KRB5: Drop privileges in the child, not the back end

In future patches, sssd_be will be running as a non-privileged user, who
will execute the setuid krb5_child. In this case, the child will start
as root and drop the privileges as soon as possible.

However, we need to also remove the privilege drop in sssd_be, because
if we dropped to the user who is authenticating, we wouldn't be even
allowed to execute krb5_child. The krb5_child permissions should be
4750, owned by root.sssd, to make sure only root and sssd can execute
the child and if executed by sssd, the child will run as root.

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

Reviewed-by: Sumit Bose <[email protected]>
Reviewed-by: Lukáš Slebodník <[email protected]>

src/providers/krb5/krb5_child.c

@@ -1840,11 +1840,60 @@ static int k5c_setup_fast(struct krb5_req *kr, bool demand)
     return EOK;
 }
 
+enum k5c_fast_opt {
+    K5C_FAST_NEVER,
+    K5C_FAST_TRY,
+    K5C_FAST_DEMAND,
+};
+
+static errno_t check_use_fast(enum k5c_fast_opt *_fast_val)
+{
+    char *use_fast_str;
+    enum k5c_fast_opt fast_val;
+
+    use_fast_str = getenv(SSSD_KRB5_USE_FAST);
+    if (use_fast_str == NULL || strcasecmp(use_fast_str, "never") == 0) {
+        DEBUG(SSSDBG_CONF_SETTINGS, "Not using FAST.\n");
+        fast_val = K5C_FAST_NEVER;
+    } else if (strcasecmp(use_fast_str, "try") == 0) {
+        fast_val = K5C_FAST_TRY;
+    } else if (strcasecmp(use_fast_str, "demand") == 0) {
+        fast_val = K5C_FAST_DEMAND;
+    } else {
+        DEBUG(SSSDBG_CRIT_FAILURE,
+                "Unsupported value [%s] for krb5_use_fast.\n",
+                use_fast_str);
+        return EINVAL;
+    }
+
+    *_fast_val = fast_val;
+    return EOK;
+}
+
 static int k5c_setup(struct krb5_req *kr, uint32_t offline)
 {
     krb5_error_code kerr;
-    char *use_fast_str;
     int parse_flags;
+    enum k5c_fast_opt fast_val;
+
+    kerr = check_use_fast(&fast_val);
+    if (kerr != EOK) {
+        return kerr;
+    }
+
+    if (offline || (fast_val == K5C_FAST_NEVER && kr->validate == false)) {
+        /* If krb5_child was started as setuid, but we don't need to
+         * perform either validation or FAST, just drop privileges to
+         * the user who is logging in. The same applies to the offline case
+         */
+        kerr = become_user(kr->uid, kr->gid);
+        if (kerr != 0) {
+            DEBUG(SSSDBG_CRIT_FAILURE, "become_user failed.\n");
+            return kerr;
+        }
+    }
+    DEBUG(SSSDBG_TRACE_INTERNAL,
+          "Running as [%"SPRIuid"][%"SPRIgid"].\n", geteuid(), getegid());
 
     kr->realm = getenv(SSSD_KRB5_REALM);
     if (kr->realm == NULL) {
@@ -1931,18 +1980,12 @@ static int k5c_setup(struct krb5_req *kr, uint32_t offline)
     if (!offline) {
         set_canonicalize_option(kr->options);
 
-        use_fast_str = getenv(SSSD_KRB5_USE_FAST);
-        if (use_fast_str == NULL || strcasecmp(use_fast_str, "never") == 0) {
-            DEBUG(SSSDBG_CONF_SETTINGS, "Not using FAST.\n");
-        } else if (strcasecmp(use_fast_str, "try") == 0) {
-            kerr = k5c_setup_fast(kr, false);
-        } else if (strcasecmp(use_fast_str, "demand") == 0) {
-            kerr = k5c_setup_fast(kr, true);
-        } else {
-            DEBUG(SSSDBG_CRIT_FAILURE,
-                  "Unsupported value [%s] for krb5_use_fast.\n",
-                   use_fast_str);
-            return EINVAL;
+        if (fast_val != K5C_FAST_NEVER) {
+            kerr = k5c_setup_fast(kr, fast_val == K5C_FAST_DEMAND);
+            if (kerr != EOK) {
+                DEBUG(SSSDBG_OP_FAILURE, "Cannot set up FAST\n");
+                return kerr;
+            }
         }
     }
 

src/providers/krb5/krb5_child_handler.c

@@ -284,14 +284,6 @@ static errno_t fork_child(struct tevent_req *req)
     pid = fork();
 
     if (pid == 0) { /* child */
-        if (state->kr->run_as_user) {
-            ret = become_user(state->kr->uid, state->kr->gid);
-            if (ret != EOK) {
-                DEBUG(SSSDBG_CRIT_FAILURE, "become_user failed.\n");
-                return ret;
-            }
-        }
-
         err = exec_child(state,
                          pipefd_to_child, pipefd_from_child,
                          KRB5_CHILD, state->kr->krb5_ctx->child_debug_fd);