pledge.2

.\" $OpenBSD: pledge.2,v 1.69 2023/10/30 06:11:04 jmc Exp $
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.\" Copyright (c) 2015 Nicholas Marriott <nicm@openbsd.org>
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.Dd $Mdocdate: October 30 2023 $
.Dt PLEDGE 2
.Os
.Sh NAME
.Nm pledge
.Nd restrict system operations
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn pledge "const char *promises" "const char *execpromises"
.Sh DESCRIPTION
The
.Fn pledge
system call forces the current process into a restricted-service operating mode.
A few subsets are available, roughly described as computation, memory
management, read-write operations on file descriptors, opening of files,
networking (and notably separate, DNS resolution).
In general, these modes were selected by studying the operation
of many programs using libc and other such interfaces, and setting
.Fa promises
or
.Fa execpromises .
.Pp
Use of
.Fn pledge
in an application will require at least some study and understanding
of the interfaces called.
Subsequent calls to
.Fn pledge
can reduce the abilities further, but abilities can never be regained.
.Pp
A process which attempts a restricted operation is killed with an uncatchable
.Dv SIGABRT ,
delivering a core file if possible.
A process currently running with pledge has state
.Sq p
in
.Xr ps 1
output; a process that was terminated due to a pledge violation
is accounted by
.Xr lastcomm 1
with the
.Sq P
flag.
.Pp
A
.Fa promises
value of
.Qq \&
restricts the process to the
.Xr _exit 2
system call.
This can be used for pure computation operating on memory shared
with another process.
.Pp
Passing
.Dv NULL
to
.Fa promises
or
.Fa execpromises
specifies to not change the current value.
.Pp
Some system calls, when allowed, have restrictions applied to them:
.Bl -ohang -offset indent
.It Xr access 2 :
May check for existence of
.Pa /etc/localtime .
.It Xr adjtime 2 :
Read-only, for
.Xr ntpd 8 .
.It Xo
.Xr chmod 2 ,
.Xr fchmod 2 ,
.Xr fchmodat 2 ,
.Xr chown 2 ,
.Xr lchown 2 ,
.Xr fchown 2 ,
.Xr fchownat 2 ,
.Xr mkfifo 2 ,
and
.Xr mknod 2 :
.Xc
Setuid/setgid/sticky bits are ignored.
The user or group cannot be changed on a file.
.It Xr ioctl 2 :
Only the
.Dv FIONREAD ,
.Dv FIONBIO ,
.Dv FIOCLEX ,
and
.Dv FIONCLEX
operations are allowed by default.
Various ioctl requests are allowed against specific file descriptors
based upon the requests
.Cm audio ,
.Cm bpf ,
.Cm disklabel ,
.Cm drm ,
.Cm inet ,
.Cm pf ,
.Cm route ,
.Cm wroute ,
.Cm tape ,
.Cm tty ,
.Cm video ,
and
.Cm vmm .
.It Xo
.Xr mmap 2
and
.Xr mprotect 2 :
.Xc
.Dv PROT_EXEC
isn't allowed.
.It Xr open 2 :
May open
.Pa /etc/localtime
and any files below
.Pa /usr/share/zoneinfo .
.It Xr profil 2 :
Can only disable profiling.
.It Fn pledge :
Can only reduce permissions for
.Fa promises
and
.Fa execpromises .
.It Xr sysctl 2 :
A small set of read-only operations are allowed, sufficient to
support:
.Xr getdomainname 3 ,
.Xr gethostname 3 ,
.Xr getifaddrs 3 ,
.Xr uname 3 ,
and system sensor readings.
.El
.Pp
The
.Fa promises
argument is specified as a string, with space separated keywords:
.Bl -tag -width "prot_exec" -offset indent
.It Cm stdio
The following system calls are permitted.
.Xr sendto 2
is only permitted if its destination socket address is
.Dv NULL .
As a result, all the expected functionalities of libc stdio work.
