tauri_runtime_cef/

environment.rs

// Copyright 2019-2024 Tauri Programme within The Commons Conservancy
// SPDX-License-Identifier: Apache-2.0
// SPDX-License-Identifier: MIT

//! The diagnostic environment variables Chromium and CEF read behind the application's
//! back, and the policy that decides whether they are honoured.
//!
//! Chromium is a browser, and a browser is expected to let whoever runs it capture its
//! own traffic and redirect its own crash reports. An application that embeds Chromium
//! inherits those hooks without asking for them, and in a shipped application they are
//! not developer conveniences any more: anyone who can set a variable in the
//! application's environment can decrypt every TLS session it makes, or point its
//! minidumps — which carry process memory — at a server of their choosing.
//!
//! [`DebugEnvironment`] is the switch. Neither variable group is reachable through a CEF
//! setting, so each is answered where Chromium reads it.
//!
//! # TLS key logging is answered on the command line
//!
//! `content/browser/network_service_instance_impl.cc` consults `SSLKEYLOGFILE` only when
//! `--ssl-key-log-file` is absent, and an empty switch value logs a warning and creates no
//! key logger. Appending the empty switch is therefore a complete answer that needs no
//! change to the process environment, and the runtime appends it only when the variable is
//! actually set — so the warning appears exactly when somebody was trying to log keys.
//!
//! # The crash reporter overrides are answered in the environment
//!
//! CEF reads its three crash variables from `BasicStartupComplete`, before any hook the
//! embedder can install, so there is nothing to append and the variables have to go. That
//! write is [`std::env::remove_var`], which is why it happens once, during runtime
//! initialization, and only for a variable that is actually set.

/// Whether Chromium and CEF may read their diagnostic environment variables.
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
#[non_exhaustive]
pub enum DebugEnvironment {
  /// Honour them in development builds (`tauri::is_dev()`), refuse them in release
  /// builds.
  #[default]
  Auto,
  /// Always honour them.
  ///
  /// Appropriate for a build whose users are expected to debug it — an internal tool, a
  /// QA build — and wrong for one handed to the public.
  Allow,
  /// Always refuse them, in every build profile.
  Deny,
}

/// The environment variable whose answer is the `--ssl-key-log-file` switch.
///
/// The network service writes the pre-master secret of every TLS session to the file it
/// names, which is exactly what a packet capture needs to decrypt the application's
/// traffic in full.
const SSL_KEY_LOG_FILE: &str = "SSLKEYLOGFILE";

/// CEF's crash reporter overrides, and what each one does.
///
/// * `CEF_CRASH_REPORTER_SERVER_URL` — replaces the crash report upload URL from
///   `crash_reporter.cfg`. A minidump carries stack and heap memory, so a redirected URL
///   is a memory exfiltration channel.
/// * `CEF_CRASH_REPORTER_RATE_LIMIT_ENABLED` — lifts the upload rate limit that keeps a
///   crash loop from becoming a flood.
/// * `BREAKPAD_DUMP_LOCATION` — redirects where minidumps are written on Windows.
///
/// All three are only consulted when the application ships a `crash_reporter.cfg`, so
/// removing them costs nothing when it does not.
const CRASH_REPORTER_VARIABLES: &[(&str, &str)] = &[
  (
    "CEF_CRASH_REPORTER_SERVER_URL",
    "redirecting crash report uploads, which carry process memory",
  ),
  (
    "CEF_CRASH_REPORTER_RATE_LIMIT_ENABLED",
    "overriding the crash report upload rate limit",
  ),
  (
    "BREAKPAD_DUMP_LOCATION",
    "redirecting where minidumps are written",
  ),
];

/// Whether `policy` refuses the variables in a build where `is_dev` says what it says.
///
/// Kept separate from acting on the answer so the decision can be unit tested without
/// touching the process environment.
fn refuses_debug_variables(policy: DebugEnvironment, is_dev: bool) -> bool {
  match policy {
    DebugEnvironment::Auto => !is_dev,
    DebugEnvironment::Allow => false,
    DebugEnvironment::Deny => true,
  }
}

