NAME
cap_enter,
cap_getmode —
Capability mode system calls
LIBRARY
library “libc”
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
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.