.Pp
.Xr clock_getres 2 ,
.Xr clock_gettime 2 ,
.Xr close 2 ,
.Xr closefrom 2 ,
.Xr dup 2 ,
.Xr dup2 2 ,
.Xr dup3 2 ,
.Xr fchdir 2 ,
.Xr fcntl 2 ,
.Xr fstat 2 ,
.Xr fsync 2 ,
.Xr ftruncate 2 ,
.Xr getdents 2 ,
.Xr getdtablecount 2 ,
.Xr getegid 2 ,
.Xr getentropy 2 ,
.Xr geteuid 2 ,
.Xr getgid 2 ,
.Xr getgroups 2 ,
.Xr getitimer 2 ,
.Xr getlogin 2 ,
.Xr getpgid 2 ,
.Xr getpgrp 2 ,
.Xr getpid 2 ,
.Xr getppid 2 ,
.Xr getresgid 2 ,
.Xr getresuid 2 ,
.Xr getrlimit 2 ,
.Xr getrtable 2 ,
.Xr getsid 2 ,
.Xr getthrid 2 ,
.Xr gettimeofday 2 ,
.Xr getuid 2 ,
.Xr issetugid 2 ,
.Xr kevent 2 ,
.Xr kqueue 2 ,
.Xr kqueue1 2 ,
.Xr lseek 2 ,
.Xr madvise 2 ,
.Xr minherit 2 ,
.Xr mmap 2 ,
.Xr mprotect 2 ,
.Xr mquery 2 ,
.Xr munmap 2 ,
.Xr nanosleep 2 ,
.Xr pipe 2 ,
.Xr pipe2 2 ,
.Xr poll 2 ,
.Xr pread 2 ,
.Xr preadv 2 ,
.Xr profil 2 ,
.Xr pwrite 2 ,
.Xr pwritev 2 ,
.Xr read 2 ,
.Xr readv 2 ,
.Xr recvfrom 2 ,
.Xr recvmsg 2 ,
.Xr select 2 ,
.Xr sendmsg 2 ,
.Xr sendsyslog 2 ,
.Xr sendto 2 ,
.Xr setitimer 2 ,
.Xr shutdown 2 ,
.Xr sigaction 2 ,
.Xr sigprocmask 2 ,
.Xr sigreturn 2 ,
.Xr socketpair 2 ,
.Xr umask 2 ,
.Xr wait4 2 ,
.Xr waitid 2 ,
.Xr write 2 ,
.Xr writev 2
.It Cm rpath
A number of system calls are allowed if they only cause
read-only effects on the filesystem:
.Pp
.Xr chdir 2 ,
.Xr getcwd 3 ,
.Xr openat 2 ,
.Xr fstatat 2 ,
.Xr faccessat 2 ,
.Xr readlinkat 2 ,
.Xr lstat 2 ,
.Xr chmod 2 ,
.Xr fchmod 2 ,
.Xr fchmodat 2 ,
.Xr chflags 2 ,
.Xr chflagsat 2 ,
.Xr chown 2 ,
.Xr fchown 2 ,
.Xr fchownat 2 ,
.Xr fstat 2 ,
.Xr getfsstat 2
.It Cm wpath
A number of system calls are allowed and may cause
write-effects on the filesystem:
.Pp
.Xr getcwd 3 ,
.Xr openat 2 ,
.Xr fstatat 2 ,
.Xr faccessat 2 ,
.Xr readlinkat 2 ,
.Xr lstat 2 ,
.Xr chmod 2 ,
.Xr fchmod 2 ,
.Xr fchmodat 2 ,
.Xr chflags 2 ,
.Xr chflagsat 2 ,
.Xr chown 2 ,
.Xr fchown 2 ,
.Xr fchownat 2 ,
.Xr fstat 2
.It Cm cpath
A number of system calls and sub-modes are allowed, which may
create new files or directories in the filesystem:
.Pp
.Xr rename 2 ,
.Xr renameat 2 ,
.Xr link 2 ,
.Xr linkat 2 ,
.Xr symlink 2 ,
.Xr symlinkat 2 ,
.Xr unlink 2 ,
.Xr unlinkat 2 ,
.Xr mkdir 2 ,
.Xr mkdirat 2 ,
.Xr rmdir 2
.It Cm dpath
A number of system calls are allowed to create special files:
.Pp
.Xr mkfifo 2 ,
.Xr mknod 2
.It Cm tmppath
A number of system calls are allowed to do operations in the
.Pa /tmp
directory, including create, read, or write:
.Pp
.Xr lstat 2 ,
.Xr chmod 2 ,
.Xr chflags 2 ,
.Xr chown 2 ,
.Xr unlink 2 ,
.Xr fstat 2
.It Cm inet
The following system calls are allowed to operate in the
.Dv AF_INET
and
.Dv AF_INET6
domains
(though
.Xr setsockopt 2
has been substantially reduced in functionality):
.Pp
.Xr socket 2 ,
.Xr listen 2 ,
.Xr bind 2 ,
.Xr connect 2 ,
.Xr accept4 2 ,
.Xr accept 2 ,
.Xr getpeername 2 ,
.Xr getsockname 2 ,
.Xr setsockopt 2 ,
.Xr getsockopt 2
.It Cm mcast
In combination with
.Cm inet
give back functionality to
.Xr setsockopt 2
for operating on multicast sockets.
