privsep: Add support for priviledge separation

Not enabled by default - enable with ./configure --enable-privsep
Requires a user added to the system - default _dhcpcd

Several processes will be spawned off the main state engine:
a privileged actioneer and a generic network proxy.
Only the privileged actioneer process will retain root permissions.

When required, the privileged actioneer will also spawn
BPF listeners for BOOTP (DHCP) and ARP.
The BOOTP BPF listener should be a short lived process.
On kernels with RFC 5227 support, the ARP BPF listener will only
be used for ARPing and announcing a preferred address and will
also be a short lived process.

When not running in master mode, an address listener will be
spawned for each address (with the exception of RA dervived addresses)
dhcpcd cares about.

TODO:
 * Solaris support.
 * ARP BPF address filtering.

1 files changed, 661 insertions, 0 deletions

diff --git a/src/privsep.c b/src/privsep.c
new file mode 100644
index 00000000..ed2a0a24
--- /dev/null
+++ b/src/privsep.c
@@ -0,0 +1,661 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+/*
+ * Priviledge Separation for dhcpcd
+ * Copyright (c) 2006-2019 Roy Marples <roy@marples.name>
+ * All rights reserved
+
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/*
+ * The current design is this:
+ * Spawn a priv process to carry out privileged actions and
+ * spawning unpriv process to initate network connections such as BPF
+ * or address specific listener.
+ * Spawn an unpriv process to send/receive common network data.
+ * Then drop all privs and start running.
+ */
+
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+#ifdef AF_LINK
+#include <net/if_dl.h>
+#endif
+
+#include <assert.h>
+#include <pwd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <grp.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "arp.h"
+#include "common.h"
+#include "control.h"
+#include "dhcp.h"
+#include "dhcp6.h"
+#include "eloop.h"
+#include "ipv6nd.h"
+#include "logerr.h"
+#include "privsep.h"
+
+#ifdef HAVE_UTIL_H
+#include <util.h>
+#endif
+
+pid_t
+ps_dostart(struct dhcpcd_ctx *ctx,
+    pid_t *priv_pid, int *priv_fd,
+    void (*recv_msg)(void *), void (*recv_unpriv_msg),
+    void *recv_ctx, int (*callback)(void *), void (*signal_cb)(int, void *),
+    unsigned int flags)
+{
+	struct passwd *pw;
+	int stype;
+	int fd[2];
+	pid_t pid;
+
+	ctx->options |= DHCPCD_PRIVSEP;
+
+	if (!(flags & PSF_DROPPRIVS)) {
+		pw = NULL;
+		goto create_sp;
+	}
+
+	errno = 0;
+	if ((pw = getpwnam(DHCPCD_USER)) == NULL) {
+		if (errno == 0) {
+			if (ctx == recv_ctx) /* Only log the once. */
+				logerrx("no such user %s", DHCPCD_USER);
+		} else
+			logerr("getpwnam");
+		ctx->options &= ~DHCPCD_PRIVSEP;
+		return -1;
+	}
+
+	if (priv_pid == NULL) {
+		gid_t gid = (gid_t)-1;
+
+		/* Main process - change ownership of stuff we need to
+		 * drop at exit. */
+		if (pw != NULL) {
+			if (chown(ctx->pidfile, pw->pw_uid, gid) == -1)
+				logerr("chown `%s'", ctx->pidfile);
+			if (chown(DBDIR, pw->pw_uid, gid) == -1)
+				logerr("chown `%s'", DBDIR);
+			if (chown(RUNDIR, pw->pw_uid, gid) == -1)
+				logerr("chown `%s'", RUNDIR);
