Use SO_PEERCRED on the PAM socket

This is the second attempt to let the PAM client and the PAM responder
exchange their credentials, i.e. uid, gid and pid. Because this approach
does not require any message interchange between the client and the
server the protocol version number is not changed.

On the client side the connection is terminated it the responder is not
run by root. On the server side the effective uid and gid and the pid of
the client are available for future use.

The following additional changes are made by this patch:
- the checks of the ownership and the permissions on the PAM sockets are
  enhanced
- internal error codes are introduced on the client side to generate
  more specific log messages if an error occurs

6 files changed, 162 insertions, 3 deletions

diff --git a/src/external/platform.m4 b/src/external/platform.m4
index 71b4f2c85..ee0093789 100644
--- a/src/external/platform.m4
+++ b/src/external/platform.m4
@@ -27,3 +27,15 @@ fi
 AM_CONDITIONAL([HAVE_FEDORA], [test x"$osname" == xfedora])
 AM_CONDITIONAL([HAVE_REDHAT], [test x"$osname" == xredhat])
 AM_CONDITIONAL([HAVE_SUSE], [test x"$osname" == xsuse])
+
+AC_CHECK_MEMBERS([struct ucred.pid, struct ucred.uid, struct ucred.gid], , ,
+                 [[#define _GNU_SOURCE
+                   #include <sys/socket.h>]])
+
+if test x"$ac_cv_member_struct_ucred_pid" = xyes -a \
+        x"$ac_cv_member_struct_ucred_uid" = xyes -a \
+        x"$ac_cv_member_struct_ucred_gid" = xyes ; then
+    AC_DEFINE([HAVE_UCRED], [1], [Define if struct ucred is available])
+else
+    AC_MSG_WARN([struct ucred is not available])
+fi
diff --git a/src/responder/common/responder.h b/src/responder/common/responder.h
index ea6ba5831..deb1e5a3c 100644
--- a/src/responder/common/responder.h
+++ b/src/responder/common/responder.h
@@ -101,6 +101,9 @@ struct cli_ctx {
     struct cli_request *creq;
     struct cli_protocol_version *cli_protocol_version;
     int priv;
+    int32_t client_euid;
+    int32_t client_egid;
+    int32_t client_pid;
 };
 
 struct sss_cmd_table {
diff --git a/src/responder/common/responder_common.c b/src/responder/common/responder_common.c
index ff27f62cf..54d6c49e1 100644
--- a/src/responder/common/responder_common.c
+++ b/src/responder/common/responder_common.c
@@ -19,6 +19,9 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+/* for struct ucred */
+#define _GNU_SOURCE
+
 #include <stdio.h>
 #include <unistd.h>
 #include <fcntl.h>
@@ -29,7 +32,8 @@
 #include <string.h>
 #include <sys/time.h>
 #include <errno.h>
-#include "popt.h"
+#include <popt.h>
+#include "config.h"
 #include "util/util.h"
 #include "db/sysdb.h"
 #include "confdb/confdb.h"
@@ -61,6 +65,40 @@ static int client_destructor(struct cli_ctx *ctx)
     return 0;
 }
 
+static errno_t get_client_cred(struct cli_ctx *cctx)
+{
+#ifdef HAVE_UCRED
+    int ret;
+    struct ucred client_cred;
+    socklen_t client_cred_len = sizeof(client_cred);
+
+    cctx->client_euid = -1;
+    cctx->client_egid = -1;
+    cctx->client_pid = -1;
+
+    ret = getsockopt(cctx->cfd, SOL_SOCKET, SO_PEERCRED, &client_cred,
+                     &client_cred_len);
+    if (ret != EOK) {
+        ret = errno;
+        DEBUG(1, ("getsock failed [%d][%s].\n", ret, strerror(ret)));
+        return ret;
+    }
+    if (client_cred_len != sizeof(struct ucred)) {
+        DEBUG(1, ("getsockopt returned unexpected message size.\n"));
+        return ENOMSG;
+    }
+
+    cctx->client_euid = client_cred.uid;
+    cctx->client_egid = client_cred.gid;
+    cctx->client_pid = client_cred.pid;
+
+    DEBUG(9, ("Client creds: euid[%d] egid[%d] pid[%d].\n",
+              cctx->client_euid, cctx->client_egid, cctx->client_pid));
+#endif
+
+    return EOK;
+}
+
 static void client_send(struct cli_ctx *cctx)
 {
     int ret;
@@ -214,6 +252,12 @@ static void accept_priv_fd_handler(struct tevent_context *ev,
 
     cctx->priv = 1;
 
