PLEDGE(2)

NAME
pledge - restrict system operations

SYNOPSIS
#include <unistd.h>
int
pledge(const char *promises, const char *execpromises);

DESCRIPTION
The pledge() system call separates the POSIX feature set into a group of approximately 3 dozen subsystems. By calling pledge() the program can declare which subsystems it will need in the future in a space-separated string called promises. Subsystems not listed become unavailable, and most attempts to use operations in that subsystem result in the process being killed with an uncatchable SIGABRT, delivering a core file if possible.

Subsequent calls to pledge() can reduce the subsystems which still work, but previously revoked subsystems cannot be re-activated.

Passing NULL to promises or execpromises specifies to not change the current value.

A few changes to POSIX behaviour come into effect on the first call to pledge(), regardless of the promise arguments:

adjtime(2):
Time cannot be changed. Only the olddelta argument works.

chmod(2), fchmod(2), fchmodat(2), chown(2), lchown(2), fchown(2), fchownat(2), mkfifo(2), and mknod(2): Setuid/setgid/sticky bits are ignored. User or group cannot be changed. The fattr promise relaxes this behaviour slightly.

ioctl(2):
Only the FIONREAD, FIONBIO, FIOCLEX, and FIONCLEX operations are allowed by default. All other ioctl operations are blocked, except for ones which are enabled by specific promises: audio, bpf, disklabel, drm, inet, pf, route, wroute, tape, tty, video, and vmm.

mmap(2) and mprotect(2): PROT_EXEC is not allowed, unless the prot_exec promise is requested.

__pledge_open(2):
A few system files can be opened directly by libc internal code using this hidden symbol system call when specific promises are requested, but applications cannot open those files themselves. This works even when rpath or wpath are absent and unveil(2) cannot block opening the files. These are the promises which can read special files: promises.

stdio
    /etc/localtime (file or zoneinfo symbolic link)
    /usr/share/zoneinfo/<file>
    /dev/null (for read and/or write)

tty /dev/tty (for read and/or write)

getpw
    /etc/spwd.db (refused with EPERM)
    /etc/pwd.db /etc/group /etc/netid

dns /etc/resolv.conf /etc/hosts /etc/services /etc/protocols

pledge():
Can only reduce permissions for promises and execpromises.

sysctl(2):
A small set of read-only operations are allowed, sufficient to support: getdomainname(3), gethostname(3), getifaddrs(3), uname(3), and system sensor readings. Some promises expose more read-only operations.

The promises argument is specified as a string, with space separated keywords. Using "" restricts the process to the _exit(2) system call (this can be used for pure computation operating on memory shared with another process).

stdio
    This promise provides access to basic memory management, actions upon already open file descriptors, inspection of various process attributes, timer resources, and other subsystems generally considered safe because they only occur inside the process.. Many of these interfaces are then used inside higher-level libc functionality, which is why the name stdio was chosen. The following system calls are permitted:

    clock_getres(2), clock_gettime(2), close(2), closefrom(2), dup(2), dup2(2), dup3(2), fchdir(2), fcntl(2), fstat(2), fsync(2), ftruncate(2), getdtablecount(2), getegid(2), getentropy(2), geteuid(2), getgid(2), getgroups(2), getitimer(2), getlogin(2), getpgid(2), getpgrp(2), getpid(2), getppid(2), getresgid(2), getresuid(2), getrlimit(2), getrtable(2), getsid(2), getthrid(2), gettimeofday(2), getuid(2), issetugid(2), kevent(2), kqueue(2), kqueue1(2), lseek(2), madvise(2), minherit(2), mmap(2), mprotect(2), mquery(2), munmap(2), nanosleep(2), pipe(2), pipe2(2), poll(2), pread(2), preadv(2), profil(2), pwrite(2), pwritev(2), read(2), readv(2), recvfrom(2), recvmsg(2), select(2), sendmsg(2), sendsyslog(2), sendto(2), setitimer(2), shutdown(2), sigaction(2), sigprocmask(2), sigreturn(2), socketpair(2), umask(2), wait4(2), waitid(2), write(2), writev(2)

    sendto(2) is only permitted if the destination socket address is NULL.

rpath
    A number of system calls are allowed which allow path traversal, reading struct stat, and opening files for read.

wpath
    Similar to rpath, but files can be opened for write.

cpath
    Similar to wpath, but files can also be created or removed.

dpath
    Similar to cpath, but special files can be created using:

    mkfifo(2), mknod(2)

tmppath
    No longer available. This pledge was designed to satisfy the mkstemp(3) family of functions. The limited filesystem access it provided is now disabled, so the promise has been removed and will return EINVAL. It should be replaced by either allowing use of the whole filesystem, meaning "rpath wpath cpath", or use of unveil(2) with path "/tmp" and permissions "rwc".

inet
    The following system calls are allowed to operate in the AF_INET and AF_INET6 domains (though setsockopt(2) has been substantially reduced in functionality):

    socket(2), listen(2), bind(2), connect(2), accept4(2), accept(2), getpeername(2), getsockname(2), setsockopt(2), getsockopt(2)

mcast
    In combination with inet give back functionality to setsockopt(2) for operating on multicast sockets.

fattr
    The following system calls are allowed to make explicit changes to fields in struct stat relating to a file:

    utimes(2), futimes(2), utimensat(2), futimens(2), chmod(2), fchmod(2), fchmodat(2), chflags(2), chflagsat(2), chown(2), fchownat(2), lchown(2), fchown(2), utimes(2)

chown
    The chown(2) family is allowed to change the user or group on a file.