+			if (ctx->options & DHCPCD_MASTER) {
+				if (chown(ctx->control_sock,
+				    pw->pw_uid, gid) == -1)
+					logerr("chown `%s'", ctx->control_sock);
+				if (chown(UNPRIVSOCKET, pw->pw_uid, gid) == -1)
+					logerr("chown `%s'", UNPRIVSOCKET);
+			}
+		}
+		goto dropprivs;
+	}
+
+create_sp:
+	stype = SOCK_CLOEXEC;
+	/* ps_root_fd needs to be a blocking socket so we can read
+	 * the result of the command sent. */
+	if (priv_fd != &ctx->ps_root_fd)
+		stype |= SOCK_NONBLOCK;
+	if (socketpair(AF_UNIX, SOCK_DGRAM | stype, 0, fd) == -1) {
+		logerr("socketpair");
+		return -1;
+	}
+
+	switch (pid = fork()) {
+	case -1:
+		logerr("fork");
+		return -1;
+	case 0:
+		*priv_fd = fd[1];
+		close(fd[0]);
+		break;
+	default:
+		*priv_pid = pid;
+		*priv_fd = fd[0];
+		close(fd[1]);
+		if (recv_unpriv_msg != NULL &&
+		    eloop_event_add(ctx->eloop, *priv_fd,
+		    recv_unpriv_msg, recv_ctx) == -1)
+		{
+			logerr("%s: eloop_event_add", __func__);
+			return -1;
+		}
+		return pid;
+	}
+
+	ctx->options |= DHCPCD_UNPRIV | DHCPCD_FORKED;
+	control_close(ctx);
+	pidfile_clean();
+	eloop_clear(ctx->eloop);
+
+	if (eloop_signal_set_cb(ctx->eloop,
+	    dhcpcd_signals, dhcpcd_signals_len, signal_cb, ctx) == -1)
+	{
+		logerr("%s: eloop_signal_set_cb", __func__);
+		goto errexit;
+	}
+	if (eloop_signal_mask(ctx->eloop, &ctx->sigset) == -1) {
+		logerr("%s: eloop_signal_mask", __func__);
+		goto errexit;
+	}
+
+	if (eloop_event_add(ctx->eloop, *priv_fd, recv_msg, recv_ctx) == -1)
+	{
+		logerr("%s: eloop_event_add", __func__);
+		goto errexit;
+	}
+
+	/* We are not root */
+	if (priv_fd != &ctx->ps_root_fd) {
+		ps_freeprocesses(ctx, recv_ctx);
+		close(ctx->ps_root_fd);
+		ctx->ps_root_fd = -1;
+	}
+
+	if (priv_fd != &ctx->ps_inet_fd) {
+		close(ctx->ps_inet_fd);
+		ctx->ps_inet_fd = -1;
+	}
+
+	if (callback(recv_ctx) == -1)
+		goto errexit;
+
+	if (pw == NULL)
+		return 0;
+
+	if (!(flags & PSF_DROPPRIVS)) {
+		if (chdir(pw->pw_dir) == -1)
+			logerr("%s: chdir `%s'", __func__, pw->pw_dir);
+		return 0;
+	}
+
+	if (chroot(pw->pw_dir) == -1)
+		logerr("%s: chroot `%s'", __func__, pw->pw_dir);
+
+dropprivs:
+	if (setgroups(1, &pw->pw_gid) == -1 ||
+	     setgid(pw->pw_gid) == -1 ||
+	     setuid(pw->pw_uid) == -1)
+		logerr("failed to drop privileges");
+
+	return 0;
+
+errexit:
+	/* Failure to start root or inet processes is fatal. */
+	if (priv_fd == &ctx->ps_root_fd || priv_fd == &ctx->ps_inet_fd)
+		ps_sendcmd(ctx, *priv_fd, PS_STOP, 0, NULL, 0);
+	shutdown(*priv_fd, SHUT_RDWR);
+	*priv_fd = -1;
+	return -1;
+}
+
+int
+ps_dostop(struct dhcpcd_ctx *ctx, pid_t *pid, int *fd)
+{
+	int status;
+
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: pid %d fd %d", __func__, *pid, *fd);
+#endif
+	if (*pid == 0)
+		return 0;
+	eloop_event_delete(ctx->eloop, *fd);
+	if (ps_sendcmd(ctx, *fd, PS_STOP, 0, NULL, 0) == -1 &&
+	    errno != ECONNRESET)
+		logerr(__func__);
+	if (shutdown(*fd, SHUT_RDWR) == -1)
+		logerr(__func__);
+	close(*fd);
+	*fd = -1;
+	/* We won't have permission for all processes .... */
+#if 0
+	if (kill(*pid, SIGTERM) == -1)
+		logerr(__func__);
+#endif
+	status = 0;
+	/* Wait for the process to finish */
