Add deferred authentication support to plugin-auth-pam

If OpenVPN signals deferred authentication support (by setting
the internal environment variables "auth_control_file" and
"deferred_auth_pam"), do not wait for PAM stack to finish.  Instead,
the privileged PAM process returns RESPONSE_DEFER via the control
socket, which gets turned into OPENVPN_PLUGIN_FUNC_DEFERRED towards
openvpn.

The PAM process will then fork() and handle all the PAM auth in
the new process, signalling success/failure back by means of the
auth_control_file (forking twice, to simplify wait() handling).

With the extra fork(), multiple deferred authentications can run at
the same time - otherwise the first one would block the next auth
call (because the child would not be ready again to read from the
control socket).

Lightly tested on Linux.

Signed-off-by: Gert Doering <gert@greenie.muc.de>

--
v2:
  - only do deferred auth if "deferred_auth_pam" is set (env)
  - put deferred auth logic into do_deferred_pam_auth()
  - line-wrap lines where needed
  - close "background end" of socketpair in deferred auth process
  - remove leftover /* plugin_log() */ lines from initial testing
  - tested over a few hundred "15s delayed" authentication cycles

v3:
  - uncrustify new code
  - do not abort background process if do_deferred_pam_auth() fails
    (this can only happen if fork() fails, which is assumed to be
    temporary, or if something is wrong with the socketpair which we
    should notice on the next read()) --> change do_deferred_pam_auth()
    to "void"
  - add documentation to README.auth-pam and Changes.rst
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20200715090105.22296-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20361.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

1 files changed, 119 insertions, 3 deletions

diff --git a/src/plugins/auth-pam/auth-pam.c b/src/plugins/auth-pam/auth-pam.c
index 9a11876..f537652 100644
--- a/src/plugins/auth-pam/auth-pam.c
+++ b/src/plugins/auth-pam/auth-pam.c
@@ -62,6 +62,7 @@
 #define RESPONSE_INIT_FAILED      11
 #define RESPONSE_VERIFY_SUCCEEDED 12
 #define RESPONSE_VERIFY_FAILED    13
+#define RESPONSE_DEFER            14
 
 /* Pointers to functions exported from openvpn */
 static plugin_log_t plugin_log = NULL;
@@ -69,7 +70,7 @@ static plugin_secure_memzero_t plugin_secure_memzero = NULL;
 static plugin_base64_decode_t plugin_base64_decode = NULL;
 
 /* module name for plugin_log() */
-static char * MODULE = "AUTH-PAM";
+static char *MODULE = "AUTH-PAM";
 
 /*
  * Plugin state, used by foreground
@@ -524,12 +525,31 @@ openvpn_plugin_func_v1(openvpn_plugin_handle_t handle, const int type, const cha
         const char *password = get_env("password", envp);
         const char *common_name = get_env("common_name", envp) ? get_env("common_name", envp) : "";
 
+        /* should we do deferred auth?
+         *  yes, if there is "auth_control_file" and "deferred_auth_pam" env
+         */
+        const char *auth_control_file = get_env("auth_control_file", envp);
+        const char *deferred_auth_pam = get_env("deferred_auth_pam", envp);
+        if (auth_control_file != NULL && deferred_auth_pam != NULL)
+        {
+            if (DEBUG(context->verb))
+            {
+                plugin_log(PLOG_NOTE, MODULE, "do deferred auth '%s'",
+                           auth_control_file);
+            }
+        }
+        else
+        {
+            auth_control_file = "";
+        }
+
         if (username && strlen(username) > 0 && password)
         {
             if (send_control(context->foreground_fd, COMMAND_VERIFY) == -1
                 || send_string(context->foreground_fd, username) == -1
                 || send_string(context->foreground_fd, password) == -1
-                || send_string(context->foreground_fd, common_name) == -1)
+                || send_string(context->foreground_fd, common_name) == -1
+                || send_string(context->foreground_fd, auth_control_file) == -1)
             {
                 plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "Error sending auth info to background process");
             }
@@ -540,6 +560,14 @@ openvpn_plugin_func_v1(openvpn_plugin_handle_t handle, const int type, const cha
                 {
                     return OPENVPN_PLUGIN_FUNC_SUCCESS;
                 }
+                if (status == RESPONSE_DEFER)
+                {
+                    if (DEBUG(context->verb))
+                    {
+                        plugin_log(PLOG_NOTE, MODULE, "deferred authentication");
+                    }
+                    return OPENVPN_PLUGIN_FUNC_DEFERRED;
+                }
                 if (status == -1)
                 {
                     plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "Error receiving auth confirmation from background process");
@@ -783,12 +811,87 @@ pam_auth(const char *service, const struct user_pass *up)
 }
 
