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, 360 insertions, 0 deletions

diff --git a/src/privsep-root.c b/src/privsep-root.c
new file mode 100644
index 00000000..80bf37ac
--- /dev/null
+++ b/src/privsep-root.c
@@ -0,0 +1,360 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+/*
+ * Priviledge Separation for dhcpcd, privileged actioneer
+ * 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.
+ */
+
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+#include <assert.h>
+#include <errno.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "common.h"
+#include "dhcpcd.h"
+#include "eloop.h"
+#include "if.h"
+#include "logerr.h"
+#include "privsep.h"
+#include "script.h"
+
+struct psr_error
+{
+	ssize_t psr_result;
+	int psr_errno;
+	char psr_pad[sizeof(ssize_t) - sizeof(int)];
+};
+
+ssize_t
+ps_root_readerror(struct dhcpcd_ctx *ctx)
+{
+	struct psr_error psr;
+	ssize_t len;
+
+	/* ps_root_fd should be a blocking socket. */
+	len = read(ctx->ps_root_fd, &psr, sizeof(psr));
+	if (len == 0 || len == -1) {
+		logerr(__func__);
+		return len;
+	}
+	if ((size_t)len < sizeof(psr)) {
+		logerrx("%s: psr_error truncated", __func__);
+		return -1;
+	}
+
+	errno = psr.psr_errno;
+	return psr.psr_result;
+}
+
+static ssize_t
+ps_root_writeerror(struct dhcpcd_ctx *ctx, ssize_t result)
+{
+	struct psr_error psr = {
+		.psr_result = result,
+		.psr_errno = errno,
+	};
+
+	return write(ctx->ps_root_fd, &psr, sizeof(psr));
+}
+
+static ssize_t
+ps_root_doioctl(unsigned long req, void *data, size_t len)
+{
+	int s, err;
+
+	s = socket(PF_INET, SOCK_DGRAM, 0);
+	if (s != -1)
+		err = ioctl(s, req, data, len);
+	else
+		err = -1;
+	if (s != -1)
+		close(s);
+	return err;
+}
+
+static ssize_t
+ps_root_run_script(struct dhcpcd_ctx *ctx, const void *data, size_t len)
+{
+	const char *envbuf = data;
+	char * const argv[] = { UNCONST(data), NULL };
+	pid_t pid;
+	int status;
+
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: IN %zu", __func__, len);
+#endif
+
+	if (len == 0)
+		return 0;
+
+	/* Script is the first one, find the environment buffer. */
+	while (*envbuf != '\0') {
+		if (len == 0)
+			return EINVAL;
+		envbuf++;
+		len--;
+	}
+
+	if (len != 0) {
+		envbuf++;
+		len--;
+	}
+
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: run script: %s", __func__, argv[0]);
+#endif
+
+	if (script_buftoenv(ctx, UNCONST(envbuf), len) == NULL)
+		return -1;
+
+	pid = script_exec(ctx, argv, ctx->script_env);
+	if (pid == -1)
+		return -1;
+	/* Wait for the script to finish */
+	while (waitpid(pid, &status, 0) == -1) {
+		if (errno != EINTR) {
+			logerr(__func__);
+			status = 0;
+			break;
+		}
+	}
+	return status;
+}
+
+static ssize_t
+ps_root_recvmsgcb(void *arg, struct ps_msghdr *psm, struct msghdr *msg)
+{
+	struct dhcpcd_ctx *ctx = arg;
+	uint8_t cmd;
+	struct ps_process *psp;
+	struct iovec *iov = msg->msg_iov;
+	void *data = iov->iov_base;
+	size_t len = iov->iov_len;
+	ssize_t err;
+
+	cmd = (uint8_t)(psm->ps_cmd & ~(PS_START | PS_STOP));
+	psp = ps_findprocess(ctx, &psm->ps_id);
+
+#ifdef PRIVSEP_DEBUG
+	logerrx("%s: IN cmd %x, psp %p", __func__, psm->ps_cmd, psp);
+#endif
+
+	if (!(psm->ps_cmd & PS_START) && psp != NULL) {
+		if (psm->ps_cmd & PS_STOP) {
+			int ret = ps_dostop(ctx, &psp->psp_pid, &psp->psp_fd);
+
+			ps_freeprocess(psp);
+			return ret;
+		}
+		return ps_sendpsmmsg(ctx, psp->psp_fd, psm, msg);
+	}
+
+	if (psm->ps_cmd & PS_STOP && psp == NULL)
+		return 0;
+
+	/* All these should just be PS_START */
+	switch (cmd) {
+#ifdef INET
+#ifdef ARP
+	case PS_BPF_ARP:	/* FALLTHROUGH */
+#endif
+	case PS_BPF_BOOTP:
+		return ps_bpf_cmd(ctx, psm, msg);