+	while (waitpid(*pid, &status, 0) == -1) {
+		if (errno != EINTR) {
+			logerr("%s: waitpid", __func__);
+			status = 0;
+			break;
+		}
+#ifdef PRIVSEP_DEBUG
+		else
+			logerr("%s: waitpid ", __func__);
+#endif
+	}
+	*pid = 0;
+
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: status %d", __func__, status);
+#endif
+
+	return status;
+}
+
+int
+ps_start(struct dhcpcd_ctx *ctx)
+{
+	pid_t pid;
+
+	TAILQ_INIT(&ctx->ps_processes);
+
+	switch (pid = ps_root_start(ctx)) {
+	case -1:
+		return -1;
+	case 0:
+		return 0;
+	default:
+		logdebugx("spawned privileged actioneer on PID %d", pid);
+	}
+
+	/* No point in spawning the generic network listener if we're
+	 * not going to use it. */
+	if (!(ctx->options & (DHCPCD_MASTER | DHCPCD_IPV6RS)))
+		goto dropprivs;
+
+	switch (pid = ps_inet_start(ctx)) {
+	case -1:
+		if (errno == ENXIO)
+			return 0;
+		return -1;
+	case 0:
+		return 0;
+	default:
+		logdebugx("spawned network proxy on PID %d", pid);
+	}
+
+dropprivs:
+	/* Drop privs now. */
+	ps_dostart(ctx, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+	    PSF_DROPPRIVS);
+
+	return 1;
+}
+
+int
+ps_stop(struct dhcpcd_ctx *ctx)
+{
+	int r, ret = 0;
+
+	if (ctx->options & DHCPCD_FORKED || ctx->eloop == NULL)
+		return 0;
+
+	r = ps_root_stop(ctx);
+	if (r != 0)
+		ret = r;
+	r = ps_inet_stop(ctx);
+	if (r != 0)
+		ret = r;
+
+	ctx->options &= ~DHCPCD_PRIVSEP;
+	return ret;
+}
+
+void
+ps_freeprocess(struct ps_process *psp)
+{
+#ifdef INET
+	struct ipv4_state *istate = IPV4_STATE(&psp->psp_ifp);
+
+	if (istate != NULL) {
+		free(istate->buffer);
+		free(istate);
+	}
+#endif
+
+	TAILQ_REMOVE(&psp->psp_ctx->ps_processes, psp, next);
+	if (psp->psp_fd != -1)
+		close(psp->psp_fd);
+	free(psp);
+}
+
+static void
+ps_free(struct dhcpcd_ctx *ctx)
+{
+	struct ps_process *psp;
+	bool stop = ctx->ps_root_pid == getpid();
+
+	while ((psp = TAILQ_FIRST(&ctx->ps_processes)) != NULL) {
+		if (stop)
+			ps_dostop(ctx, &psp->psp_pid, &psp->psp_fd);
+		ps_freeprocess(psp);
+	}
+}
+
+int
+ps_unrollmsg(struct msghdr *msg, struct ps_msghdr *psm,
+    const void *data, size_t len)
+{
+	uint8_t *datap, *namep, *controlp;
+
+	namep = UNCONST(data);
+	controlp = namep + psm->ps_namelen;
+	datap = controlp + psm->ps_controllen;
+
+	if (psm->ps_namelen != 0) {
+		if (psm->ps_namelen > len) {
+			errno = EINVAL;
+			return -1;
+		}
+		msg->msg_name = namep;
+		len -= psm->ps_namelen;
+	} else
+		msg->msg_name = NULL;
+	msg->msg_namelen = psm->ps_namelen;
+
+	if (psm->ps_controllen != 0) {
+		if (psm->ps_controllen > len) {
+			errno = EINVAL;
+			return -1;
+		}
+		msg->msg_control = controlp;
+		len -= psm->ps_controllen;
+	} else
+		msg->msg_control = NULL;
+	msg->msg_controllen = psm->ps_controllen;
+
+	if (len != 0) {
+		msg->msg_iovlen = 1;
+		msg->msg_iov[0].iov_base = datap;
+		msg->msg_iov[0].iov_len = len;
+	} else {
+		msg->msg_iovlen = 0;
+		msg->msg_iov[0].iov_base = NULL;
+		msg->msg_iov[0].iov_len = 0;
+	}
+	return 0;
+}
+
+
+ssize_t
+ps_sendpsmmsg(struct dhcpcd_ctx *ctx, int fd,
+    struct ps_msghdr *psm, const struct msghdr *msg)
+{
+	assert(msg == NULL || msg->msg_iovlen == 1);
+
+	struct iovec iov[] = {
+		{ .iov_base = UNCONST(psm), .iov_len = sizeof(*psm) },
