XAC

Abstract

• XAC is a FreeBSD MAC framework kernel module
• Performs access control based on
  • SHA512 hash of executable files, and
  • st_dev/i_num of files and directories
• It can effectively limit processes running as root
• Secure configuration hinges on a secret pin
• A version of this module is currently being used in production.

Use Cases

You can define access control (sandboxing) at the level of individual processes and executables, and effectively limit each process to the resources it really needs to access. This is mostly useful for defense against privilege escalation attacks, and also in systems that have services/programs that normally run as root.

Compared to Capsicum

It is usually not trivial to sandbox a program with capsicum, unless the program is intentionally written to operate under capsicum in capability mode. There are methods that use preloading to interpose and manage libc calls to make an unsuspecting program compatible with capsicum, but usually they are not reliable. Notably, you can't efficiently interpose direct system calls that are not routed through libc (e.g. Go executables).

XAC can be used to transparently sandbox an executable without modifying the program or interposing its library calls.

Effectiveness

Based on the contextual assumption of second-preimage resistance of the employed hash function (SHA512 in this case), we can effectively use the hash of an executable to uniquely identify it. If an executable is modified, it will automatically lose all of its privileges under XAC (and at the same time it will be automatically exempted from limitations imposed on it). So if we are using XAC in a blacklisting manner it is important that we also use a module like Veriexec that prevents execution of unauthorized executables.

However, if XAC is used in a white-listing manner, Veriexec is not required. The administrator can deny all accesses to a resource, except by an executable that has a specified hash.

Access Control for root Processes

XAC has two properties that make it effective for enforcing access control on processes running as root.

1. XAC policy enforcement does exempt the processes running under root
2. xactl control process requires a pin code to carry out its operations

The second property entails that only an entity that has knowledge of the pin code can tamper with the XAC module operations. The pin is required for changing/reloading the ruleset, disabling the kernel module, etc.

Internals

TODO

Architecture

TODO

Sub-Modules

1. xac_mac
2. xac_config_manager
3. xac_log

System Calls

XAC defined two sets of system calls. One for management, configuration, and probing of the XAC module itself, and another set of system calls for configuration of a sandboxing environment for a process.

Syscall name	Category	Authorized processes	Available in Sandboxing mode?
MAC_XAC_SYSCALL_RELOAD	Management	xactl	No
MAC_XAC_SYSCALL_ENABLE	Management	xactl	No
MAC_XAC_SYSCALL_DISABLE	Management	xactl	No
MAC_XAC_SYSCALL_STATS	Information	xactl	No
MAC_XAC_SYSCALL_LOGLEVEL	Management	xactl	No
MAC_XAC_SYSCALL_DUMP	Information	xactl	No
MAC_XAC_SYSCALL_VERSION	Information	All	Yes
MAC_XAC_SYSCALL_SELFBOX_RULE	Sandboxing	All	No
MAC_XAC_SYSCALL_SELFBOX_ENTER	Sandboxing	All	No

userspace library

libxac.so serves as a convenient wrapper around XAC kernel module system calls for userspace processes, that want to use its sandboxing capabilities.

Ruleset File Format

A XAC ruleset consists of a set of subject (SUB) rules and object (OBJ) rules.

SUB rules are defined around subjects and declare which objects they are allowed to access in what mode. In context of XAC, subjects are uniquely identified via the SHA512 hash of their executable files on disk.

Syntax of a SUB rule:

SUB /path/to/executable
    [!] * | /path/to/object/1 [R][W][X] [LOG]
    [!] * | /path/to/object/2 [R][W][X] [LOG]
    [!] * | /path/to/object/3 [R][W][X] [LOG]
    ...

OBJ rules are defined around objects and declare which subjects are not/allowed to access them. In context of XAC objects are uniquely identified via their device id and inode number.

Syntax of a OBJ rule:

OBJ /path/to/object/file
    [!] * | /path/to/suject/1 [R][W][X] [LOG]
    [!] * | /path/to/subject/2 [R][W][X] [LOG]
    [!] * | /path/to/subject/3 [R][W][X] [LOG]
    ...

The ! sign negates the sense of the match. * sign can be used to denote ANY subject/object. RWX tags denote the access modes READ, WRITE, and EXECUTE. Finally, the LOG keyword specifies that if this rule is used as the final verdict to allow/disallow an action in XAC, then a log entry should be generated.

The kernel module does not use this textual ruleset directly, but rather this file should first be translated to the binary format used by the kernel. This step also involved translation of subjects paths to their SHA512 hashes, and also translation of object paths to their (device id, inode number) pair. In this process a reverse mapping table from subject/object identities to their paths is created and stored in ruleset-usr.symtab which is later used by the logging facility to translate XAC logs to a more usable format.

This can be done via xactl as follows:

xactl -c /path/to/rules/file

Once the rulset is compiled to the binary format usable by the kernel module, which is normally stored in ruleset-usr.bin, it can be uploaded to the kernel:

xactl -r

Also the kernel module will automatically load this binary ruleset at boot time right after the root file system is mounted.

ptrace / ktrace

Debugging can amount to modification of the executaion of a binary and thus is globally banned under XAC. Because, XAC access control depends on integrity of an executable and its hash digest.

Logging

Currently XAC access logs are passed through standard kernel logging facility. Eventually, I am going to create a specialized logging interface (i.e. a character device /dev/xaclog) for XAC which can be used to filter through XAC logs more efficiently without cluttering system log. Based on symtab generated during compilation of the ruleset, these logs can be translated into a format readable by system admin and other programs.

Profiling Mode (work in progress)

Profiling mode allows us to automatically create a baseline ruleset under which a program can operate normally.

xactl -P /path/to/executable [args]

Examples

Selfboxing

A process can use libxac xacsb_allow_*() functions to configure a sandbox on the current process and then make a one-way transition in enforcing mode via xacsb_enter(). For example to configure a sandbox that only allows READ access to TARGET_FILE we can do this:

#include <sys/vnode.h>
#include "xac_lib.h"

xacsb_allow_path(TARGET_FILE, VREAD);
xacsb_enter();

Once sandbox enforcing mode is enabled, the process has no way to leave the sandbox. All processes executed from thereon, via exec family of system calls and all children fork()ed will also reside in the sandbox.

Ambient Rulesets

Ambient rulesets are enforced based on hash of executables and dev/inode of object files. These rules are persistent and are automatically enforced when an executable with a matching hash is detected.

An example ruleset:

OBJ /etc/mac_xac/
    /usr/local/bin/xactl RWX LOG
    ! * RWX

OBJ /etc/mac_xac/pin
    /usr/local/bin/xactl RW LOG
    ! * RWX

OBJ /etc/mac_xac/xac_usr.conf
    /usr/local/bin/xactl RW LOG
    /bin/cat R
    ! * RWX

OBJ /etc/mac_xac/ruleset-usr.bin
    /usr/local/bin/xactl W LOG
    ! * RWX

OBJ /etc/mac_xac/ruleset-usr.symtab
    /usr/local/bin/xactl W LOG
    ! * RWX

OBJ /usr/local/bin/xactl
    /usr/local/bin/xactl RWX
    ! * RW LOG

SUB /usre/local/bin/xxd
    /var/log/binlog RWX
    ! * RWX LOG

Roadmap

• Add support for wildcards in configuration.
• Implement an specialized logging interface.
• Implement a permissive profiling mode.
• Add support for identification of objects based on extended file attributes.
• Support recursive directory references in configuration file and selfboxing mode.
• Come up with a more user friendly format for the ruleset file.