core: check suid/sgid dynamically instead of checking file

nix/nixos-configurations/vm-suid.nix

@@ -90,7 +90,7 @@ nixpkgs.lib.nixosSystem {
         defaultEditor = true;
       };
 
-      environment.systemPackages = with pkgs; [ jq ];
+      environment.systemPackages = with pkgs; [ jq pkgs.muscl ];
     })
   ];
 }

src/core/bootstrap.rs

@@ -10,7 +10,7 @@ use tracing_subscriber::prelude::*;
 
 use crate::{
     core::common::{
-        DEFAULT_CONFIG_PATH, DEFAULT_SOCKET_PATH, UnixUser, executable_is_suid_or_sgid,
+        DEFAULT_CONFIG_PATH, DEFAULT_SOCKET_PATH, UnixUser, executing_in_suid_sgid_mode,
     },
     server::{
         config::{MysqlConfig, ServerConfig},
@@ -81,7 +81,7 @@ pub fn bootstrap_server_connection_and_drop_privileges(
 ) -> anyhow::Result<StdUnixStream> {
     if will_connect_to_external_server(server_socket_path.as_ref(), config.as_ref())? {
         assert!(
-            !executable_is_suid_or_sgid()?,
+            !executing_in_suid_sgid_mode()?,
             "The executable should not be SUID or SGID when connecting to an external server"
         );
 
@@ -178,7 +178,7 @@ fn bootstrap_internal_server_and_drop_privs(
     config_path: Option<PathBuf>,
 ) -> anyhow::Result<StdUnixStream> {
     if let Some(config_path) = config_path {
-        if !executable_is_suid_or_sgid()? {
+        if !executing_in_suid_sgid_mode()? {
             anyhow::bail!("Executable is not SUID/SGID - refusing to start internal sever");
         }
 
@@ -195,7 +195,7 @@ fn bootstrap_internal_server_and_drop_privs(
 
     let config_path = PathBuf::from(DEFAULT_CONFIG_PATH);
     if fs::metadata(&config_path).is_ok() {
-        if !executable_is_suid_or_sgid()? {
+        if !executing_in_suid_sgid_mode()? {
             anyhow::bail!("Executable is not SUID/SGID - refusing to start internal sever");
         }
         tracing::debug!("Starting server with default config at {:?}", config_path);

src/core/common.rs

@@ -67,28 +67,20 @@ fn get_unix_groups(user: &LibcUser) -> anyhow::Result<Vec<LibcGroup>> {
     Ok(groups)
 }
 
-/// Check if the current executable is SUID or SGID.
-///
-/// If the check fails, an error is returned.
+/// Check if the current executable is running in SUID/SGID mode
 #[cfg(feature = "suid-sgid-mode")]
-pub fn executable_is_suid_or_sgid() -> anyhow::Result<bool> {
-    use std::{fs, os::unix::fs::PermissionsExt};
-    let result = std::env::current_exe()
-        .context("Failed to get current executable path")
-        .and_then(|executable| {
-            fs::metadata(executable).context("Failed to get executable metadata")
-        })
-        .context("Failed to check SUID/SGID bits on executable")
-        .map(|metadata| {
-            let mode = metadata.permissions().mode();
-            mode & 0o4000 != 0 || mode & 0o2000 != 0
-        })?;
-    Ok(result)
+pub fn executing_in_suid_sgid_mode() -> anyhow::Result<bool> {
+    let euid = nix::unistd::geteuid();
+    let uid = nix::unistd::getuid();
+    let egid = nix::unistd::getegid();
+    let gid = nix::unistd::getgid();
+
+    Ok(euid != uid || egid != gid)
 }
 
 #[cfg(not(feature = "suid-sgid-mode"))]
 #[inline]
-pub fn executable_is_suid_or_sgid() -> anyhow::Result<bool> {
+pub fn executing_in_suid_sgid_mode() -> anyhow::Result<bool> {
     Ok(false)
 }
 

src/main.rs

@@ -16,7 +16,7 @@ use futures_util::StreamExt;
 use crate::{
     core::{
         bootstrap::bootstrap_server_connection_and_drop_privileges,
-        common::{ASCII_BANNER, KIND_REGARDS, executable_is_suid_or_sgid},
+        common::{ASCII_BANNER, KIND_REGARDS, executing_in_suid_sgid_mode},
         protocol::{Response, create_client_to_server_message_stream},
     },
     server::{command::ServerArgs, landlock::landlock_restrict_server},
@@ -151,7 +151,7 @@ fn main() -> anyhow::Result<()> {
 fn handle_dynamic_completion() -> anyhow::Result<Option<()>> {
     if std::env::var_os("COMPLETE").is_some() {
         #[cfg(feature = "suid-sgid-mode")]
-        if executable_is_suid_or_sgid()? {
+        if executing_in_suid_sgid_mode()? {
             use crate::core::bootstrap::drop_privs;
             drop_privs()?
         }
@@ -202,7 +202,7 @@ fn handle_server_command(args: &Args) -> anyhow::Result<Option<()>> {
     match args.command {
         Command::Server(ref command) => {
             assert!(
-                !executable_is_suid_or_sgid()?,
+                !executing_in_suid_sgid_mode()?,
                 "The executable should not be SUID or SGID when running the server manually"
             );