SSSD: Load a user to run a service as from configuration

Related:
    https://fedorahosted.org/sssd/ticket/2370

Adds a option, user to run as, that is specified in the [sssd] section. When
this option is specified, SSSD will run as this user and his private
group. When these are not specified, SSSD will run as the configure-time
user and group (usually root).

Currently all services and providers are started as root. There is a
temporary svc_supported_as_nonroot() function that returns true for a
service if that service runs and was tested as nonroot and false
otherwise. Currently this function always returns false, but will be
amended in future patches.

Reviewed-by: Pavel Reichl <preichl@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>

6 files changed, 73 insertions, 0 deletions

diff --git a/src/confdb/confdb.h b/src/confdb/confdb.h
index 34d461065..159aa9f2c 100644
--- a/src/confdb/confdb.h
+++ b/src/confdb/confdb.h
@@ -69,6 +69,7 @@
 #define CONFDB_MONITOR_KRB5_RCACHEDIR "krb5_rcache_dir"
 #define CONFDB_MONITOR_DEFAULT_DOMAIN "default_domain_suffix"
 #define CONFDB_MONITOR_OVERRIDE_SPACE "override_space"
+#define CONFDB_MONITOR_USER_RUNAS "user"
 
 /* Both monitor and domains */
 #define CONFDB_NAME_REGEX   "re_expression"
diff --git a/src/config/SSSDConfig/__init__.py.in b/src/config/SSSDConfig/__init__.py.in
index 6c9553086..b4560ea2b 100644
--- a/src/config/SSSDConfig/__init__.py.in
+++ b/src/config/SSSDConfig/__init__.py.in
@@ -56,6 +56,7 @@ option_strings = {
     'full_name_format' : _('Printf-compatible format for displaying fully-qualified names'),
     'krb5_rcache_dir' : _('Directory on the filesystem where SSSD should store Kerberos replay cache files.'),
     'default_domain_suffix' : _('Domain to add to names without a domain component.'),
+    'user' : _('The user to drop privileges to'),
 
     # [nss]
     'enum_cache_timeout' : _('Enumeration cache timeout length (seconds)'),
diff --git a/src/config/SSSDConfigTest.py b/src/config/SSSDConfigTest.py
index 2d12bc02a..78e22f6ef 100755
--- a/src/config/SSSDConfigTest.py
+++ b/src/config/SSSDConfigTest.py
@@ -280,6 +280,7 @@ class SSSDConfigTestSSSDService(unittest.TestCase):
             're_expression',
             'full_name_format',
             'krb5_rcache_dir',
+            'user',
             'default_domain_suffix',
             'debug_level',
             'debug_timestamps',
diff --git a/src/config/etc/sssd.api.conf b/src/config/etc/sssd.api.conf
index a20f5aa44..c16769a39 100644
--- a/src/config/etc/sssd.api.conf
+++ b/src/config/etc/sssd.api.conf
@@ -23,6 +23,7 @@ sbus_timeout = int, None, false
 re_expression = str, None, false
 full_name_format = str, None, false
 krb5_rcache_dir = str, None, false
+user = str, None, false
 default_domain_suffix = str, None, false
 
 [nss]
diff --git a/src/man/sssd.conf.5.xml b/src/man/sssd.conf.5.xml
index d57341661..d6bc42ca0 100644
--- a/src/man/sssd.conf.5.xml
+++ b/src/man/sssd.conf.5.xml
@@ -297,6 +297,19 @@
                         </listitem>
                     </varlistentry>
                     <varlistentry>
+                        <term>user (string)</term>
+                        <listitem>
+                            <para>
+                                The user to drop the privileges to where
+                                appropriate to avoid running as the
+                                root user.
+                            </para>
+                            <para>
+                                Default: not set, process will run as root
+                            </para>
+                        </listitem>
+                    </varlistentry>
+                    <varlistentry>
                         <term>default_domain_suffix (string)</term>
                         <listitem>
                             <para>
diff --git a/src/monitor/monitor.c b/src/monitor/monitor.c
index edd1c2dfc..df1cd5ca1 100644
--- a/src/monitor/monitor.c
+++ b/src/monitor/monitor.c
@@ -170,6 +170,10 @@ struct mt_ctx {
     struct sss_sigchild_ctx *sigchld_ctx;
     bool is_daemon;
     pid_t parent_pid;
+
+    /* For running unprivileged services */
+    uid_t uid;
+    gid_t gid;
 };
 
