NAME

Capsicum — lightweight OS capability and sandbox framework

SYNOPSIS

options CAPABILITY_MODE
options CAPABILITIES

DESCRIPTION

Capsicum is a lightweight OS capability and sandbox framework implementing a hybrid capability system model. Capsicum is designed to blend capabilities with UNIX. This approach achieves many of the benefits of least-privilege operation, while preserving existing UNIX APIs and performance, and presents application authors with an adoption path for capability-oriented design.

Capabilities are unforgeable tokens of authority that can be delegated and must be presented to perform an action. Capsicum makes file descriptors into capabilities.

Capsicum can be used for application and library compartmentalisation, the decomposition of larger bodies of software into isolated (sandboxed) components in order to implement security policies and limit the impact of software vulnerabilities.

Capsicum provides two core kernel primitives:

capability mode

A process mode, entered by invoking cap_enter(2), in which access to global OS namespaces (such as the file system and PID namespaces) is restricted; only explicitly delegated rights, referenced by memory mappings or file descriptors, may be used. Once set, the flag is inherited by future children processes, and may not be cleared.

Access to system calls in capability mode is restricted: some system calls requiring global namespace access are unavailable, while others are constrained. For instance, sysctl(2) can be used to query process-local information such as address space layout, but also to monitor a system's network connections. sysctl(2) is constrained by explicitly marking ≈60 of over 15000 parameters as permitted in capability mode; all others are denied.

The system calls which require constraints are sysctl(2), shm_open(2) (which is permitted to create anonymous memory objects but not named ones) and the openat(2) family of system calls. The openat(2) calls already accept a file descriptor argument as the directory to perform the open(2), rename(2), etc. relative to; in capability mode the openat(2) family of system calls are constrained so that they can only operate on objects “under” the provided file descriptor.

capabilities

Limit operations that can be called on file descriptors. For example, a file descriptor returned by open(2) may be refined using cap_rights_limit(2) so that only read(2) and write(2) can be called, but not fchmod(2). The complete list of the capability rights can be found in the rights(4) manual page.

In some cases, Capsicum requires use of alternatives to traditional POSIX APIs in order to name objects using capabilities rather than global namespaces:

process descriptors

File descriptors representing processes, allowing parent processes to manage child processes without requiring access to the PID namespace; described in greater detail in procdesc(4).

anonymous shared memory

An extension to the POSIX shared memory API to support anonymous swap objects associated with file descriptors; described in greater detail in shm_open(2).

In some cases, Capsicum limits the valid values of some parameters to traditional APIs in order to restrict access to global namespaces:

process IDs

Processes can only act upon their own process ID with syscalls such as cpuset_setaffinity(2).

FreeBSD provides some additional functionality to support application sandboxing that is not part of Capsicum itself:

capsicum_helpers(3)

A set of a inline functions which simplify modifying programs to use Capsicum.

libcasper(3)

A library that provides services for sandboxed applications, such as operating on files specified on a command line or establishing network connections.

SEE ALSO

cap_enter(2), cap_fcntls_limit(2), cap_getmode(2), cap_ioctls_limit(2), cap_rights_limit(2), fchmod(2), open(2), pdfork(2), pdgetpid(2), pdkill(2), pdwait4(2), read(2), shm_open(2), write(2), cap_rights_get(3), capsicum_helpers(3), libcasper(3), procdesc(4)

HISTORY

Capsicum first appeared in FreeBSD 9.0, and was developed at the University of Cambridge.

AUTHORS

Capsicum was developed by Robert Watson <rwatson@FreeBSD.org> and Jonathan Anderson <jonathan@FreeBSD.org> at the University of Cambridge, and Ben Laurie <benl@FreeBSD.org> and Kris Kennaway <kris@FreeBSD.org> at Google, Inc., and Pawel Jakub Dawidek <pawel@dawidek.net>. Portions of this manual page are drawn from Robert N. M. Watson, Jonathan Anderson, Ben Laurie, and Kris Kennaway, “Capsicum: practical capabilities for UNIX”, USENIX Security Symposium, August 2010, DOI: 10.5555/1929820.1929824.