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Add privsep article.

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Florian Obser 2023-02-18 18:21:51 +01:00
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#+TITLE: Privilege drop, privilege separation, and restricted-service operating mode in OpenBSD
#+DATE: 2023-01-30
* Prologue
My main focus in OpenBSD are privilege separated network daemons
running in restricted-service operation mode. I gave talks at [[https://www.bsdcan.org][BSDCan]]
and [[https://fosdem.org][FOSDEM]] in the [[file:index.org::*External Writings & Presentations][past]] about how I used these techniques to write
[[https://man.openbsd.org/slaacd.8][slaacd(8)]] and [[https://man.openbsd.org/unwind.8][unwind(8)]]. While I do not think of myself as a one-trick
pony, I have written some more: [[https://man.openbsd.org/slowcgi.8][slowcgi(8)]], [[https://man.openbsd.org/rad.8][rad(8)]], [[https://man.openbsd.org/dhcpleased.8][dhcpleased(8)]], and
[[https://github.com/fobser/gelatod][gelatod(8)]]. I also wrote the first version of what later turned into
[[https://man.openbsd.org/resolvd.8][resolvd(8)]].
At one point I claimed that it would take me about a week to
transmogrify one daemon into a new one.
* Why
Privilege drop, privilege separation, and restricted-service operating
mode are exploit mitigations. When[fn:: not if!] an attacker finds a
bug we try to stop them from causing damage. The mitigations we are
talking about here are aimed at attackers that achieved arbitrary
code execution. Due to other [[https://www.openbsd.org/innovations.html][mitigations]] that is quite difficult to
pull off. These are the last line of defence. We try to remove as many
resources from the attacker to play with and try to crash the program
as quickly as possible if an attacker touches something they are not
supposed to.
* Privilege drop
Privilege drop is probably the weakest mitigation discussed in this
article. It is a very old technique, but still important for
set-user-ID root binaries.
Theo de Raadt [[http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sbin/ping/ping.c.diff?r1=1.6&r2=1.7][refactored]] [[https://man.openbsd.org/ping.8][ping(8)]] over 26 years ago to open a raw
socket early and then drop root privileges. This prevents a local user
from elevating their privileges when finding a bug in ping(8):
#+begin_src diff
@@ -191,6 +191,14 @@
char rspace[3 + 4 * NROUTES + 1]; /* record route space */
#endif
+ if (!(proto = getprotobyname("icmp")))
+ errx(1, "unknown protocol icmp");
+ if ((s = socket(AF_INET, SOCK_RAW, proto->p_proto)) < 0)
+ err(1, "socket");
+
+ /* revoke privs */
+ setuid(getuid());
+
preload = 0;
datap = &outpack[8 + sizeof(struct timeval)];
while ((ch = getopt(argc, argv, "DI:LRS:c:dfh:i:l:np:qrs:T:t:vw:")) != EOF)
@@ -235,6 +243,8 @@
loop = 0;
break;
case 'l':
+ if (getuid() != 0)
+ errx(1, "must be root to specify preload");
preload = strtol(optarg, NULL, 0);
if (preload < 0)
errx(1, "bad preload value: %s", optarg);
@@ -323,12 +333,6 @@
*datap++ = i;
ident = getpid() & 0xFFFF;
-
- if (!(proto = getprotobyname("icmp")))
- errx(1, "unknown protocol icmp");
- if ((s = socket(AF_INET, SOCK_RAW, proto->p_proto)) < 0)
- err(1, "socket");
- hold = 1;
if (options & F_SADDR) {
if (IN_MULTICAST(ntohl(to->sin_addr.s_addr)))
#+end_src
20 years later we realized that we would not drop privileges when
ping(8) is invoked as root. We would just "drop" from root to root. We
can protect ourselves from a malicious ping target by [[http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sbin/ping/ping.c.diff?r1=1.214&r2=1.215][dropping]] to a
dedicated user:
#+begin_src diff
@@ -272,8 +275,12 @@
/* revoke privs */
uid = getuid();
- if (setresuid(uid, uid, uid) == -1)
- err(1, "setresuid");
+ if ((pw = getpwnam(PING_USER)) == NULL)
+ errx(1, "no %s user", PING_USER);
+ if (setgroups(1, &pw->pw_gid) ||
+ setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) ||
+ setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid))
+ err(1, "unable to revoke privs");
preload = 0;
datap = &outpack[ECHOLEN + ECHOTMLEN];
#+end_src
ping(8) needs a raw socket to be able to send ICMP echo request
packets. This is an operation that only root is allowed to do[fn::This
prevents normal users from sending arbitrary IP packets, for example
with spoofed IP addresses or from privileged (<1024) source
ports.]. Once that socket is open though, ping(8) no longer needs to do
any other privileged operation. It can hold on to the socket for later
use and drop root privileges.