+    ret = get_client_cred(cctx);
+    if (ret != EOK) {
+        DEBUG(2, ("get_client_cred failed, "
+                  "client cred may not be available.\n"));
+    }
+
     cctx->cfde = tevent_add_fd(ev, cctx, cctx->cfd,
                                TEVENT_FD_READ, client_fd_handler, cctx);
     if (!cctx->cfde) {
@@ -240,6 +284,7 @@ static void accept_fd_handler(struct tevent_context *ev,
     struct resp_ctx *rctx = talloc_get_type(ptr, struct resp_ctx);
     struct cli_ctx *cctx;
     socklen_t len;
+    int ret;
 
     cctx = talloc_zero(rctx, struct cli_ctx);
     if (!cctx) {
@@ -265,6 +310,12 @@ static void accept_fd_handler(struct tevent_context *ev,
         return;
     }
 
+    ret = get_client_cred(cctx);
+    if (ret != EOK) {
+        DEBUG(2, ("get_client_cred failed, "
+                  "client cred may not be available.\n"));
+    }
+
     cctx->cfde = tevent_add_fd(ev, cctx, cctx->cfd,
                                TEVENT_FD_READ, client_fd_handler, cctx);
     if (!cctx->cfde) {
diff --git a/src/sss_client/common.c b/src/sss_client/common.c
index 6732c24fc..237b90aba 100644
--- a/src/sss_client/common.c
+++ b/src/sss_client/common.c
@@ -23,6 +23,9 @@
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+/* for struct ucred */
+#define _GNU_SOURCE
+
 #include <nss.h>
 #include <security/pam_modules.h>
 #include <errno.h>
@@ -36,6 +39,10 @@
 #include <string.h>
 #include <fcntl.h>
 #include <poll.h>
+
+#include <libintl.h>
+#define _(STRING) dgettext (PACKAGE, STRING)
+#include "config.h"
 #include "sss_cli.h"
 
 /* common functions */
@@ -632,6 +639,29 @@ enum nss_status sss_nss_make_request(enum sss_cli_command cmd,
     return sss_nss_make_request_nochecks(cmd, rd, repbuf, replen, errnop);
 }
 
+errno_t check_server_cred(int sockfd)
+{
+#ifdef HAVE_UCRED
+    int ret;
+    struct ucred server_cred;
+    socklen_t server_cred_len = sizeof(server_cred);
+
+    ret = getsockopt(sockfd, SOL_SOCKET, SO_PEERCRED, &server_cred,
+                     &server_cred_len);
+    if (ret != 0) {
+        return errno;
+    }
+
+    if (server_cred_len != sizeof(struct ucred)) {
+        return ESSS_BAD_CRED_MSG;
+    }
+
+    if (server_cred.uid != 0 || server_cred.gid != 0) {
+        return ESSS_SERVER_NOT_TRUSTED;
+    }
+#endif
+    return 0;
+}
 int sss_pam_make_request(enum sss_cli_command cmd,
                       struct sss_cli_req_data *rd,
                       uint8_t **repbuf, size_t *replen,
@@ -653,17 +683,66 @@ int sss_pam_make_request(enum sss_cli_command cmd,
         if (ret != 0) return PAM_SERVICE_ERR;
         if ( ! (stat_buf.st_uid == 0 &&
                 stat_buf.st_gid == 0 &&
-                (stat_buf.st_mode&(S_IFSOCK|S_IRUSR|S_IWUSR)) == stat_buf.st_mode)) {
+                S_ISSOCK(stat_buf.st_mode) &&
+                (stat_buf.st_mode & ~S_IFMT) == 0600 )) {
+            *errnop = ESSS_BAD_PRIV_SOCKET;
             return PAM_SERVICE_ERR;
         }
 
