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This Wiki page describes sandbox usage and requirements for CEF.

Note to Editors: Changes made to this Wiki page without prior approval via the CEF Forum or Issue Tracker may be lost or reverted.

Overview

The Chromium/CEF multi-process model uses separate processes for the renderer, network stack, GPU, etc. This model allows Chromium to restrict access to system resources based on the sub-process type. For example, a renderer sub-process that executes JavaScript code does not need direct access to the file system or network stack. Allowing unnecessary access can be a security concern if a sub-process is compromised while executing content (e.g. JavaScript code) from a hostile website.

The Chromium sandbox is used to apply per-process access restrictions based on the process type. With the sandbox enabled a compromised sub-process will often be terminated before it can do any harm. CEF supports the Chromium sandbox on each platform as described below.

Usage

Sandbox capabilities are OS-specific and different requirements apply to each platform.

Windows

To enable the sandbox on Windows the following requirements must be met:

  1. Use the same executable for the browser process and all sub-processes. The CefSettings.browser_subprocess_path setting cannot be used in combination with the sandbox.
  2. Link the executable with the cef_sandbox static library.
  3. Call the cef_sandbox_info_create() function from within the executable (not from a separate DLL) and pass the resulting pointer into both theCefExecutProcess() and CefInitialize() functions via the windows_sandbox_info parameter.

See cef_sandbox_win.h for additional details. See cefsimple_win.cc for an example implementation.

macOS

To enable the sandbox on macOS the following requirements must be met:

  1. Link the helper process executable with the cef_sandbox static library.
  2. Call the cef_sandbox_initialize() function at the beginning of the helper executable main() function and before loading the CEF framework library.

See cef_sandbox_mac.h and cef_library_loader.h for additional details. See cefsimple_mac.mm and process_helper_mac.cc for an example implementation.

The V2 sandbox was enabled starting with 3538 branch. See this announcement for a description of the related changes.

Linux

Different approaches are used based on system capabilities. See this document for details.

Building cef_sandbox (Windows/macOS)

Linking the cef_sandbox static library is required, as described above, for using the sandbox on Windows and macOS. This library must be built with special configuration settings in order to link successfully with third-party applications. For example, Chromium's custom bundled libc++ version must be disabled to avoid linker conflicts when using the default platform runtime library.

The cef_sandbox library build is usually performed by the automate_git.py tool as part of the official binary distribution build (see the AutomatedBuildSetup Wiki page). The special sandbox-specific configuration comes from the GetConfigArgsSandbox function in the gn_args.py tool and the sandbox build is performed by automate_git.py in a separate out\[Config]_sandbox output directory. A final post-processing action by the make_distrib.py tool combines multiple lib files into a single cef_sandbox.lib (Windows) or cef_sandbox.a (macOS) file which is included with the binary distribution.

The cef_sandbox library can also be built locally for development and testing purposes as described below. This example creates a build that is compatible with the official 32-bit Windows distribution available for download from Spotify. To perform this build you must first set up a local Chromium/CEF checkout as described on the MasterBuildQuickStart Wiki page and check out the correct branch/version by passing the appropriate --branch=XXXX or --checkout=HHHHHHHH command-line flags to automate_git.py. You can then build the cef_sandbox library as follows:

# Discover what `args.gn` contents are required:
cd c:\path\to\chromium\src\cef
set GN_DEFINES=is_official_build=true
python3 tools\gn_args.py

# Create the `chromium\src\out\Debug_GN_x64_sandbox\args.gn` file with the specified contents.

# Build the cef_sandbox target in the new output directory:
cd c:\path\to\chromium\src
set DEPOT_TOOLS_WIN_TOOLCHAIN=0
gn gen out\Debug_GN_x64_sandbox
ninja -C out\Debug_GN_x64_sandbox cef_sandbox

# Create a binary distribution for the cef_sandbox build:
cd c:\path\to\chromium\src\cef\tools
make_distrib.bat --allow-partial --sandbox --ninja-build --x64-build --no-archive --no-symbols --no-docs

The resulting chromium\src\cef\binary_distrib\cef_binary_*_windows64_sandbox\Debug\cef_sandbox.lib file can then be tested in combination with an existing official binary distribution at the same version.

If a different platform or configuration is desired you can find the appropriate GN_DEFINES on the AutomatedBuildSetup Wiki page.

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