Another use for privilege drop is in daemons to restrict file-system
access by [[https://man.openbsd.org/chroot.2][chroot(2)]]'ing to =/var/empty=. The daemon needs root
privileges to call chroot(2), but afterwards it can run without
elevated permissions. To the process it looks like there is only the
=/= directory where it does not have any permissions.
The standard pattern can be seen in [[https://github.com/openbsd/src/blob/master/usr.sbin/rad/frontend.c#L200][frontend.c]] of [[https://man.openbsd.org/rad.8][rad(8)]]:
#+begin_src C
if ((pw = getpwnam(RAD_USER)) == NULL)
fatal("getpwnam");
if (chroot(pw->pw_dir) == -1)
fatal("chroot");
if (chdir("/") == -1)
fatal("chdir(\"/\")");
if (setgroups(1, &pw->pw_gid) ||
setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) ||
setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid))
fatal("can't drop privileges");
#+end_src
We first get a user with [[https://man.openbsd.org/getpwnam.3][getpwnam(3)]] to drop to. The user has
=/var/empty= configured as its home directory so we can use that in
chroot(2). Next we [[https://man.openbsd.org/chdir.2][chdir(2)]] to the new file-system root to have a
valid current working directory. This prevents us accidentally marking
a file-system as busy depending on from where the daemon was started,
preventing unmounting file-systems while the daemon is running.
We then drop privileges by putting the user into a single group using
[[https://man.openbsd.org/setgroups.2][setgroups(2)]]. The calls to [[https://man.openbsd.org/setresuid.2][setresgid(2) and setresuid(2)]] set the
real, effective and saved group and user IDs. This safely drops from
=root:wheel= to (in this case) =_rad:_rad= with no way to escalate
back to =root=.
This technique is probably used in most, if not all, OpenBSD's
privilege separated daemons.
* Restricted-service operating mode
With privilege drop in ping(8) we prevent a local unprivileged user to
gain superuser or root privileges. If there were a [[https://www.freebsd.org/security/advisories/FreeBSD-SA-22:15.ping.asc][bug in message
parsing]][fn::I do not want to heckle FreeBSD, it is just that it is a
good illustration for what we are currently discussing. FreeBSD's
ping(8) is using capsicum, so it is well locked away, too. And it is
not like I am not making any [[https://ftp.openbsd.org/pub/OpenBSD/patches/7.0/common/017_slaacd.patch.sig][mistakes]]...], a malicious ping target, or
even host in the middle, could still read and exfiltrate ssh
private keys. ping(8) runs as my user-id. It can read all files my
user can read, it can open network connections to any host on the
internet, it can execute arbitrary programs, heck it can talk to my
GPU. That is a lot of power that it does not need. It only needs to
write to the terminal and send and receive ICMP packets.
We could lock ping(8) away using chroot(2), that at least takes away
file-system access. But what can we do about programs that need
file-system access but should not execute other programs or talk to
the Internet? Like [[https://man.openbsd.org/file.1][file(1)]] for example.
Decades ago Niels Provos developed [[http://man.openbsd.org/OpenBSD-5.9/man4/systrace.4][systrace(4)]] but it turned out that
it was difficult to use. The only user in OpenBSD base was [[http://cvsweb.openbsd.org/src/usr.bin/ssh/Attic/sandbox-systrace.c?rev=1.18&content-type=text/x-cvsweb-markup][sshd(8)]].