 static int start_service(struct mt_svc *mt_svc);
@@ -910,6 +914,29 @@ static char *check_services(char **services)
     return NULL;
 }
 
+static int get_service_user(struct mt_ctx *ctx)
+{
+    errno_t ret;
+    char *user_str;
+
+    ret = confdb_get_string(ctx->cdb, ctx, CONFDB_MONITOR_CONF_ENTRY,
+                            CONFDB_MONITOR_USER_RUNAS,
+                            SSSD_USER, &user_str);
+    if (ret != EOK) {
+        DEBUG(SSSDBG_FATAL_FAILURE, "Failed to get the user to run as\n");
+        return ret;
+    }
+
+    ret = sss_user_by_name_or_uid(user_str, &ctx->uid, &ctx->gid);
+    talloc_free(user_str);
+    if (ret != EOK) {
+        DEBUG(SSSDBG_FATAL_FAILURE, "Failed to set allowed UIDs.\n");
+        return ret;
+    }
+
+    return EOK;
+}
+
 static int get_monitor_config(struct mt_ctx *ctx)
 {
     int ret;
@@ -955,6 +982,12 @@ static int get_monitor_config(struct mt_ctx *ctx)
         ctx->num_services++;
     }
 
+    ret = get_service_user(ctx);
+    if (ret != EOK) {
+        DEBUG(SSSDBG_CRIT_FAILURE, "Failed to get the unprivileged user\n");
+        return ret;
+    }
+
     ret = confdb_get_domains(ctx->cdb, &ctx->domains);
     if (ret != EOK) {
         DEBUG(SSSDBG_FATAL_FAILURE, "No domains configured.\n");
@@ -1020,6 +1053,14 @@ static errno_t get_ping_config(struct mt_ctx *ctx, const char *path,
     return EOK;
 }
 
+/* This is a temporary function that returns false if the service
+ * being started was only tested when running as root.
+ */
+static bool svc_supported_as_nonroot(const char *svc_name)
+{
+    return false;
+}
+
 static int get_service_config(struct mt_ctx *ctx, const char *name,
                               struct mt_svc **svc_cfg)
 {
@@ -1027,6 +1068,8 @@ static int get_service_config(struct mt_ctx *ctx, const char *name,
     char *path;
     struct mt_svc *svc;
     time_t now = time(NULL);
+    uid_t uid = 0;
+    gid_t gid = 0;
 
     *svc_cfg = NULL;
 
@@ -1066,6 +1109,11 @@ static int get_service_config(struct mt_ctx *ctx, const char *name,
         return ret;
     }
 
+    if (svc_supported_as_nonroot(svc->name)) {
+        uid = ctx->uid;
+        gid = ctx->gid;
+    }
+
     if (!svc->command) {
         svc->command = talloc_asprintf(
             svc, "%s/sssd_%s", SSSD_LIBEXEC_PATH, svc->name
@@ -1075,6 +1123,14 @@ static int get_service_config(struct mt_ctx *ctx, const char *name,
             return ENOMEM;
         }
 
+        svc->command = talloc_asprintf_append(svc->command,
+                " --uid %"SPRIuid" --gid %"SPRIgid,
+                uid, gid);
+        if (!svc->command) {
+            talloc_free(svc);
+            return ENOMEM;
+        }
+
         if (cmdline_debug_level != SSSDBG_UNRESOLVED) {
             svc->command = talloc_asprintf_append(
                 svc->command, " -d %#.4x", cmdline_debug_level