+#endif
+#ifdef INET
+	case PS_BOOTP:
+		return ps_inet_cmd(ctx, psm, msg);
+#endif
+#ifdef INET6
+#ifdef DHCP6
+	case PS_DHCP6:	/* FALLTHROUGH */
+#endif
+	case PS_ND:
+		return ps_inet_cmd(ctx, psm, msg);
+#endif
+	default:
+		break;
+	}
+
+	assert(msg->msg_iovlen == 1);
+
+	switch (psm->ps_cmd) {
+	case PS_IOCTL:
+		err = ps_root_doioctl(psm->ps_flags, data, len);
+		break;
+	case PS_SCRIPT:
+		err = ps_root_run_script(ctx, data, len);
+		break;
+	default:
+		err = ps_root_os(psm, msg);
+		break;
+	}
+
+	return ps_root_writeerror(ctx, err);
+}
+
+/* Receive from state engine, do an action. */
+static void
+ps_root_recvmsg(void *arg)
+{
+	struct dhcpcd_ctx *ctx = arg;
+
+	if (ps_recvpsmsg(ctx, ctx->ps_root_fd, ps_root_recvmsgcb, ctx) == -1)
+		logerr(__func__);
+}
+
+static int
+ps_root_startcb(void *arg)
+{
+	struct dhcpcd_ctx *ctx = arg;
+
+	setproctitle("[privileged actioneer]");
+	ctx->ps_root_pid = getpid();
+
+	return 0;
+}
+
+static void
+ps_root_signalcb(int sig, void *arg)
+{
+	struct dhcpcd_ctx *ctx = arg;
+
+	/* Ignore SIGINT, respect PS_STOP command or SIGTERM. */
+	if (sig == SIGINT)
+		return;
+
+	logerrx("process %d unexpectedly terminating on signal %d",
+	    getpid(), sig);
+	if (ctx->ps_root_pid == getpid()) {
+		shutdown(ctx->ps_root_fd, SHUT_RDWR);
+		shutdown(ctx->ps_data_fd, SHUT_RDWR);
+	}
+	eloop_exit(ctx->eloop, sig == SIGTERM ? EXIT_SUCCESS : EXIT_FAILURE);
+}
+
+static ssize_t
+ps_root_dispatchcb(void *arg, struct ps_msghdr *psm, struct msghdr *msg)
+{
+	struct dhcpcd_ctx *ctx = arg;
+	ssize_t err;
+
+	err = ps_bpf_dispatch(ctx, psm, msg);
+	if (err == -1 && errno == ENOTSUP)
+		err = ps_inet_dispatch(ctx, psm, msg);
+	return err;
+}
+
+static void
+ps_root_dispatch(void *arg)
+{
+	struct dhcpcd_ctx *ctx = arg;
+
+	if (ps_recvpsmsg(ctx, ctx->ps_data_fd, ps_root_dispatchcb, ctx) == -1)
+		logerr(__func__);
+}
+
+pid_t
+ps_root_start(struct dhcpcd_ctx *ctx)
+{
+	int fd[2];
+	pid_t pid;
+
+#define	SOCK_CXNB	SOCK_CLOEXEC | SOCK_NONBLOCK
+	if (socketpair(AF_UNIX, SOCK_DGRAM | SOCK_CXNB, 0, fd) == -1)
+		return -1;
+
+	pid = ps_dostart(ctx, &ctx->ps_root_pid, &ctx->ps_root_fd,
+	    ps_root_recvmsg, NULL, ctx,
+	    ps_root_startcb, ps_root_signalcb, 0);
+
+	if (pid == 0) {
+		ctx->ps_data_fd = fd[1];
+		close(fd[0]);
+	} else if (pid != -1) {
+		ctx->ps_data_fd = fd[0];
+		close(fd[1]);
+		if (eloop_event_add(ctx->eloop, ctx->ps_data_fd,
+		    ps_root_dispatch, ctx) == -1)
+			logerr(__func__);
+	}
+	return pid;
+}
+
+int
+ps_root_stop(struct dhcpcd_ctx *ctx)
+{
+
+	return ps_dostop(ctx, &ctx->ps_root_pid, &ctx->ps_root_fd);
+}
+
+ssize_t
+ps_root_script(const struct interface *ifp, const void *data, size_t len)
+{
+	char buf[PS_BUFLEN], *p = buf;
+	size_t blen = PS_BUFLEN, slen = strlen(ifp->options->script) + 1;
+
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: sending script: %zu %s len %zu",
+	    __func__, slen, ifp->options->script, len);
+#endif
+
+	if (slen > blen) {
+		errno = ENOBUFS;
+		return -1;
+	}
+	memcpy(p, ifp->options->script, slen);
+	p += slen;
+	blen -= slen;
+
+	if (len > blen) {
+		errno = ENOBUFS;
+		return -1;
+	}
+	memcpy(p, data, len);
+
+#ifdef PRIVSEP_DEBUG
+	logdebugx("%s: sending script data: %zu", __func__, slen + len);
+#endif
+
+	if (ps_sendcmd(ifp->ctx, ifp->ctx->ps_root_fd, PS_SCRIPT, 0,
+	    buf, slen + len) == -1)
+		return -1;
+
+	return ps_root_readerror(ifp->ctx);
+}
+
+ssize_t
+ps_root_ioctl(struct dhcpcd_ctx *ctx, unsigned long req, void *data, size_t len)
+{
+	if (ps_sendcmd(ctx, ctx->ps_root_fd, PS_IOCTL, req, data, len) == -1)
+		return -1;
+	return ps_root_readerror(ctx);
+}