+		{ .iov_base = NULL, },	/* name */
+		{ .iov_base = NULL, },	/* control */
+		{ .iov_base = NULL, },	/* payload */
+	};
+	int iovlen = __arraycount(iov);
+	ssize_t len;
+
+	if (msg != NULL) {
+		struct iovec *iovp = &iov[1];
+
+		assert(msg->msg_iovlen == 1);
+
+		psm->ps_namelen = msg->msg_namelen;
+		psm->ps_controllen = (socklen_t)msg->msg_controllen;
+
+		iovp->iov_base = msg->msg_name;
+		iovp->iov_len = msg->msg_namelen;
+		iovp++;
+		iovp->iov_base = msg->msg_control;
+		iovp->iov_len = msg->msg_controllen;
+		iovp++;
+		iovp->iov_base = msg->msg_iov[0].iov_base;
+		iovp->iov_len = msg->msg_iov[0].iov_len;
+		iovlen = __arraycount(iov);
+	} else
+		iovlen = 1;
+
+	len = writev(fd, iov, iovlen);
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: %zd", __func__, len);
+#endif
+	if ((len == -1 || len == 0) && ctx->options & DHCPCD_FORKED)
+		eloop_exit(ctx->eloop, len == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
+	return len;
+}
+
+ssize_t
+ps_sendpsmdata(struct dhcpcd_ctx *ctx, int fd,
+    struct ps_msghdr *psm, const void *data, size_t len)
+{
+	struct iovec iov[] = {
+		{ .iov_base = UNCONST(data), .iov_len = len },
+	};
+	struct msghdr msg = {
+		.msg_iov = iov, .msg_iovlen = 1,
+	};
+
+	return ps_sendpsmmsg(ctx, fd, psm, &msg);
+}
+
+
+ssize_t
+ps_sendmsg(struct dhcpcd_ctx *ctx, int fd, uint8_t cmd, unsigned long flags,
+    const struct msghdr *msg)
+{
+	assert(msg->msg_iovlen == 1);
+
+	struct ps_msghdr psm = {
+		.ps_cmd = cmd,
+		.ps_flags = flags,
+		.ps_namelen = msg->msg_namelen,
+		.ps_controllen = (socklen_t)msg->msg_controllen,
+		.ps_datalen = msg->msg_iov[0].iov_len,
+	};
+
+	return ps_sendpsmmsg(ctx, fd, &psm, msg);
+}
+
+ssize_t
+ps_sendcmd(struct dhcpcd_ctx *ctx, int fd, uint8_t cmd, unsigned long flags,
+    const void *data, size_t len)
+{
+	struct ps_msghdr psm = {
+		.ps_cmd = cmd,
+		.ps_flags = flags,
+	};
+	struct iovec iov[] = {
+		{ .iov_base = UNCONST(data), .iov_len = len }
+	};
+	struct msghdr msg = {
+		.msg_iov = iov, .msg_iovlen = 1,
+	};
+
+	return ps_sendpsmmsg(ctx, fd, &psm, &msg);
+}
+
+static ssize_t
+ps_sendcmdmsg(int fd, uint8_t cmd, const struct msghdr *msg)
+{
+	struct ps_msghdr psm = { .ps_cmd = cmd };
+	uint8_t data[PS_BUFLEN], *p = data;
+	struct iovec iov[] = {
+		{ .iov_base = &psm, .iov_len = sizeof(psm) },
+		{ .iov_base = data, .iov_len = 0 },
+	};
+	size_t dl = sizeof(data);
+
+	if (msg->msg_namelen != 0) {
+		if (msg->msg_namelen > dl)
+			goto nobufs;
+		psm.ps_namelen = msg->msg_namelen;
+		memcpy(p, msg->msg_name, msg->msg_namelen);
+		p += msg->msg_namelen;
+		dl -= msg->msg_namelen;
+	}
+
+	if (msg->msg_controllen != 0) {
+		if (msg->msg_controllen > dl)
+			goto nobufs;
+		psm.ps_controllen = (socklen_t)msg->msg_controllen;
+		memcpy(p, msg->msg_control, msg->msg_controllen);
+		p += msg->msg_controllen;
+		dl -= msg->msg_controllen;
+	}
+
+	psm.ps_datalen = msg->msg_iov[0].iov_len;
+	if (psm.ps_datalen > dl)
+		goto nobufs;
+
+	iov[1].iov_len = psm.ps_namelen + psm.ps_controllen + psm.ps_datalen;
+	if (psm.ps_datalen != 0)
+		memcpy(p, msg->msg_iov[0].iov_base, psm.ps_datalen);
+	return writev(fd, iov, __arraycount(iov));
