Пользовательский PAM для SSHD на C

Я пытаюсь понять, как работает PAM, создавая бэкдор SSH. Следуя этому примеру https://github.com/beatgammit/simple-pam, я уже знаю, как чтобы сделать функциональный модуль PAM, но я хочу прочитать токен жесткого кода из CIN. Я пробовал следующее:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <security/pam_appl.h>
#include <security/pam_modules.h>

/* expected hook */
PAM_EXTERN int pam_sm_setcred( pam_handle_t *pamh, int flags, int argc, const char **argv ) {
    return PAM_SUCCESS;
}

PAM_EXTERN int pam_sm_acct_mgmt(pam_handle_t *pamh, int flags, int argc, const char **argv) {
    printf("Acct mgmt\n");
    return PAM_SUCCESS;
}

/* expected hook, this is where custom stuff happens */
PAM_EXTERN int pam_sm_authenticate( pam_handle_t *pamh, int flags,int argc, const char **argv ) {
    int retval;

    const char* pUsername;
    retval = pam_get_user(pamh, &pUsername, "Username: ");

    printf("Welcome %s\n", pUsername);

    if (retval != PAM_SUCCESS) {
        return retval;
    }

    if (strcmp(pUsername, "luciannitescu") != 0) {
        return PAM_AUTH_ERR;
    }

    char securetoken[8] = "aaaaaaa";
    char password[8];
    printf("Enter your secure token:\n");
    scanf(password);

    if (strcmp(password, securetoken) != 0)
    {
        return PAM_AUTH_ERR;
    }
    return PAM_SUCCESS;
}

При выполнении аутентификации ssh я получаю сообщение об отказе в разрешении, и если я удаляю следующие строки кода, все работает:

char securetoken[8] = "aaaaaaa";
    char password[8];
    printf("Enter your secure token:\n");
    scanf(password);

    if (strcmp(password, securetoken) != 0)
    {
        return PAM_AUTH_ERR;
    }

Неудачная аутентификация:

ssh [email protected] -v
OpenSSH_7.6p1 Ubuntu-4, OpenSSL 1.0.2n  7 Dec 2017
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to 127.0.0.1 [127.0.0.1] port 22.
debug1: Connection established.
debug1: identity file /home/lucian/.ssh/id_rsa type 0
debug1: key_load_public: No such file or directory
debug1: identity file /home/lucian/.ssh/id_rsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/lucian/.ssh/id_dsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/lucian/.ssh/id_dsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/lucian/.ssh/id_ecdsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/lucian/.ssh/id_ecdsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/lucian/.ssh/id_ed25519 type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/lucian/.ssh/id_ed25519-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_7.6p1 Ubuntu-4
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.2p2 Ubuntu-4ubuntu2.4
debug1: match: OpenSSH_7.2p2 Ubuntu-4ubuntu2.4 pat OpenSSH* compat 0x04000000
debug1: Authenticating to 127.0.0.1:22 as 'luciannitescu'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: [email protected]
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: [email protected] MAC: <implicit> compression: none
debug1: kex: client->server cipher: [email protected] MAC: <implicit> compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:...
debug1: Host '127.0.0.1' is known and matches the ECDSA host key.
debug1: Found key in /home/lucian/.ssh/known_hosts:22
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<rsa-sha2-256,rsa-sha2-512>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Offering public key: RSA SHA256:... /home/lucian/.ssh/id_rsa
debug1: Authentications that can continue: publickey,password
debug1: Trying private key: /home/lucian/.ssh/id_dsa
debug1: Trying private key: /home/lucian/.ssh/id_ecdsa
debug1: Trying private key: /home/lucian/.ssh/id_ed25519
debug1: Next authentication method: password
[email protected]'s password: 
debug1: Authentications that can continue: publickey,password
Permission denied, please try again.
[email protected]'s password: 
debug1: Authentications that can continue: publickey,password
Permission denied, please try again.
[email protected]'s password: 
debug1: Authentications that can continue: publickey,password
debug1: No more authentication methods to try.
[email protected]: Permission denied (publickey,password).