flock
    File locking via fcntl(2), flock(2), lockf(3), and open(2) is allowed. No distinction is made between shared and exclusive locks. This promise is required for unlock as well as lock.

unix
    The following system calls are allowed to operate in the AF_UNIX domain:

    socket(2), listen(2), bind(2), connect(2), accept4(2), accept(2), getpeername(2), getsockname(2), setsockopt(2), getsockopt(2)

    The bind(2) call can create AF_UNIX sockets at any path even without cpath, and connect(2) can connect at any path even without rpath or wpath.

dns Some low-level behaviours required by the DNS resolver described in res_init(3) are permitted. This includes __pledge_open(2) reading hosts(5), protocols(5), resolv.conf(5), and services(5), and exposing a few networking system calls: socket(2), connect(2), sendto(2), recvfrom(2) which can only operate on the specific socket type SOCK_DNS. The library resolver opens sockets with SOCK_DNS only on port 53, so the kernel can differentiate these operations from regular sockets operations.

getpw
    This uses the special features of __pledge_open(2) to read required system files to support the getpwnam(3), getgrnam(3), getgrouplist(3), and initgroups(3) family of functions, including lookups via the yp(8) protocol for YP and LDAP databases.

sendfd
    Allows sending of file descriptors using sendmsg(2). File descriptors referring to directories may not be passed.

recvfd
    Allows receiving of file descriptors using recvmsg(2). File descriptors referring to directories may not be passed.

tape
    Allow MTIOCGET and MTIOCTOP operations against tape drives.

tty In addition to allowing read-write operations on /dev/tty, this opens up a variety of ioctl(2) requests used by tty devices. If tty is accompanied with rpath, revoke(2) is permitted. Otherwise only the following ioctl(2) requests are permitted:

    TIOCSPGRP, TIOCGETA, TIOCGPGRP, TIOCGWINSZ, TIOCSWINSZ, TIOCSBRK, TIOCCDTR, TIOCSETA, TIOCSETAW, TIOCSETAF, TIOCUCNTL

proc
    Allows the following process relationship operations:

    fork(2), vfork(2), kill(2), getpriority(2), setpriority(2), setrlimit(2), setpgid(2), setsid(2). kill(2) may still operate on the process group with pid 0.

exec
    Allows a process to call execve(2). Coupled with the proc promise, this allows a process to fork and execute another program. If execpromises has been previously set the new program begins with those promises, unless setuid/setgid bits are set in which case execution is blocked with EACCES. Otherwise the new program starts running without pledge active, and hopefully makes a new pledge soon.

prot_exec
    Allows the use of PROT_EXEC with mmap(2) and mprotect(2).

settime
    Allows the setting of system time, via the settimeofday(2), adjtime(2), and adjfreq(2) system calls.

ps  Allows enough sysctl(2) interfaces to allow inspection of processes operating on the system using programs like ps(1).

vminfo
    Allows enough sysctl(2) interfaces to allow inspection of the system's virtual memory by programs like top(1) and vmstat(8).

id  Allows the following system calls which can change the rights of a process:

    setuid(2), seteuid(2), setreuid(2), setresuid(2), setgid(2), setegid(2), setregid(2), setresgid(2), setgroups(2), setlogin(2), setrlimit(2), getpriority(2), setpriority(2), setrtable(2)

pf  Allows a subset of ioctl(2) operations on the pf(4) device:

    DIOCADDRULE, DIOCGETSTATUS, DIOCNATLOOK, DIOCRADDTABLES, DIOCRCLRADDRS, DIOCRCLRTABLES, DIOCRCLRTSTATS, DIOCRGETTSTATS, DIOCRSETADDRS, DIOCXBEGIN, DIOCXCOMMIT

route
    Allow inspection of the routing table.

wroute
    Allow changes to the routing table.

audio
    Allows a subset of ioctl(2) operations on audio(4) devices (see sio_open(3) for more information):

    AUDIO_GETPOS, AUDIO_GETPAR, AUDIO_SETPAR, AUDIO_START, AUDIO_STOP, AUDIO_MIXER_DEVINFO, AUDIO_MIXER_READ, AUDIO_MIXER_WRITE

video
    Allows a subset of ioctl(2) operations on video(4) devices:

    VIDIOC_DQBUF, VIDIOC_ENUM_FMT, VIDIOC_ENUM_FRAMEINTERVALS, VIDIOC_ENUM_FRAMESIZES, VIDIOC_G_CTRL, VIDIOC_G_PARM, VIDIOC_QBUF, VIDIOC_QUERYBUF, VIDIOC_QUERYCAP, VIDIOC_QUERYCTRL, VIDIOC_S_CTRL, VIDIOC_S_FMT, VIDIOC_S_PARM, VIDIOC_STREAMOFF, VIDIOC_STREAMON, VIDIOC_TRY_FMT, VIDIOC_REQBUFS

bpf Allow BIOCGSTATS operation for statistics collection from a bpf(4) device.

unveil
    Allow unveil(2) to be called.

error
    Rather than killing the process upon violation, indicate error with ENOSYS.

    Also when pledge() is called with higher promises or execpromises, those changes will be ignored and return success. This is useful when a parent enforces execpromises but an execve'd child has a different idea.

A process currently running with pledge has state ‘p’ in ps(1) output; a process that was terminated due to a pledge violation is accounted by lastcomm(1) with the ‘P’ flag.

RETURN VALUES

ERRORS
pledge() will fail if:

[EFAULT]
    promises or execpromises points outside the process's allocated address space.

[EINVAL]
    promises is malformed or contains invalid keywords.

[EPERM]
    This process is attempting to increase permissions.

HISTORY
The pledge() system call first appeared in OpenBSD 5.9.