Security/Sandbox

Overview

Security Sandboxing makes use of child processes as a security boundary. The process model, i.e. how Firefox is split into various processes and how these processes interact between each other is common to all platforms. For more information see the Electrolysis wiki page. The security aspects of a sandboxed child process are implemented on a per-platform basis. See the Platform Specifics section below for more information.

Current Status

Sandbox	Trunk		Aurora		Beta		Release
	Level		Level	Version	Level	Version	Level	Version
Windows (content)	Level 2		Level 1	Fx50	Level 1	Fx50
Windows (GMP)	enabled		enabled		enabled		enabled
Windows 64bit (NPAPI Plugin)	enabled		enabled		enabled		enabled
OSX (content)	Level 2
OSX (GMP)	enabled		enabled		enabled		enabled
Linux (content)	Level 2
Linux (GMP)	enabled		enabled		enabled		enabled

DXR search for the current sandbox preferences.

Windows

Content

Sandbox Feature	Level 0	Level 1	Level 2	Level 10	Level 20
Job Level	JOB_NONE	JOB_NONE	JOB_INTERACTIVE	JOB_RESTRICTED	JOB_LOCKDOWN
Access Token Level	USER_NON_ADMIN	USER_NON_ADMIN	USER_INTERACTIVE	USER_LIMITED	USER_LOCKDOWN
Alternate Desktop				yes	yes
Initial Integrity Level	INTEGRITY_LEVEL_MEDIUM	INTEGRITY_LEVEL_LOW	INTEGRITY_LEVEL_LOW	INTEGRITY_LEVEL_LOW	INTEGRITY_LEVEL_LOW
Delayed Integrity Level	INTEGRITY_LEVEL_MEDIUM	INTEGRITY_LEVEL_LOW	INTEGRITY_LEVEL_LOW	INTEGRITY_LEVEL_LOW	INTEGRITY_LEVEL_UNTRUSTED
Mitigations		MITIGATION_BOTTOM_UP_ASLR MITIGATION_HEAP_TERMINATE MITIGATION_SEHOP MITIGATION_DEP_NO_ATL_THUNK MITIGATION_DEP	MITIGATION_BOTTOM_UP_ASLR MITIGATION_HEAP_TERMINATE MITIGATION_SEHOP MITIGATION_DEP_NO_ATL_THUNK MITIGATION_DEP	MITIGATION_BOTTOM_UP_ASLR MITIGATION_HEAP_TERMINATE MITIGATION_SEHOP MITIGATION_DEP_NO_ATL_THUNK MITIGATION_DEP	MITIGATION_BOTTOM_UP_ASLR MITIGATION_HEAP_TERMINATE MITIGATION_SEHOP MITIGATION_DEP_NO_ATL_THUNK MITIGATION_DEP
Delayed Mitigations		MITIGATION_STRICT_HANDLE_CHECKS MITIGATION_DLL_SEARCH_ORDER	MITIGATION_STRICT_HANDLE_CHECKS MITIGATION_DLL_SEARCH_ORDER	MITIGATION_STRICT_HANDLE_CHECKS MITIGATION_DLL_SEARCH_ORDER	MITIGATION_STRICT_HANDLE_CHECKS MITIGATION_DLL_SEARCH_ORDER

Windows Feature Header

Gecko Media Plugin

Sandbox Feature	Level
Job Level	JOB_LOCKDOWN
Access Token Level	USER_LOCKDOWN, USER_RESTRICTED[1]
Initial Integrity Level	INTEGRITY_LEVEL_LOW
Delayed Integrity Level	INTEGRITY_LEVEL_UNTRUSTED
Alternate desktop	yes
Mitigations	MITIGATION_BOTTOM_UP_ASLR MITIGATION_HEAP_TERMINATE MITIGATION_SEHOP MITIGATION_DEP_NO_ATL_THUNK MITIGATION_DEP
Delayed Mitigations	MITIGATION_STRICT_HANDLE_CHECKS MITIGATION_DLL_SEARCH_ORDER

[1] depends on the media plugin

64-bit Plugin

Released with known issues that break certain Flash features. If you're testing 64-bit builds and come across Flash issues please file a bug.

