bpf.c

/*	$OpenBSD: bpf.c,v 1.13 2004/05/05 14:28:58 deraadt Exp $	*/

/* BPF socket interface code, originally contributed by Archie Cobbs. */

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 2021 Franco Fichtner <[email protected]>
 * Copyright (c) 1995, 1996, 1998, 1999
 * The Internet Software Consortium.    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.
 * 3. Neither the name of The Internet Software Consortium nor the names
 *    of its contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM 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 INTERNET SOFTWARE CONSORTIUM 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.
 *
 * This software has been written for the Internet Software Consortium
 * by Ted Lemon <[email protected]> in cooperation with Vixie
 * Enterprises.  To learn more about the Internet Software Consortium,
 * see ``http://www.vix.com/isc''.  To learn more about Vixie
 * Enterprises, see ``http://www.vix.com''.
 */

#include <sys/cdefs.h>
#include "dhcpd.h"
#include "privsep.h"
#include <sys/capsicum.h>
#include <sys/ioctl.h>
#include <sys/uio.h>

#include <net/bpf.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/udp.h>
#include <netinet/if_ether.h>

#include <capsicum_helpers.h>

#define BPF_FORMAT "/dev/bpf%d"

/*
 * Called by get_interface_list for each interface that's discovered.
 * Opens a packet filter for each interface and adds it to the select
 * mask.
 */
int
if_register_bpf(struct interface_info *info, int flags)
{
	char filename[50];
	int sock, b;

	/* Open a BPF device */
	for (b = 0;; b++) {
		snprintf(filename, sizeof(filename), BPF_FORMAT, b);
		sock = open(filename, flags);
		if (sock < 0) {
			if (errno == EBUSY)
				continue;
			else
				error("Can't find free bpf: %m");
		} else
			break;
	}

	/* Set the BPF device to point at this interface. */
	if (ioctl(sock, BIOCSETIF, info->ifp) < 0)
		error("Can't attach interface %s to bpf device %s: %m",
		    info->name, filename);

	/* Tag the packets with the proper VLAN PCP setting. */
	if (info->client->config->vlan_pcp != 0) {
		if (ioctl(sock, BIOCSETVLANPCP,
		    &info->client->config->vlan_pcp) < 0)
			error( "Can't set the VLAN PCP tag on interface %s: %m",
			    info->name);
	}

	return (sock);
}

/*
 * Packet write filter program:
 * 'ip and udp and src port bootps and dst port (bootps or bootpc)'
 */
static const struct bpf_insn dhcp_bpf_wfilter[] = {
	BPF_STMT(BPF_LD + BPF_B + BPF_IND, 14),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, (IPVERSION << 4) + 5, 0, 12),

	/* Make sure this is an IP packet... */
	BPF_STMT(BPF_LD + BPF_H + BPF_ABS, 12),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETHERTYPE_IP, 0, 10),

	/* Make sure it's a UDP packet... */
	BPF_STMT(BPF_LD + BPF_B + BPF_ABS, 23),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 8),

	/* Make sure this isn't a fragment... */
	BPF_STMT(BPF_LD + BPF_H + BPF_ABS, 20),
	BPF_JUMP(BPF_JMP + BPF_JSET + BPF_K, IP_MF|IP_OFFMASK, 6, 0),

	/* Get the IP header length... */
	BPF_STMT(BPF_LDX + BPF_B + BPF_MSH, 14),

	/* Make sure it's from the right port... */
	BPF_STMT(BPF_LD + BPF_H + BPF_IND, 14),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, LOCAL_PORT, 0, 3),

	/* Make sure it is to the right ports ... */
	BPF_STMT(BPF_LD + BPF_H + BPF_IND, 16),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, REMOTE_PORT, 0, 1),

	/* If we passed all the tests, ask for the whole packet. */
	BPF_STMT(BPF_RET+BPF_K, (u_int)-1),

	/* Otherwise, drop it. */
	BPF_STMT(BPF_RET+BPF_K, 0),
};

void
if_register_send(struct interface_info *info)
{
	cap_rights_t rights;
	struct bpf_version v;
	struct bpf_program p;
	int sock, on = 1;

	/* Open a BPF device and hang it on this interface... */
	info->wfdesc = if_register_bpf(info, O_WRONLY);

	/* Make sure the BPF version is in range... */
	if (ioctl(info->wfdesc, BIOCVERSION, &v) < 0)
		error("Can't get BPF version: %m");

	if (v.bv_major != BPF_MAJOR_VERSION ||
	    v.bv_minor < BPF_MINOR_VERSION)
		error("Kernel BPF version out of range - recompile dhcpd!");

