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B2G/Architecture/System Security

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Contents

Terminology

Web application: An HTML/JS application started within a content process. All user-facing applications on B2G are web applications.

b2g process: This is the main process of B2G, it controls web application's access to resources, the API, etc. This is a high-privileged process (i.e., runs as root)

Content process : This is a sub-process spawned by the b2g process, and which communicates with the b2g process. It represents a web application. This is a low-privileged process (i.e., run as regular user and has a very limited access and view of/to the operating system).

IPDL: Intercommunication Protocol Definition Language, see [1].

AOSP: Android Open Source Project.

Proposed <*>: This means the section has NOT yet been implemented in b2g and is being discussed. In that case, a status, priority and a proposed ETA is also included.

system call: An interface to talk between the user-space(processes) and the kernel. There is no other way for a user-space process to talk to the kernel.

DAC, MAC: Discretionary Access Control (up to the user) and Mandatory Access Control (enforced by the kernel)

B2G System Security Model

Current implementation is documented at https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS/Security/System_security

Global tracking bug https://bugzilla.mozilla.org/show_bug.cgi?id=862082

Goals and scope of this document

  • Extend the security measures documented on https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS/Security/System_security
  • Limit and enforce the scope of resources that can be accessed or used by a web application
  • Ensure several layers of security are being correctly used in the operating system
  • Limit and contain the impact of vulnerabilities caused by security bugs, system-wide
  • Web application permissions and any application related security feature are not detailed here
  • Expose a road-map of the upcoming system-related security features

Road Map

Feature Implementation Status Priority/Importance
Seccomp https://bugzilla.mozilla.org/show_bug.cgi?id=790923 (ASAP) High
GL Proxy N/A High
Supervisor process https://bugzilla.mozilla.org/show_bug.cgi?id=845191 (Soon) High
RBAC (MAC) Medium
Disk Encryption https://bugzilla.mozilla.org/show_bug.cgi?id=777917 Medium
ASLR https://bugzilla.mozilla.org/show_bug.cgi?id=777948 (Soon) Medium
Compiler hardening https://bugzilla.mozilla.org/show_bug.cgi?id=620058 Medium
JIT hardening https://bugzilla.mozilla.org/show_bug.cgi?id=677272 Low
Implement automated compliance testing of security features N/A Medium
Integrity measurements Low

Features implementation details, risk analysis, discussion

Features get scoped here, before going into a tracking bug if they're selected to be worked on. Each feature should in general include some sort of risk analysis (or a threat model), and some implementation details.

Supervisor process

Risks

  • Parent process (b2g) is compromised, gives full device access (run as root with no restriction).

Implementation

  • Create a new, small footprint process called "Supervisor".
    • Supervisor provides the following features:
      • Start system update (Any kind of - Gecko and full system updates)
      • Shutdown, Reboot system
      • Adjust process priorities (nice/renice)
      • Adjust OOM killer values (oom_adjust)
      • Possibly, load kernel modules at process startup. If not, the init process should take care of starting the necessary scripts for this task.
      • Drop privileges of spawned subprocesses
    • Supervisor DOES NOT provide the following features:
      • XPCOM
      • JS runtime
      • Any other such gecko feature. This is not gecko. It MAY link to libxul and use a subset of features for IPC communication only.
  • the b2g process should run as system:system instead of root:root.
  • the supervisor process should run as root:root.
  • Select an IPC mechanism. It may use libxul for this (such as IPDL).

See also bugs (up for discussion):


RBAC (Role Based Access Control)

Risks

  • Ability from the attacker to run arbitrary code on the device once a process has been compromised
  • Ability from the attacker to use a process in an unintended way and access resources that the Linux DAC cannot control access to
  • In some cases, ability from the attacker to exploit the kernel through vulnerable system calls, that the processes normally wouldn't use

Implementation

RBAC is implemented by various frameworks, including SELinux, RSBAC RC, and GrSecurity RBAC.

These frameworks are generally called Mandatory Access Control frameworks (MAC), allow setting white-lists of systems calls on any process, or group of processes, based on roles and types. Roles are assigned to the processes and users, types to the resources they access. This allows the framework to control the access with little to no modification of the running program, unlike seccomp. Both SELinux and seccomp enforce their policies by controlling system calls at the kernel level.

  • Allows for extremely flexible configurations
  • Restrictions are always enforced by the kernel
  • Restrictions can also be configured for any process and therefore sand-boxing of the non-b2g processes (wpa_supplicant, init, etc.)
  • Restrictions can be configured for the B2G process, even thus it's running as root
  • Restrictions to the content-processes make little sense when seccomp-bpf is already being used.
    • Further decrease performance
    • Does not lock down the content-process more strictly than seccomp
    • Similar checks being performed
  • Possible to target only some processes (targeted policy), albeit a complete policy (all processes, no exception) is preferred from the security point of view.
  • Sand-box escape scenarios:
    • The security provided by the framework depends entirely on the rules/policy applied to the system
    • Any kernel vulnerability triggered via an allowed system call - this may also lead to the ability to disable the MAC framework
    • b2g process vulnerability triggered via IPDL
  • Misc & caveats:
    • Requires a custom kernel with SELinux enabled, or other kernel patch based solution built and enabled, until Android 4.3 and 4.4 based Gonk, which has SELinux enabled kernels, userspace tools and an Android-only policy.
    • Security policy can be extensive and eventually require modifications to run on different devices.
    • Security policy from Android most likely needs large changes to run with B2G, and to be taken advantage of for B2G