Sandbox Feature	Level
Job Level	JOB_UNPROTECTED
Access Token Level	USER_INTERACTIVE
Initial Integrity Level	INTEGRITY_LEVEL_LOW
Delayed Integrity Level	INTEGRITY_LEVEL_LOW
Alternate desktop	no
Mitigations	MITIGATION_BOTTOM_UP_ASLR MITIGATION_HEAP_TERMINATE MITIGATION_SEHOP MITIGATION_DEP_NO_ATL_THUNK MITIGATION_DEP
Delayed Mitigations

• Firefox/win64 Wiki Page
• Release Announcement
• Windows 64-bit builds

OSX

Content

Filter rules

Gecko Media Plugins

Filter rules

Linux

Content

Filter ruleset

Filesystem access policy

Gecko Media Plugin

Filter ruleset

Platform Specifics

Windows

Source Code Overview

The core of the Windows sandbox is Google's chromium sandbox. Relative to the root of mozilla-central, the sandbox exists at:

security/sandbox

The chromium sandbox is based on the chromium base libraries (Google's code) which are located at:

security/sandbox/chromium/base

There is also partial/shim code to get the base code compiling with our SDK build settings or to limit the base code by reducing dependencies at:

security/sandbox/chromium-shim/base

The chromium Windows sandbox itself (Google's code) is located at:

security/sandbox/chromium/sandbox/win

Processes Overview

There are 2 processes when dealing with a sandboxed application:

1. The broker: The parent process that starts sandboxed children
2. The target: The child process that is sandboxed

Both processes make use of the chromium sandbox library, but they make use of it indirectly through 2 libraries (Mozilla code). This indirect use of the library is due to header conflicts with the ipc layer where it has a different, much older, non compatible, copy of the chromium base library:

1. For the broker, ./security/sandbox/win/src/sandboxbroker
2. For the target, ./security/sandbox/win/src/sandboxtarget

Build settings

To disable building the sandbox completely build with this in your mozconfig:
ac_add_options --disable-sandbox

To disable just the content sandbox parts:
ac_add_options --disable-content-sandbox

Environment variables

Disable content process sandbox	MOZ_DISABLE_CONTENT_SANDBOX
Disable media plugin sandbox	MOZ_DISABLE_GMP_SANDBOX
Disable NPAPI process sandbox	MOZ_DISABLE_NPAPI_SANDBOX
Allow weaker than default sandbox level	MOZ_ALLOW_WEAKER_SANDBOX
Enable sandbox logging	MOZ_WIN_SANDBOX_LOGGING

Key source code locations

The sandboxed target process lowers its own privileges after initialization via these calls:
Content process
GMP process
NPAPI process

Level descriptions header:
http://mxr.mozilla.org/mozilla-central/source/security/sandbox/chromium/sandbox/win/src/security_level.h

The call that starts the sandboxed process in Firefox is:
https://dxr.mozilla.org/mozilla-central/rev/918df3a0bc1c/ipc/glue/GeckoChildProcessHost.cpp#1030

All of the code that sets policies can be found here:
http://dxr.mozilla.org/mozilla-central/source/security/sandbox/win/src/sandboxbroker/sandboxBroker.cpp

OSX

The OSX sandbox is based on the TrustedBSD MAC Framework. It is undocumented and considered private by Apple.

• Mozilla OSX Sandbox Ruleset wiki page
• OSX Sandbox
• OSX sandbox policy language

Linux

Seccomp stands for secure computing mode. It's a simple sandboxing tool in the Linux kernel, available since Linux version 2.6.12. When enabling seccomp, the process enters a "secure mode" where a very small number of system calls are available (exit(), read(), write(), sigreturn()). Writing code to work in this environment is difficult; for example, dynamic memory allocation (using brk() or mmap(), either directly or to implement malloc()) is not possible.

Seccomp-BPF is a more recent extension to seccomp, which allows filtering system calls with BPF (Berkeley Packet Filter) programs. Most of our Linux user base have systems that support seccomp-bpf.

These filters can be used to allow or deny an arbitrary set of system calls, as well as filter on system call arguments (numeric values only; pointer arguments can't be dereferenced). Additionally, instead of simply terminating the process, the filter can raise a signal, which allows the signal handler to simulate the effect of a disallowed system call (or simply gather more information on the failure for debugging purposes). Seccomp-bpf is available since Linux version 3.5 and is usable on the ARM architecture since Linux version 3.10. Several backports are available for earlier kernel versions.