	/* Set up the bpf write filter program structure. */
	p.bf_insns = __DECONST(struct bpf_insn *, dhcp_bpf_wfilter);
	p.bf_len = nitems(dhcp_bpf_wfilter);

	if (ioctl(info->wfdesc, BIOCSETWF, &p) < 0)
		error("Can't install write filter program: %m");

	if (ioctl(info->wfdesc, BIOCLOCK, NULL) < 0)
		error("Cannot lock bpf");

	cap_rights_init(&rights, CAP_WRITE);
	if (caph_rights_limit(info->wfdesc, &rights) < 0)
		error("Can't limit bpf descriptor: %m");

	/*
	 * Use raw socket for unicast send.
	 */
	if ((sock = socket(AF_INET, SOCK_RAW, IPPROTO_UDP)) == -1)
		error("socket(SOCK_RAW): %m");
	if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &on,
	    sizeof(on)) == -1)
		error("setsockopt(IP_HDRINCL): %m");
	info->ufdesc = sock;
}

/*
 * Packet filter program...
 */
static const struct bpf_insn dhcp_bpf_filter[] = {
	/* Use relative index (0) for IP packet... */
	BPF_STMT(BPF_LDX + BPF_W + BPF_IMM, 0),

	/*
	 * Test whether this is a VLAN packet...
	 *
	 * In case the server packet is using a VLAN ID
	 * of 0, meaning an untagged priority was set, the
	 * response shall be read and replied to.
	 */
	BPF_STMT(BPF_LD + BPF_H + BPF_IND, 12),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETHERTYPE_VLAN, 0, 4),

	/* Test whether it has a VID of 0 */
	BPF_STMT(BPF_LD + BPF_H + BPF_IND, 14),
	BPF_STMT(BPF_ALU + BPF_AND + BPF_K, EVL_VLID_MASK),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 0, 17),

	/* Correct the relative index for VLAN packet (4)... */
	BPF_STMT(BPF_LDX + BPF_W + BPF_IMM, 4),

	/* Make sure this is an IP packet... */
	BPF_STMT(BPF_LD + BPF_H + BPF_IND, 12),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETHERTYPE_IP, 0, 14),

	/* Make sure it's a UDP packet... */
	BPF_STMT(BPF_LD + BPF_B + BPF_IND, 23),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 12),

	/* Make sure this isn't a fragment... */
	BPF_STMT(BPF_LD + BPF_H + BPF_IND, 20),
	BPF_JUMP(BPF_JMP + BPF_JSET + BPF_K, IP_MF|IP_OFFMASK, 10, 0),

	/*
	 * Get the IP header length...
	 *
	 * To find the correct position of the IP header
	 * length field store the index (0 or 4) in the
	 * accumulator and compare it with 0.
	 */
	BPF_STMT(BPF_MISC + BPF_TXA, 0),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 0, 2),
	/* Store IP header length of IP packet in index. */
	BPF_STMT(BPF_LDX + BPF_B + BPF_MSH, 14),
	/* Skip over following VLAN handling instruction. */
	BPF_JUMP(BPF_JMP + BPF_JA, 1, 0, 0),
	/* Store IP header length of VLAN packet in index. */
	BPF_STMT(BPF_LDX + BPF_B + BPF_MSH, 18),
	/* Add IP header length to previous relative index. */
	BPF_STMT(BPF_ALU + BPF_ADD + BPF_X, 0),
	/* Move result back to index to reach UDP header below. */
	BPF_STMT(BPF_MISC + BPF_TAX, 0),

	/* Make sure it's to the right port... */
	BPF_STMT(BPF_LD + BPF_H + BPF_IND, 16),
	BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, LOCAL_PORT, 0, 1),

	/* If we passed all the tests, ask for the whole packet. */
	BPF_STMT(BPF_RET+BPF_K, (u_int)-1),

	/* Otherwise, drop it. */
	BPF_STMT(BPF_RET+BPF_K, 0),
};

void
if_register_receive(struct interface_info *info)
{
	static const unsigned long cmds[2] = { SIOCGIFFLAGS, SIOCGIFMEDIA };
	cap_rights_t rights;
	struct bpf_version v;
	struct bpf_program p;
	int flag = 1, sz;

	/* Open a BPF device and hang it on this interface... */
	info->rfdesc = if_register_bpf(info, O_RDONLY);