.It Cm fattr
The following system calls are allowed to make explicit changes
to fields in
.Vt struct stat
relating to a file:
.Pp
.Xr utimes 2 ,
.Xr futimes 2 ,
.Xr utimensat 2 ,
.Xr futimens 2 ,
.Xr chmod 2 ,
.Xr fchmod 2 ,
.Xr fchmodat 2 ,
.Xr chflags 2 ,
.Xr chflagsat 2 ,
.Xr chown 2 ,
.Xr fchownat 2 ,
.Xr lchown 2 ,
.Xr fchown 2 ,
.Xr utimes 2
.It Cm chown
The
.Xr chown 2
family is allowed to change the user or group on a file.
.It Cm flock
File locking via
.Xr fcntl 2 ,
.Xr flock 2 ,
.Xr lockf 3 ,
and
.Xr open 2
is allowed.
No distinction is made between shared and exclusive locks.
This promise is required for unlock as well as lock.
.It Cm unix
The following system calls are allowed to operate in the
.Dv AF_UNIX
domain:
.Pp
.Xr socket 2 ,
.Xr listen 2 ,
.Xr bind 2 ,
.Xr connect 2 ,
.Xr accept4 2 ,
.Xr accept 2 ,
.Xr getpeername 2 ,
.Xr getsockname 2 ,
.Xr setsockopt 2 ,
.Xr getsockopt 2
.It Cm dns
Subsequent to a successful
.Xr open 2
of
.Pa /etc/resolv.conf ,
a few system calls become able to allow DNS network transactions:
.Pp
.Xr sendto 2 ,
.Xr recvfrom 2 ,
.Xr socket 2 ,
.Xr connect 2
.It Cm getpw
This allows read-only opening of files in
.Pa /etc
for the
.Xr getpwnam 3 ,
.Xr getgrnam 3 ,
.Xr getgrouplist 3 ,
and
.Xr initgroups 3
family of functions, including lookups via the
.Xr yp 8
protocol for YP and LDAP databases.
.It Cm sendfd
Allows sending of file descriptors using
.Xr sendmsg 2 .
File descriptors referring to directories may not be passed.
.It Cm recvfd
Allows receiving of file descriptors using
.Xr recvmsg 2 .
File descriptors referring to directories may not be passed.
.It Cm tape
Allow
.Dv MTIOCGET
and
.Dv MTIOCTOP
operations against tape drives.
.It Cm tty
In addition to allowing read-write operations on
.Pa /dev/tty ,
this opens up a variety of
.Xr ioctl 2
requests used by tty devices.
If
.Cm tty
is accompanied with
.Cm rpath ,
.Xr revoke 2
is permitted.
Otherwise only the following
.Xr ioctl 2
requests are permitted:
.Pp
.Dv TIOCSPGRP ,
.Dv TIOCGETA ,
.Dv TIOCGPGRP ,
.Dv TIOCGWINSZ ,
.Dv TIOCSWINSZ ,
.Dv TIOCSBRK ,
.Dv TIOCCDTR ,
.Dv TIOCSETA ,
.Dv TIOCSETAW ,
.Dv TIOCSETAF ,
.Dv TIOCUCNTL
.It Cm proc
Allows the following process relationship operations:
.Pp
.Xr fork 2 ,
.Xr vfork 2 ,
.Xr kill 2 ,
.Xr getpriority 2 ,
.Xr setpriority 2 ,
.Xr setrlimit 2 ,
.Xr setpgid 2 ,
.Xr setsid 2
.It Cm exec
Allows a process to call
.Xr execve 2 .
Coupled with the
.Cm proc
promise, this allows a process to fork and execute another program.
If
.Fa 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
.Er EACCES .
Otherwise the new program starts running without pledge active,
and hopefully makes a new pledge soon.