For limitations that apply to the semantics of system calls (e.g., “can this process access the filesystem”, not “can this process use system call #83”) we require unprivileged user namespaces, which a large majority of desktop users don't support. Specifically: chroot()ing into a deleted directory to revoke FS access, and namespace unsharing for networking, SysV IPC if possible, and process IDs.

• seccomp-bpf filtering rules for various processes

Bug Lists

Milestones

• Windows Content Process
  • sbwc1
    • Roll out level 1 sandbox policy to release.
  • sbwc2
    • Roll out level 2 sandbox policy to release.
    • Do we have file system write access restrictions to add here?
    • file:/// isolation?
  • sbwc3
    • Roll out level 10 sandbox policy to release.
    • file system read access restrictions from content
• OSX Content Process
  • sbmc1
    • Roll out weak OSX security sandbox access ruleset
    • Prevent file system write access
  • sbmc2
    • Home directory read access
    • file:/// isolation?
  • sbmc3
    • TBD: Triage existing sandbox rules and define set to remove in milestone 3
    • File access: system /tmp and /var/folders/ and any other individual directories
• Linux Content Process
  • sblc1
    • enable (heavily perforated) seccomp-bpf filter by default in Nightly
  • sblc2
    • file system broker
    • remove/restrict file system write access
  • sblc3
    • remove/restrict file system read access
    • file:/// isolation?
  • sblc4
    • remove/restrict socket access/modification and solve X11 problem
  • sblc5
    • make use of chroot and user namespaces
• Windows 64-bit NPAPI
  • sbwn1

Triage Lists

• Triage list: http://is.gd/Mfb8L9
  • Lists any bug with sb?
  • Lists sandboxing component bugs that are not tracked by a milestone
  • Ignores sb+, sb-, and sb? bugs with needinfos
  • meta bugs
• sb? Triage List: http://is.gd/B3KscF
  • does not include needinfo bugs
• sb? needinfos: http://is.gd/dnSyBs

Tracking Bugs

• Sandboxing Component
• bug 925570 Process sandboxing work
• bug 1011491 Tighten Windows GMP sandbox policies
• bug 1066612 Apply Chromium sandbox "process-level mitigations" to GMP process
• bug 1105816 Tighten Windows content sandbox policy
• bug 1083344 Tighten rules for Mac OS content process sandbox on 10.9 and 10.10
• bug 1149706 Remaining issues for content process sandboxing on the Mac
• bug 1123755 Tighten Windows NPAPI process sandbox policy
• bug 1196384 (sandbox-fs) Deny file system access for content processes
• bug 1124817 Move file access and API usage away from content process
• bug 1280415 Content sandbox issues on nightly desktop Firefox

Communication

Weekly Team Meeting	Thursday at 8:00am PT Vidyo: "PlatInt" room Invitation: Contact Jim Mathies to get added to the meeting invite list. Meeting Notes Archive
IRC	Server: irc.mozilla.org Channel: #boxing
Newsgroup/Mailing List	boxing@lists.mozilla.org

People

Engineering Management	Brad Lassey (blassey) Jim Mathies (jimm)
Project Management	TBD
QA	Tracy Walker (Quality Assurance Lead)
Development Team	Haik Aftandilian (haik) Julian Hector (tedd) Jim Mathies (jimm) Bob Owen (bobowen) Gian-Carlo Pascutto (gcp)
Other Teams	kang, Security/OpSec Security Engineering SecurityEngineering

Repo Module Ownership

• Cross platform
• Windows
• OSX
• Linux/B2G

Links

• Electrolysis Wiki Page (lot of additional resource links)
• Chromium Sandbox
• Apple's Sandbox guide
• "Introducing Chrome's next-generation Linux sandbox" (seccomp-bpf related)
• Native Client on Wikipedia (Links to papers on Native Client's design and use of SFI, as well as papers on SFI itself.)
• Features of Protected Mode in Internet Explorer

Research

• Ian's Internal Research page (2012)

B2G Archive

• Firefox OS System Security page on MDN
• Sandboxing on b2g overview
• seccomp b2g filter perf data

B2G has always been “sandboxed” to some extent; every app/tab gets its own content process, which uses the Android security model: a separate uid per process, no group memberships, and kernel patches that require group membership for things like network access. But privilege escalation via kernel vulnerabilities is relatively common, so we also use the seccomp-bpf system call filter to reduce the attack surface that a compromised content process can directly access.

Older

• Old Meeting Notes