Disk encryption

Risks

  • Device is stolen and attacker has full access to the user's data storage

Requirements

  • Phone should be able to dial emergency numbers even when without the decryption key

Proposed Implementation

  • Android already uses disk encryption in a relatively sane manner and their approach may be re-used, see http://source.android.com/tech/encryption/android_crypto_implementation.html - Password handling should be revisited (different encryption/unlock passwords, better derivation of the password, as Android needs a very long password to resist brute force attacks).
    • Android encrypts only the data partition
    • Uses read-only partitions when unencrypted to ensure no data is being written
    • Locking/Unlocking the bootloader wipes the device and restores it to factory settings, this is enforced by fastboot
    • Devices are shipped with the bootloader locked by default
  • A user interface must be present to set the encryption password
  • Potential UX issues and proposed solutions
    • Allow a weaker screen lock password:
      • Unlocking the phone screen is done several times a day, sometimes several times within a few minutes, thus users rarely use a secure mechanism for their screen lock
      • Users are not tempted to use a weak PIN/password for FDE, since they are only asked for the FDE password at phone startup, not

every time they want to unlock their phone and use it

  • ChromeOS uses a slightly different disk encryption mechanism using eCryptFS.
    • ChromeOS only encrypts the user's home directory.
  • FDE (Full Disk Encryption)
    • Requires a fast CPU or hardware acceleration
    • Ensures no data is left unencrypted on the flash device

Address Space Layout Randomization (ASLR)

Risks

  • Loading libraries and application code at predictable or fixed addresses leads to easy exploitation of memory corruption vulnerabilities

Proposed Implementations

  • Upgrade Gonk to Jelly Bean's build system (newer GCC version, and complete ASLR support)
    • Faster, newer GCC, smaller performance impact from ASLR
    • This provides full ASLR, no fixed or predictable addresses are used
    • Requires upgrading the build system
  • Enable ASLR support, PIE, and linker ASLR in the current build system
    • Requires patching of various components
      • Failure to do would result in only partial ASLR, which is no better than no ASLR
    • May lead to slower process startup and high performance penalties - however, recent builds of B2G pre-start a content-process, which may hide any performance penalty

Updates: Proposed Additional Implementation: Tracking of applications versions for known security patches

A version tracking mechanism is necessary in order to decide when components of B2G need to be updated due to a security vulnerability. A list of the currently installed applications in Gonk must therefore be maintained, in particular for:

  • The kernel
  • The gonk processes such as wpa_supplicant
  • The gonk libraries such as Bionic

The version tracking mechanism should automatically warn the product security group based on a security feed (CVEs, Android Security upgrades)

Issues

  • This part of B2G may differ and generally be handled by a third party, such as a vendor or carrier, thus, they must be the ones running the tracking software
  • The tracking software may however be provided to them, with guidance, for example
  • Guidance on updates may also be provided instead


Sandbox implementations that were not selected (kept for reference)

rlimit sandbox

rlimit() is a system call that can be used to deny file and process creation. Like chroot(), this may be used as long as no privileged user (such as root) is running any process that is being rlimit'ed.

  • Relatively easy to implement, supported by various operating systems
    • RLIMIT_FSIZE = 0 requires that no file is written to (within the process) - this can already works
    • RLIMIT_NOFILE = 0 requires that no new file descriptor is opened (within the process)
    • RLIMIT_NPROC = 0 requires that no new thread of process is created (within the process)
  • Does not require kernel modifications
  • Used as fall-back sand-box in other programs, such as OpenSSH
  • Sand-box escape scenario:
    • Kernel vulnerability (any)
    • Loading code directly in memory (instead of executing, for example, /bin/sh), then finding user-space vulnerabilities in other processes
    • Privilege escalation to root/privileged user, then disable rlimits
    • User-land vulnerability via any kind of IPC
    • b2g process vulnerability triggered via IPDL

chroot

chroot() is a well-known system call, which changes the view of the root filesystem of the process. This system call is not explicitly designed to secure access to the file system but may be used in this fashion as long no privileged user (such as root) is running any process within the chroot after the process has been initialized.

  • Can be initialized after the process has already accessed all its needed files and resources, although the process must

still be running as root when calling chroot() or must have all needed files located inside the chroot directory). See https://bugzilla.mozilla.org/show_bug.cgi?id=776648.

    • Linux namespaces can be used in combination with the chroot in order to reduce the amount of code changes or files copied
  • While chroot() restricts a process' view of the filesystem, it enforces no other restrictions. All system calls are still available to the process.
  • Does not require kernel modifications
  • Sand-box escape scenario:
    • Kernel vulnerability (any)
    • User-land vulnerability via any kind of IPC
    • Privilege escalation to root/privileged user, then escape via chdir('..') and chrooting back to the original root, or simply by remounting the entire device's root
    • b2g process vulnerability triggered via IPDL