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, 310 insertions, 0 deletions

diff --git a/src/privsep-bpf.c b/src/privsep-bpf.c
new file mode 100644
index 00000000..a14d8a14
--- /dev/null
+++ b/src/privsep-bpf.c
@@ -0,0 +1,310 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+/*
+ * Priviledge Separation BPF Initiator
+ * 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/socket.h>
+#include <sys/types.h>
+
+/* Need these headers just for if_ether on some OS. */
+#ifndef __NetBSD__
+#include <net/if_arp.h>
+#include <netinet/in.h>
+#endif
+#include <netinet/if_ether.h>
+
+#include <assert.h>
+#include <pwd.h>
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "arp.h"
+#include "bpf.h"
+#include "dhcp.h"
+#include "dhcp6.h"
+#include "eloop.h"
+#include "ipv6nd.h"
+#include "logerr.h"
+#include "privsep.h"
+
+static void
+ps_bpf_recvbpf(void *arg)
+{
+	struct ps_process *psp = arg;
+	unsigned int flags;
+	uint8_t buf[MTU_MAX];
+	ssize_t len;
+	struct ps_msghdr psm = {
+		.ps_id = psp->psp_id,
+		.ps_cmd = psp->psp_id.psi_cmd,
+	};
+
+	/* A BPF read can read more than one filtered packet at time.
+	 * This mechanism allows us to read each packet from the buffer. */
+	flags = 0;
+	while (!(flags & BPF_EOF)) {
+		len = bpf_read(&psp->psp_ifp, psp->psp_work_fd,
+		    buf, sizeof(buf), &flags);
+		if (len == -1)
+			logerr(__func__);
+		if (len == -1 || len == 0)
+			break;
+		len = ps_sendpsmdata(psp->psp_ctx, psp->psp_ctx->ps_data_fd,
+		    &psm, buf, (size_t)len);
+		if (len == -1 && errno != ECONNRESET)
+			logerr(__func__);
+		if (len == -1 || len == 0)
+			break;
+	}
+}
+
+static ssize_t
+ps_bpf_recvmsgcb(void *arg, __unused struct ps_msghdr *psm, struct msghdr *msg)
+{
+	struct ps_process *psp = arg;
+	struct iovec *iov = msg->msg_iov;
+
+	return bpf_send(&psp->psp_ifp, psp->psp_work_fd, psp->psp_proto,
+	    iov->iov_base, iov->iov_len);
+}
+
+static void
+ps_bpf_recvmsg(void *arg)
+{
+	struct ps_process *psp = arg;
+
+	if (ps_recvpsmsg(psp->psp_ctx, psp->psp_fd,
+	    ps_bpf_recvmsgcb, arg) == -1)
+		logerr(__func__);
+}
+
+static int
+ps_bpf_start_bpf(void *arg)
+{
+	struct ps_process *psp = arg;
+	struct dhcpcd_ctx *ctx = psp->psp_ctx;
+
+	setproctitle("[BPF %s] %s", psp->psp_protostr, psp->psp_ifname);
+
+	ps_freeprocesses(ctx, psp);
+
+	psp->psp_work_fd = bpf_open(&psp->psp_ifp, psp->psp_filter);
+	if (psp->psp_work_fd == -1)
+		logerr("%s: bpf_open",__func__);
+	else if (eloop_event_add(ctx->eloop,
+	    psp->psp_work_fd, ps_bpf_recvbpf, psp) == -1)
+		logerr("%s: eloop_event_add", __func__);
+	else
+		return 0;
+
+	eloop_exit(ctx->eloop, EXIT_FAILURE);
+	return -1;
+}
+
+static void
+ps_bpf_signal_bpfcb(int sig, void *arg)
+{
+	struct dhcpcd_ctx *ctx = arg;
+
+	eloop_exit(ctx->eloop, sig == SIGTERM ? EXIT_SUCCESS : EXIT_FAILURE);
+}
+
+ssize_t
+ps_bpf_cmd(struct dhcpcd_ctx *ctx, struct ps_msghdr *psm, struct msghdr *msg)
+{
+	uint8_t cmd;
+	struct ps_process *psp;
+	pid_t start;
+	struct iovec *iov = msg->msg_iov;
+	struct interface *ifp;
+	struct ipv4_state *istate;
+
+	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
+
+	switch (cmd) {
+#ifdef ARP