In 2015 Theo de Raadt tricked Nicholas Marriott into privilege
separating and sand-boxing file(1) using systrace. [[http://cvsweb.openbsd.org/src/usr.bin/file/Attic/sandbox.c][It lasted for half
a year]], it was that painful.
One problem with systrace(4) was that it worked on the level of
syscalls and their arguments. This is not something user-land
developers are intimately familiar with. We are interacting with libc
and do not know what kind of syscalls libc does on our behalf. Another
issue is that a program might need some [[https://man.openbsd.org/ioctl.2][ioctl(2)]]s or [[https://man.openbsd.org/sysctl.2][sysctl(2)]]s but it
should not be able to do all of them. So we need to encode
restrictions on arguments of syscalls. This gets unwieldy, fast.
There was also [[http://man.openbsd.org/OpenBSD-5.9/systrace.1][systrace(1)]] to define a policy outside of the
program. It turns out that most programs need to do some sort of
initialization where they need wide access to the system. This is
before they touch untrusted data. Once the initialization is done we
can restrict access. How much we can restrict access can depend on
command line flags. systrace(1) could not help with this, the program
would retain all the privileges it needed for initialization. They
would be fewer than all privileges, but still way to many.
As far as I know, the experience with file(1) was the last straw. Theo
set out to improve on this situation by developing tame(2), which was
later renamed to [[https://man.openbsd.org/pledge.2][pledge(2)]][fn::It turned out that it was difficult to
use tame(2) in a sentence when presenting the concept, hence the
rename to pledge(2).].
pledge(2) was developed by studying all programs in OpenBSD base and
putting their needed services into categories using broad strokes like
/memory management/, /read-write on open file descriptors/, /opening
of files/, or /networking/. If a program violates what it pledged to
do, for example trying to open a file when it did not pledge =rpath=,
it will be terminated with an uncatchable =SIGABRT=.
It is worth repeating that: If a program violates what it pledged to
do it will be *terminated* by the kernel. An attacker does not get to
play again and try something else[fn::Needless to say that I despise
init systems that restart services when they crash.].
This was an iterative process with patches floating around. A few
co-conspirators, including myself, joined the effort a bit later to
add pledge(2) to more programs. Once we hit 50 or so pledged programs,
Theo considered it mature enough for commit and work continued in
tree. Soon after, the list of programs not pledged at all was shorter
than the list of pledged programs. This is a huge success and speaks
to the usability of pledge(2). In decades we only had one program
using systrace(4) in OpenBSD base, then pledge(2) shows up and in less
than a year nearly all of OpenBSD base uses it.
To add pledge(2) to a program we need to know what it does and
potentially re-factor it to pull (hoist) one-time initialization up
before pledge(2) is called for the first time. Some programs are
sloppy in the sense that they open a certain resource the moment they
need it, this means that they retain more access than they need. As
we have seen with ping(8), if we pull opening of the raw socket before
option parsing we can drop root privileges before touching untrusted
data[fn::With a set-user-ID root program command line options are
untrusted data!].
Since pledge(2) is internal to the program we can call it once we are
done with option parsing and pledge different things depending on
given options. [[https://github.com/openbsd/src/blob/master/sbin/ping/ping.c#L770][For example, ping(8) retains the ability to do DNS
lookups depending on the =-n= flag]]:
#+begin_src C
if (options & F_HOSTNAME) {
if (pledge("stdio inet dns", NULL) == -1)
err(1, "pledge");
} else {
if (pledge("stdio inet", NULL) == -1)
err(1, "pledge");
}
#+end_src
pledge(2) is not fine-grained. It turns out that programs fall into
broad categories of what they want to do after initialization. There
are not hundreds of different promises for every obscure program, it
is not needed. To add a new promise, a rule of thumb is: At least two
programs have been identified that need a new promise. To add a
syscall to an existing promise, that is, to give more power to an
existing promise, needs careful evaluation of what all the other
programs already using the promise, gain. It is not enough to show
that it is fine for the new program, existing programs are much more
important. Another question is how much additional kernel attack
surface this exposes.