Позднее редактирование:

После изменения кода на следующее:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <security/pam_appl.h>
#include <security/pam_modules.h>

/* expected hook */
PAM_EXTERN int pam_sm_setcred( pam_handle_t *pamh, int flags, int argc, const char **argv ) {
    return PAM_SUCCESS;
}

PAM_EXTERN int pam_sm_acct_mgmt(pam_handle_t *pamh, int flags, int argc, const char **argv) {
    printf("Acct mgmt\n");

    return PAM_SUCCESS;
}

/* expected hook, this is where custom stuff happens */
PAM_EXTERN int pam_sm_authenticate( pam_handle_t *pamh, int flags,int argc, const char **argv ) {
    struct pam_message msg[1];
    struct pam_response *resp = NULL;
    struct pam_conv *conv;

    pam_get_item (pamh, PAM_CONV, (const void **) &conv);
    msg[0].msg = "Enter your secret token: ";
    msg[0].msg_style = PAM_PROMPT_ECHO_OFF;
    conv->conv(1, &msg, &resp, conv->appdata_ptr);
    // Token is now in resp->resp;

    int retval;

    const char* pUsername;
    retval = pam_get_user(pamh, &pUsername, "Username: ");

    printf("Welcome %s\n", pUsername);

    if (retval != PAM_SUCCESS) {
        return retval;
    }

    if (strcmp(pUsername, "luciannitescu") != 0) {
        return PAM_AUTH_ERR;
    }



    return PAM_SUCCESS;
}

Я не могу установить SSH-соединение:

lucian@local:~$ ssh [email protected]
[email protected]'s password: 
Connection closed by 127.0.0.1 port 22

Отладка сервера:

debug3: mm_request_send entering: type 23
debug2: userauth_pubkey: authenticated 0 pkalg rsa-sha2-512 [preauth]
debug3: userauth_finish: failure partial=0 next methods="publickey,password" [preauth]
debug3: send packet: type 51 [preauth]
debug3: receive packet: type 50 [preauth]
debug1: userauth-request for user luciannitescu service ssh-connection method password [preauth]
debug1: attempt 2 failures 1 [preauth]
debug2: input_userauth_request: try method password [preauth]
debug3: mm_auth_password entering [preauth]
debug3: mm_request_send entering: type 12 [preauth]
debug3: mm_auth_password: waiting for MONITOR_ANS_AUTHPASSWORD [preauth]
debug3: mm_request_receive_expect entering: type 13 [preauth]
debug3: mm_request_receive entering [preauth]
debug3: mm_request_receive entering
debug3: monitor_read: checking request 12
debug3: PAM: sshpam_passwd_conv called with 1 messages
mm_log_handler: write: Broken pipe
debug1: do_cleanup
debug3: PAM: sshpam_thread_cleanup entering
Segmentation fault (core dumped)

ssh c ubuntu linux pam
person Nitescu Lucian    schedule 18.10.2018    source источник

Ответы (1)


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Вы не можете использовать scanf() для чтения значения от пользователя, так как стандартный ввод не передается пользователю. Вы должны использовать ssh клавиатурно-интерактивную аутентификацию с соответствующими функциями. Это поддерживается функциями pam_conv (диалог PAM).

Страница руководства _man 3 pam_conv_ описывает интерфейс. Это работает в основном следующим образом:

PAM_EXTERN int
pam_sm_authenticate (pam_handle_t * pamh,
                 int flags, int argc, const char **argv)
{
    struct pam_message msg[1];
    struct pam_response *resp = NULL;
    struct pam_conv *conv;

    pam_get_item (pamh, PAM_CONV, (const void **) &conv);
    msg[0].msg = "Enter your secret token: ";
    msg[0].msg_style = PAM_PROMPT_ECHO_OFF;
    conv->conv(1, &msg, &resp, conv->appdata_ptr);
    // Token is now in resp->resp;
    ....
}

Вы можете использовать это как начало для вашей пользовательской функции аутентификации.

person Ctx    schedule 18.10.2018
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почему у меня закрывается соединение на порту 22? - person Nitescu Lucian; 18.10.2018
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@NitescuLucian Если у вас есть дополнительные проблемы, подробно опишите их в своем вопросе. - person Ctx; 18.10.2018
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@NitescuLucian 1) Всегда выполняйте проверку ошибок (pam_get_item()/conv-›conv()) и т. д. Я опустил это для краткости. Кроме того, вы можете запустить свой сервер с sshd -dddDp 222 и подключиться к порту 222 и посмотреть, что происходит на стороне сервера. - person Ctx; 18.10.2018
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не могли бы вы помочь мне? - person Nitescu Lucian; 18.10.2018