PAM_SYSTEM_ERR);
        }

        /* get a connection to the agent */
        if (ssh_get_authentication_socket(&fd) != 0) {
                openpam_log(PAM_LOG_DEBUG, "failed to connect to the agent");
                pam_err = PAM_SYSTEM_ERR;
                goto end;
        }

        /* look for keys to add to it */
        for (kfn = pam_ssh_keyfiles; *kfn != NULL; ++kfn) {
                pam_err = pam_get_data(pamh, *kfn, &item);
                if (pam_err == PAM_SUCCESS && item != NULL) {
                        psk = item;
                        if (ssh_add_identity(fd, psk->key, psk->comment) == 0)
                                openpam_log(PAM_LOG_DEBUG,
                                    "added %s to ssh agent", psk->comment);
                        else
                                openpam_log(PAM_LOG_DEBUG, "failed "
                                    "to add %s to ssh agent", psk->comment);
                        /* we won't need the key again, so wipe it */
                        pam_set_data(pamh, *kfn, NULL, NULL);
                }
        }
        pam_err = PAM_SUCCESS;

        /* disconnect from agent */
        ssh_close_authentication_socket(fd);

 end:
        /* switch back to original environment */
        for (env = environ; *env != NULL; ++env)
                free(*env);
        free(environ);
        environ = envlist;

        return (pam_err);
}

PAM_EXTERN int
pam_sm_open_session(pam_handle_t *pamh, int flags __unused,
    int argc __unused, const char *argv[] __unused)
{
        struct passwd *pwd;
        const char *user;
        const void *data;
        int pam_err;

        /* no keys, no work */
        if (pam_get_data(pamh, pam_ssh_have_keys, &data) != PAM_SUCCESS &&
            openpam_get_option(pamh, "want_agent") == NULL)
                return (PAM_SUCCESS);

        /* switch to user credentials */
        pam_err = pam_get_user(pamh, &user, NULL);
        if (pam_err != PAM_SUCCESS)
                return (pam_err);
        pwd = getpwnam(user);
        if (pwd == NULL)
                return (PAM_USER_UNKNOWN);
        pam_err = openpam_borrow_cred(pamh, pwd);
        if (pam_err != PAM_SUCCESS)
                return (pam_err);

        /* start the agent */
        pam_err = pam_ssh_start_agent(pamh);
        if (pam_err != PAM_SUCCESS) {
                openpam_restore_cred(pamh);
                return (pam_err);
        }

        /* we have an agent, see if we can add any keys to it */
        pam_err = pam_ssh_add_keys_to_agent(pamh);
        if (pam_err != PAM_SUCCESS) {
                /* XXX ignore failures */
        }

        openpam_restore_cred(pamh);
        return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_close_session(pam_handle_t *pamh, int flags __unused,
    int argc __unused, const char *argv[] __unused)
{
        const char *ssh_agent_pid;
        char *end;
        int status;
        pid_t pid;

        if ((ssh_agent_pid = pam_getenv(pamh, "SSH_AGENT_PID")) == NULL) {
                openpam_log(PAM_LOG_DEBUG, "no ssh agent");
                return (PAM_SUCCESS);
        }
        pid = (pid_t)strtol(ssh_agent_pid, &end, 10);
        if (*ssh_agent_pid == '\0' || *end != '\0') {
                openpam_log(PAM_LOG_DEBUG, "invalid ssh agent pid");
                return (PAM_SESSION_ERR);
        }
        openpam_log(PAM_LOG_DEBUG, "killing ssh agent %d", (int)pid);
        if (kill(pid, SIGTERM) == -1 ||
            (waitpid(pid, &status, 0) == -1 && errno != ECHILD))
                return (PAM_SYSTEM_ERR);
        return (PAM_SUCCESS);
}

PAM_MODULE_ENTRY("pam_ssh");