pledge(2) does not only protect the user of the system or systems on
the Internet from harm when a bug is found, it also protects the
kernel from user-land. Checking if a syscall is allowed happens early,
before a lot of kernel code runs.
pledge(2) can be used to gain understanding on what a program
does. We see the following pledges in file(1):
#+begin_src shell
$ cat -n file.c | fgrep 'pledge("'
171 if (pledge("stdio rpath getpw recvfd sendfd id proc", NULL) == -1)
210 if (pledge("stdio rpath sendfd", NULL) == -1)
374 if (pledge("stdio getpw recvfd id", NULL) == -1)
389 if (pledge("stdio recvfd", NULL) == -1)
#+end_src
The reader is encouraged to stop here and read [[https://github.com/openbsd/src/blob/master/usr.bin/file/file.c][the code]] around those
pledge(2) calls to figure out what file does and why it pledges those
things.
------------------------------------------------------------------------
On line 171, file(1) pledges all the things it needs. It can
read-write already open file descriptors[fn::It can only read if the
file descriptor was opened for reading and only write if the file
descriptor was opened for writing. Since file(1) cannot open files for
writing or create new files, it cannot write to disk.], open files for
reading, pass file descriptors around, figure out under which user it
runs and lookup another user. It can also fork itself.
On line 210 it forked itself and we are in the parent process. It can
shed all pledges that the forked child needs for its initialization as
well as the ability to fork another instance. The parent process can
now only open files for reading, pass those file descriptors to the
child process and read-write already open file descriptors.
On line 374 we are in the child process and we shed all pledges that
the parent needed. We need to be able to read-write open file
descriptors and receive file descriptors from the parent
process. During initialization we need to know if we are running
as root and be able to look up a user. file(1) can privilege drop to a
dedicated user when invoked as root! Once we are done with that, the
child process, which does all the magic[fn:: Pun intended.], sheds all
those additional privileges and on 389 it can only read-write existing
file descriptors and receive new file descriptors from the parent
process.
We see file(1) is privilege separated, running two processes. One
process opens files but does not look at the contents. The other
process, which is completely locked away, parses untrusted data and
informs the parent process about the result.
Neither process can talk to the Internet or write to disk. They cannot
create new files or open existing files for writing. While it can
read ssh private keys, it cannot exfiltrate them.
We can find out what file(1) can and cannot do from just those four
pledge lines, that is very powerful when starting a code review. They
can also be used to get a quick overview how file(1) operates by
reading the code adjacent to the pledge(2) calls, that is where
interesting stuff happens.
* Privilege separation
A single process design that pledges ~"stdio inet rpath"~ still has a
lot of attack surface. This is not good if it is a network daemon
running as root and enabled per default on all installations. Like
[[https://man.openbsd.org/dhcpleased.8][dhcpleased(8)]] for example. For starters it can read and exfiltrate ssh
private keys.
As we have seen with file(1), we can split up a program into multiple
communicating processes that each pledge less operations than the sum
of all pledges. We can move the risky operations of parsing untrusted
data to a process that does not have access to the Internet, nor the
file-system. That process will also not have any elevated privileges
to change the system configuration like configuring network addresses
or changing the routing table.
An attacker who finds a loophole into this least privileged process
will have a hard time creating havoc. They can only talk to more
privileged processes using a very narrow communication channel with
easy to parse[fn::When using the imsg framework, it is not even parsing, nor
data marshalling. It is raw C structs of a fixed, and known at compile
time, size.] messages.
* OpenBSD network daemons
We will look in detail at how dhcpleased(8) uses privilege separation
and pledge(2) to implement a DHCP client in a safe way. Other network
daemons follow a similar pattern and the reader should be able to
study the source code of rad(8), slaacd(8), and unwind(8) to find out
how those daemons work since they share a common ancestry.