	/* Make sure the BPF version is in range... */
	if (ioctl(info->rfdesc, BIOCVERSION, &v) < 0)
		error("Can't get BPF version: %m");

	if (v.bv_major != BPF_MAJOR_VERSION ||
	    v.bv_minor < BPF_MINOR_VERSION)
		error("Kernel BPF version out of range - recompile dhcpd!");

	/*
	 * Set immediate mode so that reads return as soon as a packet
	 * comes in, rather than waiting for the input buffer to fill
	 * with packets.
	 */
	if (ioctl(info->rfdesc, BIOCIMMEDIATE, &flag) < 0)
		error("Can't set immediate mode on bpf device: %m");

	/* Get the required BPF buffer length from the kernel. */
	if (ioctl(info->rfdesc, BIOCGBLEN, &sz) < 0)
		error("Can't get bpf buffer length: %m");
	info->rbuf_max = sz;
	info->rbuf = malloc(info->rbuf_max);
	if (!info->rbuf)
		error("Can't allocate %lu bytes for bpf input buffer.",
		    (unsigned long)info->rbuf_max);
	info->rbuf_offset = 0;
	info->rbuf_len = 0;

	/* Set up the bpf filter program structure. */
	p.bf_insns = __DECONST(struct bpf_insn *, dhcp_bpf_filter);
	p.bf_len = nitems(dhcp_bpf_filter);

	if (ioctl(info->rfdesc, BIOCSETF, &p) < 0)
		error("Can't install packet filter program: %m");

	if (ioctl(info->rfdesc, BIOCLOCK, NULL) < 0)
		error("Cannot lock bpf");

	cap_rights_init(&rights, CAP_IOCTL, CAP_EVENT, CAP_READ);
	if (caph_rights_limit(info->rfdesc, &rights) < 0)
		error("Can't limit bpf descriptor: %m");
	if (caph_ioctls_limit(info->rfdesc, cmds, 2) < 0)
		error("Can't limit ioctls for bpf descriptor: %m");
}

void
send_packet_unpriv(int privfd, struct dhcp_packet *raw, size_t len,
    struct in_addr from, struct in_addr to)
{
	struct imsg_hdr hdr;
	struct buf *buf;
	int errs;

	hdr.code = IMSG_SEND_PACKET;
	hdr.len = sizeof(hdr) +
	    sizeof(size_t) + len +
	    sizeof(from) + sizeof(to);

	if ((buf = buf_open(hdr.len)) == NULL)
		error("buf_open: %m");

	errs = 0;
	errs += buf_add(buf, &hdr, sizeof(hdr));
	errs += buf_add(buf, &len, sizeof(len));
	errs += buf_add(buf, raw, len);
	errs += buf_add(buf, &from, sizeof(from));
	errs += buf_add(buf, &to, sizeof(to));
	if (errs)
		error("buf_add: %m");

	if (buf_close(privfd, buf) == -1)
		error("buf_close: %m");
}

void
send_packet_priv(struct interface_info *interface, struct imsg_hdr *hdr, int fd)
{
	unsigned char buf[256];
	struct iovec iov[2];
	struct msghdr msg;
	struct dhcp_packet raw;
	size_t len;
	struct in_addr from, to;
	int result, bufp = 0;

	if (hdr->len < sizeof(*hdr) + sizeof(size_t))
		error("corrupted message received");
	buf_read(fd, &len, sizeof(len));
	if (hdr->len != sizeof(*hdr) + sizeof(size_t) + len +
	    sizeof(from) + sizeof(to)) {
		error("corrupted message received");
	}
	if (len > sizeof(raw))
		error("corrupted message received");
	buf_read(fd, &raw, len);
	buf_read(fd, &from, sizeof(from));
	buf_read(fd, &to, sizeof(to));

	/* Assemble the headers... */
	if (to.s_addr == INADDR_BROADCAST)
		assemble_hw_header(interface, buf, &bufp);
	assemble_udp_ip_header(buf, &bufp, from.s_addr, to.s_addr,
	    htons(REMOTE_PORT), (unsigned char *)&raw, len);

	iov[0].iov_base = buf;
	iov[0].iov_len = bufp;
	iov[1].iov_base = &raw;
	iov[1].iov_len = len;