+
+nobufs:
+	errno = ENOBUFS;
+	return -1;
+}
+
+ssize_t
+ps_recvmsg(struct dhcpcd_ctx *ctx, int rfd, uint8_t cmd, int wfd)
+{
+	struct sockaddr_storage ss = { .ss_family = AF_UNSPEC };
+	uint8_t controlbuf[sizeof(struct sockaddr_storage)] = { 0 };
+	uint8_t databuf[64 * 1024];
+	struct iovec iov[] = {
+	    { .iov_base = databuf, .iov_len = sizeof(databuf) }
+	};
+	struct msghdr msg = {
+		.msg_name = &ss, .msg_namelen = sizeof(ss),
+		.msg_control = controlbuf, .msg_controllen = sizeof(controlbuf),
+		.msg_iov = iov, .msg_iovlen = 1,
+	};
+
+	ssize_t len = recvmsg(rfd, &msg, 0);
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: recv fd %d, %zd bytes", __func__, rfd, len);
+#endif
+	if ((len == -1 || len == 0) && ctx->options & DHCPCD_FORKED) {
+		eloop_exit(ctx->eloop, len == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
+		return len;
+	}
+
+	iov[0].iov_len = (size_t)len;
+	len = ps_sendcmdmsg(wfd, cmd, &msg);
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: send fd %d, %zu bytes", __func__, wfd, len);
+#endif
+	if ((len == -1 || len == 0) && ctx->options & DHCPCD_FORKED)
+		eloop_exit(ctx->eloop, len == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
+	return len;
+}
+
+ssize_t
+ps_recvpsmsg(struct dhcpcd_ctx *ctx, int fd,
+    ssize_t (*callback)(void *, struct ps_msghdr *, struct msghdr *),
+    void *cbctx)
+{
+	struct ps_msg psm;
+	ssize_t len;
+	size_t dlen;
+	struct iovec iov[1];
+	struct msghdr msg = { .msg_iov = iov, .msg_iovlen = 1 };
+	bool stop = false;
+
+	len = read(fd, &psm, sizeof(psm));
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: %zd", __func__, len);
+#endif
+
+	if (len == -1 && (errno == ECONNRESET || errno == EBADF))
+		len = 0;
+	if (len == -1 || len == 0)
+		stop = true;
+	else {
+		dlen = (size_t)len;
+		if (dlen < sizeof(psm.psm_hdr)) {
+			errno = EINVAL;
+			return -1;
+		}
+
+		if (psm.psm_hdr.ps_cmd == PS_STOP) {
+			stop = true;
+			len = 0;
+		}
+	}
+
+	if (stop) {
+#ifdef PRIVSEP_DEBUG
+		logdebugx("process %d stopping", getpid());
+#endif
+		ps_free(ctx);
+		eloop_exit(ctx->eloop, len != -1 ? EXIT_SUCCESS : EXIT_FAILURE);
+		return len;
+	}
+	dlen -= sizeof(psm.psm_hdr);
+
+	if (ps_unrollmsg(&msg, &psm.psm_hdr, psm.psm_data, dlen) == -1)
+		return -1;
+
+	errno = 0;
+	return callback(cbctx, &psm.psm_hdr, &msg);
+}
+
+struct ps_process *
+ps_findprocess(struct dhcpcd_ctx *ctx, struct ps_id *psid)
+{
+	struct ps_process *psp;
+
+	TAILQ_FOREACH(psp, &ctx->ps_processes, next) {
+		if (memcmp(&psp->psp_id, psid, sizeof(psp->psp_id)) == 0)
+			return psp;
+	}
+	errno = ESRCH;
+	return NULL;
+}
+
+struct ps_process *
+ps_newprocess(struct dhcpcd_ctx *ctx, struct ps_id *psid)
+{
+	struct ps_process *psp;
+
+	psp = calloc(1, sizeof(*psp));
+	if (psp == NULL)
+		return NULL;
+	psp->psp_ctx = ctx;
+	memcpy(&psp->psp_id, psid, sizeof(psp->psp_id));
+	TAILQ_INSERT_TAIL(&ctx->ps_processes, psp, next);
+	return psp;
+}
+
+void
+ps_freeprocesses(struct dhcpcd_ctx *ctx, struct ps_process *notthis)
+{
+	struct ps_process *psp, *psn;
+
+	TAILQ_FOREACH_SAFE(psp, &ctx->ps_processes, next, psn) {
+		if (psp == notthis)
+			continue;
+		ps_freeprocess(psp);
+	}
+}