** Overview
dhcpleased(8) uses three communicating processes to implement
privilege separation: /parent/, /engine/, and /frontend/. The /parent/
process retains its root privileges to make changes to the system like
configuring IP addresses. This process must be protected from the
outside world and we make sure that it does not interact with
untrusted data. The /frontend/ process interacts with the outside
world, for example by sending and receiving DHCP messages. But it does
not parse DHCP messages it receives because that is untrusted data. It
is the /engine/'s job to parse DHCP messages and implement the state
machine for the DHCP protocol. The /engine/ process is completely
locked away.
dhcpleased(8) has a [[https://man.openbsd.org/dhcpleased.conf.5][configuration file]], following the typical OpenBSD
syntax. For example, to send a custom vendor-class option in a
~DHCPDISCOVER~ or ~DHCPREQUEST~ packet, the configuration looks like
this:
#+begin_src
interface vio0 {
send vendor class id "foobar"
}
#+end_src
Most configuration files on OpenBSD use this kind of syntax, from
[[https://man.openbsd.org/cwmrc.5][cwmrc(5)]] to [[https://man.openbsd.org/pf.conf.5][pf.conf(5)]].
It also comes with a control program, [[https://man.openbsd.org/dhcpleasectl.8][dhcpleasectl(8)]], to interact
with the running daemon. Many OpenBSD daemons provide a similar tool
to interact with them.
These are the features that come for free when using an existing
OpenBSD network daemon as a template to write a new one: It will be
privilege separated and using pledge(2) as a guide on how
functionality should be split up between the processes. It will have a
configuration file that uses a syntax that people familiar with
OpenBSD will find easy to use. And there is a control process to
interact with the running daemon. And finally there is a logging
framework that handles logging to syslog or =stderr=. All this
scaffolding and tooling is already there, we just need to swap out the
specific code the old daemon uses and replace them with something new.
** Initialization
dhcpleased(8) comes to life in [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.c#L144][void main(int argc, char *argv[]) in
dhcpleased.c]]. After a bit of house keeping and argument parsing it
ends up [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.c#L240][creating communications channels]] to talk to the
/frontend/ and /engine/ processes and starts those two child
processes:
#+begin_src C
if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
PF_UNSPEC, pipe_main2frontend) == -1)
fatal("main2frontend socketpair");
if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
PF_UNSPEC, pipe_main2engine) == -1)
fatal("main2engine socketpair");
/* Start children. */
engine_pid = start_child(PROC_ENGINE, saved_argv0, pipe_main2engine[1],
debug, verbose);
frontend_pid = start_child(PROC_FRONTEND, saved_argv0,
pipe_main2frontend[1], debug, verbose);
#+end_src
~start​_child()~ [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.c#L401][ensures]] that after forking the file descriptor the
child process can use to talk to the parent process has number three:
#+begin_src C
if (fd != 3) {
if (dup2(fd, 3) == -1)
#+end_src
It then [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.c#L407][sets up]] an =argv= array for [[https://man.openbsd.org/execv.3][execvp(3)]] to re-exec itself. The
flags =-E= and =-F= control if the child process runs as /frontend/ or
/engine/ process:
#+begin_src C
argv[argc++] = argv0;
switch (p) {
case PROC_MAIN:
fatalx("Can not start main process");
case PROC_ENGINE:
argv[argc++] = "-E";
break;
case PROC_FRONTEND:
argv[argc++] = "-F";
break;
}
if (debug)
argv[argc++] = "-d";
if (verbose)
argv[argc++] = "-v";
if (verbose > 1)
argv[argc++] = "-v";
argv[argc++] = NULL;
execvp(argv0, argv);
#+end_src
Using fork & exec ensures that the child processes get a different
memory layout. If there is an information leak in one process it
cannot be used by an attacker to find gadgets in a different,
potentially more privileged process.
Going back to the main function, [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.c#L200][after option parsing]] we know if we
are still in the parent process or in /engine/ or /frontend/ process:
#+begin_src C
if (engine_flag)
engine(debug, verbose);
else if (frontend_flag)
frontend(debug, verbose);
#+end_src
The ~engine()~ and ~frontend()~ functions live in [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/engine.c#L177][engine.c]] and
[[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/frontend.c#L131][frontend.c]] respectively. Neither returns to the =main()= function.
In the initialization of the child processes we drop privileges and
pledge what the process needs to run. The /engine/ process pledges
=stdio recvfd= and the /frontend/ process =stdio unix recvfd route=.