	/* Fire it off */
	if (to.s_addr == INADDR_BROADCAST)
		result = writev(interface->wfdesc, iov, 2);
	else {
		struct sockaddr_in sato;

		sato.sin_addr = to;
		sato.sin_port = htons(REMOTE_PORT);
		sato.sin_family = AF_INET;
		sato.sin_len = sizeof(sato);

		memset(&msg, 0, sizeof(msg));
		msg.msg_name = (struct sockaddr *)&sato;
		msg.msg_namelen = sizeof(sato);
		msg.msg_iov = iov;
		msg.msg_iovlen = 2;
		result = sendmsg(interface->ufdesc, &msg, 0);
	}

	if (result < 0)
		warning("send_packet: %m");
}

ssize_t
receive_packet(struct interface_info *interface, unsigned char *buf,
    size_t len, struct sockaddr_in *from, struct hardware *hfrom)
{
	int length = 0, offset = 0;
	struct bpf_hdr hdr;

	/*
	 * All this complexity is because BPF doesn't guarantee that
	 * only one packet will be returned at a time.  We're getting
	 * what we deserve, though - this is a terrible abuse of the BPF
	 * interface.  Sigh.
	 */

	/* Process packets until we get one we can return or until we've
	 * done a read and gotten nothing we can return...
	 */
	do {
		/* If the buffer is empty, fill it. */
		if (interface->rbuf_offset >= interface->rbuf_len) {
			length = read(interface->rfdesc, interface->rbuf,
			    interface->rbuf_max);
			if (length <= 0)
				return (length);
			interface->rbuf_offset = 0;
			interface->rbuf_len = length;
		}

		/*
		 * If there isn't room for a whole bpf header, something
		 * went wrong, but we'll ignore it and hope it goes
		 * away... XXX
		 */
		if (interface->rbuf_len - interface->rbuf_offset <
		    sizeof(hdr)) {
			interface->rbuf_offset = interface->rbuf_len;
			continue;
		}

		/* Copy out a bpf header... */
		memcpy(&hdr, &interface->rbuf[interface->rbuf_offset],
		    sizeof(hdr));

		/*
		 * If the bpf header plus data doesn't fit in what's
		 * left of the buffer, stick head in sand yet again...
		 */
		if (interface->rbuf_offset + hdr.bh_hdrlen + hdr.bh_caplen >
		    interface->rbuf_len) {
			interface->rbuf_offset = interface->rbuf_len;
			continue;
		}

		/* Skip over the BPF header... */
		interface->rbuf_offset += hdr.bh_hdrlen;

		/*
		 * If the captured data wasn't the whole packet, or if
		 * the packet won't fit in the input buffer, all we can
		 * do is drop it.
		 */
		if (hdr.bh_caplen != hdr.bh_datalen) {
			interface->rbuf_offset =
			    BPF_WORDALIGN(interface->rbuf_offset +
			    hdr.bh_caplen);
			continue;
		}

		/* Decode the physical header... */
		offset = decode_hw_header(interface->rbuf,
		    interface->rbuf_offset, hfrom);

		/*
		 * If a physical layer checksum failed (dunno of any
		 * physical layer that supports this, but WTH), skip
		 * this packet.
		 */
		if (offset < 0) {
			interface->rbuf_offset =
			    BPF_WORDALIGN(interface->rbuf_offset +
			    hdr.bh_caplen);
			continue;
		}
		interface->rbuf_offset += offset;
		hdr.bh_caplen -= offset;

		/* Decode the IP and UDP headers... */
		offset = decode_udp_ip_header(interface->rbuf,
		    interface->rbuf_offset, from, NULL, hdr.bh_caplen);

		/* If the IP or UDP checksum was bad, skip the packet... */
		if (offset < 0) {
			interface->rbuf_offset =
			    BPF_WORDALIGN(interface->rbuf_offset +
			    hdr.bh_caplen);
			continue;
		}
		interface->rbuf_offset += offset;
		hdr.bh_caplen -= offset;

		/*
		 * If there's not enough room to stash the packet data,
		 * we have to skip it (this shouldn't happen in real
		 * life, though).
		 */
		if (hdr.bh_caplen > len) {
			interface->rbuf_offset =
			    BPF_WORDALIGN(interface->rbuf_offset +
			    hdr.bh_caplen);
			continue;
		}

		/* Copy out the data in the packet... */
		memcpy(buf, interface->rbuf + interface->rbuf_offset,
		    hdr.bh_caplen);
		interface->rbuf_offset =
		    BPF_WORDALIGN(interface->rbuf_offset +
		    hdr.bh_caplen);
		return (hdr.bh_caplen);
	} while (!length);
	return (0);
}