 /*
+ * deferred auth handler
+ *   - fork() (twice, to avoid the need for async wait / SIGCHLD handling)
+ *   - query PAM stack via pam_auth()
+ *   - send response back to OpenVPN via "ac_file_name"
+ *
+ * parent process returns "0" for "fork() and wait() succeeded",
+ *                        "-1" for "something went wrong, abort program"
+ */
+
+static void
+do_deferred_pam_auth(int fd, const char *ac_file_name,
+                     const char *service, const struct user_pass *up)
+{
+    if (send_control(fd, RESPONSE_DEFER) == -1)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "BACKGROUND: write error on response socket [4]");
+        return;
+    }
+
+    /* double forking so we do not need to wait() for async auth kids */
+    pid_t p1 = fork();
+
+    if (p1 < 0)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "BACKGROUND: fork(1) failed");
+        return;
+    }
+    if (p1 != 0)                           /* parent */
+    {
+        waitpid(p1, NULL, 0);
+        return;                            /* parent's job succeeded */
+    }
+
+    /* child */
+    close(fd);                              /* socketpair no longer needed */
+
+    pid_t p2 = fork();
+    if (p2 < 0)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "BACKGROUND: fork(2) failed");
+        exit(1);
+    }
+
+    if (p2 != 0)                            /* new parent: exit right away */
+    {
+        exit(0);
+    }
+
+    /* grandchild */
+    plugin_log(PLOG_NOTE, MODULE, "BACKGROUND: deferred auth for '%s', pid=%d",
+               up->username, (int) getpid() );
+
+    /* the rest is very simple: do PAM, write status byte to file, done */
+    int ac_fd = open( ac_file_name, O_WRONLY );
+    if (ac_fd < 0)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "cannot open '%s' for writing",
+                   ac_file_name );
+        exit(1);
+    }
+    int pam_success = pam_auth(service, up);
+
+    if (write( ac_fd, pam_success ? "1" : "0", 1 ) != 1)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "cannot write to '%s'",
+                   ac_file_name );
+    }
+    close(ac_fd);
+    plugin_log(PLOG_NOTE, MODULE, "BACKGROUND: %s: deferred auth: PAM %s",
+               up->username, pam_success ? "succeeded" : "rejected" );
+    exit(0);
+}
+
+/*
  * Background process -- runs with privilege.
  */
 static void
 pam_server(int fd, const char *service, int verb, const struct name_value_list *name_value_list)
 {
     struct user_pass up;
+    char ac_file_name[PATH_MAX];
     int command;
 #ifdef USE_PAM_DLOPEN
     static const char pam_so[] = "libpam.so";
@@ -847,7 +950,8 @@ pam_server(int fd, const char *service, int verb, const struct name_value_list *
             case COMMAND_VERIFY:
                 if (recv_string(fd, up.username, sizeof(up.username)) == -1
                     || recv_string(fd, up.password, sizeof(up.password)) == -1
-                    || recv_string(fd, up.common_name, sizeof(up.common_name)) == -1)
+                    || recv_string(fd, up.common_name, sizeof(up.common_name)) == -1
+                    || recv_string(fd, ac_file_name, sizeof(ac_file_name)) == -1)
                 {
                     plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "BACKGROUND: read error on command channel: code=%d, exiting",
                             command);
@@ -867,6 +971,18 @@ pam_server(int fd, const char *service, int verb, const struct name_value_list *
                 /* If password is of the form SCRV1:base64:base64 split it up */
                 split_scrv1_password(&up);
 
+                /* client wants deferred auth
+                 */
+                if (strlen(ac_file_name) > 0)
+                {
+                    do_deferred_pam_auth(fd, ac_file_name, service, &up);
+                    break;
+                }
+
+
+                /* non-deferred auth: wait for pam result and send
+                 * result back via control socketpair
+                 */
                 if (pam_auth(service, &up)) /* Succeeded */
                 {
                     if (send_control(fd, RESPONSE_VERIFY_SUCCEEDED) == -1)