We then set-up the [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/frontend.c#L180][communication channel]] to the /parent/ (also known
as the /main/) process:
#+begin_src
imsg_init(&iev_main->ibuf, 3);
iev_main->handler = frontend_dispatch_main;
#+end_src
As mentioned before we force the file descriptor to number 3 in
~start​_child()~ so the child process knows how it can reach the
/parent/ process. We are using [[https://man.openbsd.org/event_init.3][libevent]] to call functions when an
event happens on a file descriptor. Here ~frontend​_dispatch​_main~ is
called when we receive a message from the /parent/ process.
There is also a function ~frontend​_dispatch​_engine~ for messages from
the /engine/ process. The naming scheme is
~RECEIVINGPROCESS​_dispatch​_SENDINGPROCESS~ and there are functions in
=engine.c= and =dhpleased.c= to have a full mesh of communication
channels between all three processes.
Now that we have started the child processes and hooked up the
communication channels between /parent/ and /frontend/ as well as
/parent/ and /engine/, it is time to send our first message from the
/parent/ to both children. The child processes pledged =recvfd=, which
allows them to receive open file descriptors over an existing open
file descriptor. The /parent/ process calls
[[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.c#L292][main​_imsg​_send​_ipc​_sockets()]] to create another socket pair and pass
the end points to /engine/ and /frontend/ to create a full mesh.
The file descriptor is received by [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/engine.c#L387][engine​_dispatch​_main()]] and
[[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/frontend.c#L232][frontend​_dispatch​_main()]] using a message type of =IMSG​_SOCKET​_IPC=.
After receiving this file descriptor, /engine/ can [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/engine.c#L446][drop]] the =recvfd=
pledge and only pledges =stdio=. It no longer expects any more file
descriptors.
The start-up of /frontend/ is a bit more complicated. It needs to
receive a route socket from the /parent/ process to learn of
interfaces gaining or losing the =autoconf= flag during runtime. Once
it received the =route= socket [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/frontend.c#L329][from the parent process]] it can get a
list of all interfaces that already have the =autoconf= flag and drop
the =route= pledge in [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/frontend.c#L650][frontend​_startup()]] afterwards:
#+begin_src C
frontend_startup(void)
{
if (!event_initialized(&ev_route))
fatalx("%s: did not receive a route socket from the main "
"process", __func__);
init_ifaces();
if (pledge("stdio unix recvfd", NULL) == -1)
fatal("pledge");
event_add(&ev_route, NULL);
}
#+end_src
It still needs to hold on to =unix= for communication with
dhcpleasectl(8) and =recvfd= for receiving [[https://man.openbsd.org/bpf.4][bpf(4)]] sockets when new
interfaces are set to =autoconf= with ifconfig(8).
The astute reader will notice that we have not talked about pledging
the /parent/ process. Unfortunately that is not possible because there
is no pledge that would allow opening and programming a new bpf(4)
socket and we need to create a new one when an interface is set to
=autoconf= while dhcpleased(8) is already running.
However, not all is lost. The parent process, which has to keep
running as root, is not touching any untrusted data. An attacker needs
to go through the /frontend/ or /engine/ process to gain a foothold in
the /parent/ process. And they need to do this via the
=main​_dispatch​_frontend= and =main​_dispatch​_engine= functions. We will
look at those in a bit to see why that is a very difficult
proposition.
But not all is lost. We can restrict the amount of havoc an attacker
can cause if they ever get all the way to the /parent/ process using
[[https://man.openbsd.org/unveil.2][unveil(2)]]:
#+begin_src
if (unveil(conffile, "r") == -1)
fatal("unveil %s", conffile);
if (unveil("/dev/bpf", "rw") == -1)
fatal("unveil /dev/bpf");
if (unveil(_PATH_LEASE, "rwc") == -1) {
no_lease_files = 1;
log_warn("disabling lease files, unveil " _PATH_LEASE);
}
if (unveil(NULL, NULL) == -1)
fatal("unveil");
#+end_src
It turns out that the parent process needs very little access to the
file system. It needs access to the dhcpleased(8) config file, the
bpf(4) device and the directory where lease files are stored. That
list of files and directory does not include access to ssh private
keys.
