oracular (2) cap_enter.2freebsd.gz

NAME

     cap_enter, cap_getmode — Capability mode system calls

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/capsicum.h>

     int
     cap_enter(void);

     int
     cap_getmode(u_int *modep);

DESCRIPTION

     cap_enter() places the current process into capability mode, a mode of execution in which processes may
     only issue system calls operating on file descriptors or reading limited global system state.  Access to
     global name spaces, such as file system or IPC name spaces, is prevented.  If the process is already in a
     capability mode sandbox, the system call is a no-op.  Future process descendants created with fork(2) or
     pdfork(2) will be placed in capability mode from inception.

     When combined with cap_rights_limit(2), cap_ioctls_limit(2), cap_fcntls_limit(2), cap_enter() may be used
     to create kernel-enforced sandboxes in which appropriately-crafted applications or application components
     may be run.

     cap_getmode() returns a flag indicating whether or not the process is in a capability mode sandbox.

RUN-TIME SETTINGS

     If the kern.trap_enotcap sysctl MIB is set to a non-zero value, then for any process executing in a
     capability mode sandbox, any syscall which results in either an ENOTCAPABLE or ECAPMODE error also
     generates the synchronous SIGTRAP signal to the thread on the syscall return.  On signal delivery, the
     si_errno member of the siginfo signal handler parameter is set to the syscall error value, and the si_code
     member is set to TRAP_CAP.

     See also the PROC_TRAPCAP_CTL and PROC_TRAPCAP_STATUS operations of the procctl(2) function for similar
     per-process functionality.

CAVEAT

     Creating effective process sandboxes is a tricky process that involves identifying the least possible
     rights required by the process and then passing those rights into the process in a safe manner.  Consumers
     of cap_enter() should also be aware of other inherited rights, such as access to VM resources, memory
     contents, and other process properties that should be considered.  It is advisable to use fexecve(2) to
     create a runtime environment inside the sandbox that has as few implicitly acquired rights as possible.

RETURN VALUES

     The cap_enter() and cap_getmode() functions return the value 0 if successful; otherwise the value -1 is
     returned and the global variable errno is set to indicate the error.

     When the process is in capability mode, cap_getmode() sets the flag to a non-zero value.  A zero value
     means the process is not in capability mode.

ERRORS

     The cap_enter() and cap_getmode() system calls will fail if:

     [ENOSYS]           The kernel is compiled without:

                        options CAPABILITY_MODE

     The cap_getmode() system call may also return the following error:

     [EFAULT]           Pointer modep points outside the process's allocated address space.

SEE ALSO

     cap_fcntls_limit(2), cap_ioctls_limit(2), cap_rights_limit(2), fexecve(2), procctl(2), cap_sandboxed(3),
     capsicum(4), sysctl(9)

HISTORY

     The cap_getmode() system call first appeared in FreeBSD 8.3.  Support for capabilities and capabilities
     mode was developed as part of the TrustedBSD Project.

AUTHORS

     These functions and the capability facility were created by Robert N. M. Watson at the University of
     Cambridge Computer Laboratory with support from a grant from Google, Inc.