         ret = sss_cli_check_socket(errnop, SSS_PAM_PRIV_SOCKET_NAME);
     } else {
+        ret = stat(SSS_PAM_SOCKET_NAME, &stat_buf);
+        if (ret != 0) return PAM_SERVICE_ERR;
+        if ( ! (stat_buf.st_uid == 0 &&
+                stat_buf.st_gid == 0 &&
+                S_ISSOCK(stat_buf.st_mode) &&
+                (stat_buf.st_mode & ~S_IFMT) == 0666 )) {
+            *errnop = ESSS_BAD_PUB_SOCKET;
+            return PAM_SERVICE_ERR;
+        }
+
         ret = sss_cli_check_socket(errnop, SSS_PAM_SOCKET_NAME);
     }
     if (ret != NSS_STATUS_SUCCESS) {
         return PAM_SERVICE_ERR;
     }
 
+    ret = check_server_cred(sss_cli_sd);
+    if (ret != 0) {
+        sss_cli_close_socket();
+        *errnop = ret;
+        return PAM_SERVICE_ERR;
+    }
+
     return sss_nss_make_request_nochecks(cmd, rd, repbuf, replen, errnop);
 }
+
+
+const char *ssscli_err2string(int err)
+{
+    const char *m;
+
+    switch(err) {
+        case ESSS_BAD_PRIV_SOCKET:
+            return _("Privileged socket has wrong ownership or permissions.");
+            break;
+        case ESSS_BAD_PUB_SOCKET:
+            return _("Public socket has wrong ownership or permissions.");
+            break;
+        case ESSS_BAD_CRED_MSG:
+            return _("Unexpected format of the server credential message.");
+            break;
+        case ESSS_SERVER_NOT_TRUSTED:
+            return _("SSSD is not run by root.");
+            break;
+        default:
+            m = strerror(err);
+            if (m == NULL) {
+                return _("An error occurred, but no description can be found.");
+            }
+            return m;
+            break;
+    }
+
+    return _("Unexpected error while looking for an error description");
+}
diff --git a/src/sss_client/pam_sss.c b/src/sss_client/pam_sss.c
index 6059dfaa1..4208faa62 100644
--- a/src/sss_client/pam_sss.c
+++ b/src/sss_client/pam_sss.c
@@ -877,10 +877,13 @@ static int send_and_receive(pam_handle_t *pamh, struct pam_items *pi,
     }
     rd.data = buf;
 
+    errnop = 0;
     ret = sss_pam_make_request(task, &rd, &repbuf, &replen, &errnop);
 
     if (ret != NSS_STATUS_SUCCESS) {
-        logger(pamh, LOG_ERR, "Request to sssd failed.");
+        if (errnop != 0) {
+            logger(pamh, LOG_ERR, "Request to sssd failed. %s", ssscli_err2string(errnop));
+        }
         pam_status = PAM_SYSTEM_ERR;
         goto done;
     }
diff --git a/src/sss_client/sss_cli.h b/src/sss_client/sss_cli.h
index f38726579..f7e58fe93 100644
--- a/src/sss_client/sss_cli.h
+++ b/src/sss_client/sss_cli.h
@@ -406,6 +406,17 @@ enum user_info_type {
  * @}
  */ /* end of group sss_pam_cli */
 
+enum sss_cli_error_codes {
+    ESSS_SSS_CLI_ERROR_START = 0x1000,
+    ESSS_BAD_PRIV_SOCKET,
+    ESSS_BAD_PUB_SOCKET,
+    ESSS_BAD_CRED_MSG,
+    ESSS_SERVER_NOT_TRUSTED,
+
+    ESS_SSS_CLI_ERROR_MAX
+};
+
+const char *ssscli_err2string(int err);
 
 enum nss_status sss_nss_make_request(enum sss_cli_command cmd,
                                      struct sss_cli_req_data *rd,