** dispatching
As we said the /parent/ process is not touching any untrusted data,
that is left to the /frontend/ and /engine/ process. The /frontend/
and /engine/ processes send imsg messages to the /parent/
process. Let's have a look at how those messages arrive and what the
/parent/ process does with them.
Messages from the /frontend/ process arrive at
[[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.c#L431][main​_dispatch​_frontend()]] in dhcpleased.c:
#+begin_src C
void
main_dispatch_frontend(int fd, short event, void *bula)
{
// [...]
uint32_t if_index;
#ifndef SMALL
int verbose;
#endif /* SMALL */
// [...]
for (;;) {
if ((n = imsg_get(ibuf, &imsg)) == -1)
fatal("imsg_get");
if (n == 0) /* No more messages. */
break;
switch (imsg.hdr.type) {
case IMSG_OPEN_BPFSOCK:
if (IMSG_DATA_SIZE(imsg) != sizeof(if_index))
fatalx("%s: IMSG_OPEN_BPFSOCK wrong length: "
"%lu", __func__, IMSG_DATA_SIZE(imsg));
memcpy(&if_index, imsg.data, sizeof(if_index));
open_bpfsock(if_index);
break;
#ifndef SMALL
case IMSG_CTL_RELOAD:
if (main_reload() == -1)
log_warnx("configuration reload failed");
else
log_warnx("configuration reloaded");
break;
case IMSG_CTL_LOG_VERBOSE:
if (IMSG_DATA_SIZE(imsg) != sizeof(verbose))
fatalx("%s: IMSG_CTL_LOG_VERBOSE wrong length: "
"%lu", __func__, IMSG_DATA_SIZE(imsg));
memcpy(&verbose, imsg.data, sizeof(verbose));
log_setverbose(verbose);
break;
#endif /* SMALL */
case IMSG_UPDATE_IF:
if (IMSG_DATA_SIZE(imsg) != sizeof(imsg_ifinfo))
fatalx("%s: IMSG_UPDATE_IF wrong length: %lu",
__func__, IMSG_DATA_SIZE(imsg));
memcpy(&imsg_ifinfo, imsg.data, sizeof(imsg_ifinfo));
read_lease_file(&imsg_ifinfo);
main_imsg_compose_engine(IMSG_UPDATE_IF, -1,
&imsg_ifinfo, sizeof(imsg_ifinfo));
break;
default:
log_debug("%s: error handling imsg %d", __func__,
imsg.hdr.type);
break;
}
imsg_free(&imsg);
}
// [...]
}
#+end_src
We see that it handles four message types =IMSG​_OPEN​_BPFSOCK=,
=IMSG​_CTL​_RELOAD=, =IMSG​_CTL​_LOG​_VERBOSE= and =IMSG​_UPDATE​_IF=.
One of them, =IMSG​_CTL​_RELOAD=, does not have any payload data, the
/parent/ process just performs a predefined action. The /frontend/
process cannot influence how the action is performed, only when it is
performed.
For the other three, the /frontend/ process sends a piece of data that
influences how the action is performed. The piece of data has a fixed,
known at compile time, length. For example =IMSG​_OPEN​_BPFSOCK= is send
by the /frontend/ process when it learns of a new network interface
gaining the /autoconf/ flag. It needs a new bpf(4) socket to send an
receive DHCP messages on that interface[fn::bpf(4) sockets are bound
to a specific interface and then locked, meaning the /frontend/
process will not be able to change anything about the socket. That
also means that the /frontend/ process cannot send and receive
arbitrary messages but only those that match the bpf filter.]. It
sends a single =uint32_t= that uniquely identifies the network
interface in the system. If it sends something else, we assume the
/frontend/ process has been compromised and is no longer trusted. The
/parent/ process terminates itself:
#+begin_src C
if (IMSG_DATA_SIZE(imsg) != sizeof(if_index))
fatalx("%s: IMSG_OPEN_BPFSOCK wrong length: "
"%lu", __func__, IMSG_DATA_SIZE(imsg));
#+end_src
We are using the same pattern of checking the data size for all the
other message types send from the /frontend/ process. The same is true
for how [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.c#L514][main​_dispatch​_engine()]] handles the messages send by the
/engine/ process.
