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linux-security-module: AppArmor Security Goal

AppArmor Security Goal

From: Crispin Cowan <crispin_at_nospam>
Date: Thu Nov 08 2007 - 21:14:45 GMT
To: Arjan van de Ven <arjan@infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.rutgers.edu>, LSM ML <linux-security-module@vger.kernel.org>, apparmor-dev <apparmor-dev@forge.novell.com>


AppArmor Security Goal
Crispin Cowan, PhD
MercenaryLinux.com

This document is intended to specify the security goal that AppArmor is intended to achieve, so that users can evaluate whether AppArmor will meet their needs, and kernel developers can evaluate whether AppArmor is living up to its claims. This document is *not* a general purpose explanation of how AppArmor works, nor is it an explanation for why one might want to use AppArmor rather than some other system.

AppArmor is intended to protect systems from attackers exploiting vulnerabilities in applications that the system hosts. The threat is that an attacker can cause a vulnerable application to do something unexpected and undesirable. AppArmor addresses this threat by confining the application to access only the resources it needs to access to execute properly, effectively imposing "least privilege" execution on the application.

Applications have access to a number of resources including files, interprocess communication, networking, capabilities, and execution of other applications. The purpose of least privilege is to bound the damage that a malicious user or code can do by removing access to all resources that the application does not need for its intended function. For instance, a policy for a web server might grant read only access to most web documents, preventing an attacker who can corrupt the web server from defacing the web pages.

An "application" is one or more related processes performing a function, e.g. the gang of processes that constitute an Apache web server, or a Postfix mail server. AppArmor *only* confines processes that the AppArmor policy says it should confine, and other processes are permitted to do anything that DAC permits. This is sometimes known as a targeted security policy.

AppArmor does not provide a "default" policy that applies to all processes. So to defend an entire host, you have to piece-wise confine each process that is exposed to potential attack. For instance, to defend a system against network attack, place AppArmor profiles around every application that accesses the network. This limits the damage a network attacker can do to the file system to only those files granted by the profiles for the network-available applications. Similarly, to defend a system against attack from the console, place AppArmor profiles around every application that accessed the keyboard and mouse. The system is "defended" in that the worst the attacker can do to corrupt the system is limited to the transitive closure of what the confined processes are allowed to access.

AppArmor currently mediates access to files, ability to use POSIX.1e Capabilities, and coarse-grained control on network access. This is sufficient to prevent a confined process from *directly* corrupting the file system. It is not sufficient to prevent a confined process from *indirectly* corrupting the system by influencing some other process to do the dirty deed. But to do so requires a complicit process that can be manipulated through another channel such as IPC. A "complicit" process is either a malicious process the attacker somehow got control of, or is a process that is actively listening to IPC of some kind and can be corrupted via IPC.

The only IPC that AppArmor mediates is access to named sockets, FIFOs, etc. that appear in the file system name space, a side effect of AppArmor's file access mediation. Future versions of AppArmor will mediate more resources, including finer grained network access controls, and controls on various forms of IPC.

AppArmor specifies the programs to be confined and the resources they can access in a syntax similar to how users are accustomed to accessing those resources. So file access controls are specified using absolute paths with respect to the name space the process is in. POSIX.1e capabilities are specified by name. Network access controls currently are specified by simply naming the protocol that can be used e.g. tcp, udp, and in the future will be more general, resembling firewall rules.

Thus the AppArmor security goal should be considered piecewise from the point of view of a single confined process: that process should only be able to access the resources specified in its profile:

  • can only access files that are reachable in its name space by path names matching its profile, and only with the permitted modes: read, append, write, memory map, execute, and link
  • can only use the POSIX.1e capabilities listed in the profile
  • can only perform the network operations listed in the profile

Security issues that AppArmor explicitly does *not* address:

  • Processes that are not confined by AppArmor are not restricted in any way by AppArmor. If an unconfined process is considered an unacceptable threat, then confine additional applications until adequate security is achieved.
  • A process that is not permitted to directly access a resource can influence some other process that does have access to the resource may do so, if the "influence" is a permitted action.
  • A confined process may only access a file if it has at least one of the files aliases specified in its profile. If a file alias is not specified in the profile then it can not be accessed by that path. The creation of aliases needs to be tightly controlled in confined applications, hard links creation should be limited to provide adequate security.
  • A confined process can operate on a file descriptor passed to it by an unconfined process, even if it manipulates a file not in the confined process's profile. To block this attack, confine the process that passed the file descriptor.
  • Process activities not currently mediated by AppArmor are permitted, e.g. confined processes can perform any IPC that DAC permits, other than signals as mediated by CAP_KILL.
  • Manipulating AppArmor policy requires being both root privileged and not being confined by AppArmor, thus there is explicitly no capability for non-privileged users to change AppArmor policy.
  • AppArmor confines processes if they are children of a confined process, or if the name of the exec'd child matches the name of an AppArmor profile. Another process could copy a program to a different path name and then execute it without confinement, but the other process would have to have permission to do so in the first place. To prevent this, confine the other process and additional applications until adequate security is achieved.
  • Mount and namespace manipulations can be used to arbitrarily change the pathnames that files appear at, and thus completely bypass AppArmor policy. To prevent this, processes confined by AppArmor are currently not permitted to call mount or manipulate name spaces at all. A future development may provide more granular controls on mount and namespace manipulations.
  • AppArmor does not slice bread, cure cancer, or bring world peace. This list may be expanded :-)
-- Crispin Cowan, Ph.D. http://crispincowan.com/~crispin CEO, Mercenary Linux http://mercenarylinux.com/ Itanium. Vista. GPLv3. Complexity at work - To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html