+	case PS_BPF_ARP:	/* FALLTHROUGH */
+#endif
+	case PS_BPF_BOOTP:
+		break;
+	default:
+		logerrx("%s: unknown command %x", __func__, psm->ps_cmd);
+		errno = ENOTSUP;
+		return -1;
+	}
+
+	if (!(psm->ps_cmd & PS_START)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	if (psp != NULL)
+		return 1;
+
+	psp = ps_newprocess(ctx, &psm->ps_id);
+	if (psp == NULL)
+		return -1;
+
+	ifp = &psp->psp_ifp;
+	assert(msg->msg_iovlen == 1);
+	assert(iov->iov_len = sizeof(*ifp));
+	memcpy(ifp, iov->iov_base, sizeof(*ifp));
+	ifp->ctx = psp->psp_ctx;
+	ifp->options = NULL;
+	memset(ifp->if_data, 0, sizeof(ifp->if_data));
+
+	if ((istate = ipv4_getstate(ifp)) == NULL) {
+		ps_freeprocess(psp);
+		return -1;
+	}
+
+	memcpy(psp->psp_ifname, ifp->name, sizeof(psp->psp_ifname));
+
+	switch (cmd) {
+#ifdef ARP
+	case PS_BPF_ARP:
+		psp->psp_proto = ETHERTYPE_ARP;
+		psp->psp_protostr = "ARP";
+		psp->psp_filter = bpf_arp;
+		break;
+#endif
+	case PS_BPF_BOOTP:
+		psp->psp_proto = ETHERTYPE_IP;
+		psp->psp_protostr = "BOOTP";
+		psp->psp_filter = bpf_bootp;
+		break;
+	}
+
+	start = ps_dostart(ctx,
+	    &psp->psp_pid, &psp->psp_fd,
+	    ps_bpf_recvmsg, NULL, psp,
+	    ps_bpf_start_bpf, ps_bpf_signal_bpfcb, PSF_DROPPRIVS);
+	switch (start) {
+	case -1:
+		ps_freeprocess(psp);
+		return -1;
+	case 0:
+		break;
+	default:
+		logdebugx("%s: spawned BPF %s on PID %d",
+		    psp->psp_ifname, psp->psp_protostr, start);
+		break;
+	}
+	return start;
+}
+
+ssize_t
+ps_bpf_dispatch(struct dhcpcd_ctx *ctx,
+    struct ps_msghdr *psm, struct msghdr *msg)
+{
+	struct iovec *iov = msg->msg_iov;
+	struct interface *ifp;
+
+	ifp = if_findindex(ctx->ifaces, psm->ps_id.psi_ifindex);
+
+	switch (psm->ps_cmd) {
+#ifdef ARP
+	case PS_BPF_ARP:
+		arp_packet(ifp, iov->iov_base, iov->iov_len);
+		break;
+#endif
+	case PS_BPF_BOOTP:
+		dhcp_packet(ifp, iov->iov_base, iov->iov_len);
+		break;
+	default:
+		errno = ENOTSUP;
+		return -1;
+	}
+
+	return 1;
+}
+
+static ssize_t
+ps_bpf_send(const struct interface *ifp, uint8_t cmd,
+    const void *data, size_t len)
+{
+	struct dhcpcd_ctx *ctx = ifp->ctx;
+	struct ps_msghdr psm = {
+		.ps_cmd = cmd,
+		.ps_id = {
+			.psi_ifindex = ifp->index,
+			.psi_cmd = (uint8_t)(cmd & ~(PS_START | PS_STOP)),
+		},
+	};
+
+	return ps_sendpsmdata(ctx, ctx->ps_root_fd, &psm, data, len);
+}
+
+#ifdef ARP
+ssize_t
+ps_bpf_openarp(const struct interface *ifp)
+{
+
+	return ps_bpf_send(ifp, PS_BPF_ARP | PS_START, ifp, sizeof(*ifp));
+}
+
+ssize_t
+ps_bpf_closearp(const struct interface *ifp)
+{
+
+	return ps_bpf_send(ifp, PS_BPF_ARP | PS_STOP, NULL, 0);
+}
+
+ssize_t
+ps_bpf_sendarp(const struct interface *ifp, const void *data, size_t len)
+{
+
+	return ps_bpf_send(ifp, PS_BPF_ARP, data, len);
+}
+#endif
+
+ssize_t
+ps_bpf_openbootp(const struct interface *ifp)
+{
+
+	return ps_bpf_send(ifp, PS_BPF_BOOTP | PS_START, ifp, sizeof(*ifp));
+}
+
+ssize_t
+ps_bpf_closebootp(const struct interface *ifp)
+{
+
+	return ps_bpf_send(ifp, PS_BPF_BOOTP | PS_STOP, NULL, 0);
+}
+
+ssize_t
+ps_bpf_sendbootp(const struct interface *ifp, const void *data, size_t len)
+{
+
+	return ps_bpf_send(ifp, PS_BPF_BOOTP, data, len);
+}