The /frontend/ process sends the =IMSG​_OPEN​_BPFSOCK= message using
[[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/frontend.c#L632][frontend​_imsg​_compose​_main()]] in =frontend.c=. The sending function
names follow a pattern of
~SENDINGPROCESS​_imsg​_compose​_RECEIVINGPROCESS~ and they are each a
thin wrapper around ~imsg​_compose​_event()~ defined in =dhcpleased.c=
which in turn is a thin wrapper around [[https://man.openbsd.org/imsg_init.3][imsg​_compose(3).]]
A good way to understand what dhcpleased(8) is doing is searching
for where all the [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/dhcpleased.h#L194][imsg​_types]] are sent and received.
** Configuration file, control process and logging framework.
We will not go into detail on how these features are implemented. They
are not that relevant to the topic of privilege separation and the
author is not an expert in LR(1) grammars or [[https://man.openbsd.org/yacc.1][yacc(1)]]. We will give
some pointers to get interested readers started to learn on their own.
The parsed configuration file is stored and passed around in a ~struct
dhcpleased​_conf~ structure. The entry point into the parser is
~parse​_config()~ in [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/parse.y#L722][parse.y]]. parse.y contains a hand written lexer as
well as the yacc(1) grammar. Changing or adding to the grammar of the
configuration file is done in three places:
1. New tokens are added to the grammar at the beginning of the file
using the =%token= keyword. Tokens are in all caps.
2. Adding rules to the grammar, using the defined tokens
3. Teaching the lexer about new tokens. This is done by adding to the
~keywords~ array in the [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/parse.y#L357][lookup()]] function.
Passing =-nv= flags to dhcpleased(8) runs a configuration test and
prints out the canonical form of the parsed configuration file. The
code for this lives in [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/printconf.c][printconf.c]]. Care should be taken to print a
valid configuration, i.e. one that can be passed to dhcpleased(8)
again.
The source code for the control process can be found in
[[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/usr.sbin/dhcpleasectl/dhcpleasectl.c][usr.sbin/dhcpleased/dhcpleasectl.c]]. Communication is done using
[[https://man.openbsd.org/unix.4][UNIX-domain sockets]] and connections are accepted by the /frontend/
process. Communication uses imsgs, and messages from the control
process are handled by [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/control.c#L221][control​_dispatch​_imsg()]] in control.c. The main
difference to the dispatch function between the dhcpleased(8)
processes is that messages with invalid data sizes are ignored instead
of exiting the daemon. This is done because everyone in the =wheel=
group can send messages to the daemon and we do not want them to be
able to make the daemon crash[fn::Or even everyone on the system in
case of unwind(8).]:
#+begin_src C
case IMSG_CTL_LOG_VERBOSE:
if (IMSG_DATA_SIZE(imsg) != sizeof(verbose))
break;
#+end_src
Here dhcpleased(8) expects single integer to set the verbosity of the
daemon. If we get something else we just ignore the message using
~break~.
Finally, the network daemons come with a logging framework that handles
logging to syslog or =stderr= when the daemons runs in the foreground
using =-d=. The code for this can be found in [[https://github.com/openbsd/src/blob/3c46ceeaef274bbef234dac63245c4b6567168d7/sbin/dhcpleased/log.c][log.c]].
* Epilogue
Writing software in C with security in mind can be a lot of fun when
standing on the shoulders of giants and having things like privilege
separation and restricted-service operating mode in your toolbox.
I wrote two daemons, dhcpleased(8) and slaacd(8), that are enabled by
default on every OpenBSD installation. We might eventually add a third
one. Having the mitigations shown here as well as all the other
mitigations constantly being added and enabled per default is what
lets me sleep at night. I am cautiously optimistic that when a bug is
found in dhcpleased(8) or slaacd(8) an attacker will have a hard time
pivoting to arbitrary code execution as root.