/// Whether a variable is set to something Chromium would act on.
fn is_set(name: &str) -> bool {
  std::env::var_os(name).is_some_and(|value| !value.is_empty())
}

/// Whether the runtime should append an empty `--ssl-key-log-file`, which is how
/// `SSLKEYLOGFILE` is refused.
///
/// Answers `false` when the variable is not set, so the switch — and the warning Chromium
/// logs for it — only appears when something was actually asking for a key log.
pub(crate) fn neutralizes_tls_key_log(policy: DebugEnvironment, is_dev: bool) -> bool {
  if !refuses_debug_variables(policy, is_dev) || !is_set(SSL_KEY_LOG_FILE) {
    return false;
  }

  log::warn!(
    "ignoring the {SSL_KEY_LOG_FILE} environment variable: it asks Chromium to log the TLS \
     session keys that decrypt this application's network traffic. Set \
     DebugEnvironment::Allow to honour it."
  );
  true
}

/// Removes CEF's crash reporter overrides from the process environment when `policy`
/// refuses them.
///
/// Must run before the first CEF call: CEF reads these from
/// `ChromeMainDelegateCef::BasicStartupComplete`, which `cef::initialize` reaches, and
/// child processes inherit the environment of the browser process that spawned them, so a
/// variable removed here is gone from the whole process tree.
///
/// # Safety
///
/// Calls [`std::env::remove_var`], which is unsound while another thread reads or writes
/// the environment concurrently. The caller must be the runtime's own initialization,
/// which runs on the main thread before CEF exists and before this runtime starts a
/// thread of its own.
pub(crate) fn remove_crash_reporter_overrides(policy: DebugEnvironment, is_dev: bool) {
  if !refuses_debug_variables(policy, is_dev) {
    return;
  }

  for (name, effect) in CRASH_REPORTER_VARIABLES {
    if !is_set(name) {
      continue;
    }

    log::warn!(
      "ignoring the {name} environment variable: it asks CEF for {effect}. Set \
       DebugEnvironment::Allow to honour it."
    );
    // SAFETY: documented on this function; the runtime applies the policy from its own
    // initialization, before CEF exists and before it starts a thread of its own.
    unsafe { std::env::remove_var(name) };
  }
}

#[cfg(test)]
mod tests {
  use super::*;

  #[test]
  fn auto_follows_the_build_profile() {
    assert!(
      !refuses_debug_variables(DebugEnvironment::Auto, true),
      "a development build keeps its debugging hooks"
    );
    assert!(
      refuses_debug_variables(DebugEnvironment::Auto, false),
      "a shipped build must not hand its TLS keys to whoever sets a variable"
    );
  }

  #[test]
  fn explicit_policies_ignore_the_build_profile() {
    for is_dev in [false, true] {
      assert!(!refuses_debug_variables(DebugEnvironment::Allow, is_dev));
      assert!(refuses_debug_variables(DebugEnvironment::Deny, is_dev));
    }
  }

  #[test]
  fn a_permissive_policy_never_touches_the_command_line() {
    // Asserted without depending on the ambient environment: `Allow` short-circuits
    // before the variable is even read.
    assert!(!neutralizes_tls_key_log(DebugEnvironment::Allow, false));
    assert!(!neutralizes_tls_key_log(DebugEnvironment::Auto, true));
  }

  #[test]
  fn every_crash_variable_is_described() {
    // The name and the effect both go into a warning the user reads.
    for (name, effect) in CRASH_REPORTER_VARIABLES {
      assert!(!name.is_empty());
      assert!(!effect.is_empty());
    }
  }

  #[test]
  fn the_tls_variable_is_not_also_removed_from_the_environment() {
    // It is answered on the command line instead, which needs no environment write.
    assert!(
      !CRASH_REPORTER_VARIABLES
        .iter()
        .any(|(name, _)| *name == SSL_KEY_LOG_FILE)
    );
  }
}