.It Cm prot_exec
Allows the use of
.Dv PROT_EXEC
with
.Xr mmap 2
and
.Xr mprotect 2 .
.It Cm settime
Allows the setting of system time, via the
.Xr settimeofday 2 ,
.Xr adjtime 2 ,
and
.Xr adjfreq 2
system calls.
.It Cm ps
Allows enough
.Xr sysctl 2
interfaces to allow inspection of processes operating on the system using
programs like
.Xr ps 1 .
.It Cm vminfo
Allows enough
.Xr sysctl 2
interfaces to allow inspection of the system's virtual memory by
programs like
.Xr top 1
and
.Xr vmstat 8 .
.It Cm id
Allows the following system calls which can change the rights of a
process:
.Pp
.Xr setuid 2 ,
.Xr seteuid 2 ,
.Xr setreuid 2 ,
.Xr setresuid 2 ,
.Xr setgid 2 ,
.Xr setegid 2 ,
.Xr setregid 2 ,
.Xr setresgid 2 ,
.Xr setgroups 2 ,
.Xr setlogin 2 ,
.Xr setrlimit 2 ,
.Xr getpriority 2 ,
.Xr setpriority 2 ,
.Xr setrtable 2
.It Cm pf
Allows a subset of
.Xr ioctl 2
operations on the
.Xr pf 4
device:
.Pp
.Dv DIOCADDRULE ,
.Dv DIOCGETSTATUS ,
.Dv DIOCNATLOOK ,
.Dv DIOCRADDTABLES ,
.Dv DIOCRCLRADDRS ,
.Dv DIOCRCLRTABLES ,
.Dv DIOCRCLRTSTATS ,
.Dv DIOCRGETTSTATS ,
.Dv DIOCRSETADDRS ,
.Dv DIOCXBEGIN ,
.Dv DIOCXCOMMIT
.It Cm route
Allow inspection of the routing table.
.It Cm wroute
Allow changes to the routing table.
.It Cm audio
Allows a subset of
.Xr ioctl 2
operations on
.Xr audio 4
devices
(see
.Xr sio_open 3
for more information):
.Pp
.Dv AUDIO_GETPOS ,
.Dv AUDIO_GETPAR ,
.Dv AUDIO_SETPAR ,
.Dv AUDIO_START ,
.Dv AUDIO_STOP ,
.Dv AUDIO_MIXER_DEVINFO ,
.Dv AUDIO_MIXER_READ ,
.Dv AUDIO_MIXER_WRITE
.It Cm video
Allows a subset of
.Xr ioctl 2
operations on
.Xr video 4
devices:
.Pp
.Dv VIDIOC_DQBUF ,
.Dv VIDIOC_ENUM_FMT ,
.Dv VIDIOC_ENUM_FRAMEINTERVALS ,
.Dv VIDIOC_ENUM_FRAMESIZES ,
.Dv VIDIOC_G_CTRL ,
.Dv VIDIOC_G_PARM ,
.Dv VIDIOC_QBUF ,
.Dv VIDIOC_QUERYBUF ,
.Dv VIDIOC_QUERYCAP ,
.Dv VIDIOC_QUERYCTRL ,
.Dv VIDIOC_S_CTRL ,
.Dv VIDIOC_S_FMT ,
.Dv VIDIOC_S_PARM ,
.Dv VIDIOC_STREAMOFF ,
.Dv VIDIOC_STREAMON ,
.Dv VIDIOC_TRY_FMT ,
.Dv VIDIOC_REQBUFS
.It Cm bpf
Allow
.Dv BIOCGSTATS
operation for statistics collection from a
.Xr bpf 4
device.
.It Cm unveil
Allow
.Xr unveil 2
to be called.
.It Cm error
Rather than killing the process upon violation, indicate error with
.Er ENOSYS .
.Pp
Also when
.Fn pledge
is called with higher
.Fa promises
or
.Fa execpromises ,
those changes will be ignored and return success.
This is useful when a parent enforces
.Fa execpromises
but an execve'd child has a different idea.
.El
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn pledge
will fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
.Fa promises
or
.Fa execpromises
points outside the process's allocated address space.
.It Bq Er EINVAL
.Fa promises
is malformed or contains invalid keywords.
.It Bq Er EPERM
This process is attempting to increase permissions.
.El
.Sh HISTORY
The
.Fn pledge
system call first appeared in
.Ox 5.9 .