The RNDIS USB Gadget may be exploited to dump contents of kernel memory space via packet filter update mechanism.
The RNDIS_MSG_SET usb control transfer request handler - rndis_set_response calls gen_ndis_set_resp passing a buffer pointer offset by BufOffset + 8. The BufOffset variable is retrieved from the RNDIS message and not validated to respect buffer boundaries. Consequently by manipulating the four byte InformationBufferOffset member of rndis_set_msg_type an attacker may offset the actual buffer by up to 0xffffffff bytes.
rndis.c - rndis_msg_parser
case RNDIS_MSG_QUERY:
return rndis_query_response(params,
(rndis_query_msg_type *)buf);
case RNDIS_MSG_SET:
return rndis_set_response(params, (rndis_set_msg_type *)buf);
rndis.c - rndis_set_response
static int rndis_set_response(struct rndis_params *params,
rndis_set_msg_type *buf)
{
u32 BufLength, BufOffset;
rndis_set_cmplt_type *resp;
rndis_resp_t *r;
r = rndis_add_response(params, sizeof(rndis_set_cmplt_type));
if (!r)
return -ENOMEM;
resp = (rndis_set_cmplt_type *)r->buf;
BufLength = le32_to_cpu(buf->InformationBufferLength);
BufOffset = le32_to_cpu(buf->InformationBufferOffset);
#ifdef VERBOSE_DEBUG
pr_debug("%s: Length: %d\n", __func__, BufLength);
pr_debug("%s: Offset: %d\n", __func__, BufOffset);
pr_debug("%s: InfoBuffer: ", __func__);
for (i = 0; i < BufLength; i++) {
pr_debug("%02x ", *(((u8 *) buf) + i + 8 + BufOffset));
}
pr_debug("\n");
#endif
resp->MessageType = cpu_to_le32(RNDIS_MSG_SET_C);
resp->MessageLength = cpu_to_le32(16);
resp->RequestID = buf->RequestID; /* Still LE in msg buffer */
if (gen_ndis_set_resp(params, le32_to_cpu(buf->OID),
((u8 *)buf) + 8 + BufOffset, BufLength, r))
resp->Status = cpu_to_le32(RNDIS_STATUS_NOT_SUPPORTED);
else
resp->Status = cpu_to_le32(RNDIS_STATUS_SUCCESS);
params->resp_avail(params->v);
return 0;
}
Next the code responsible for handling RNDIS_OID_GEN_CURRENT_PACKET_FILTER OID sets the current packet filter to the value pointed by the buf pointer. With the offset applied this allows one to retrieve two bytes at a specified address and store the value in the packet filter.
rndis.c - gen_ndis_set_resp
switch (OID) {
case RNDIS_OID_GEN_CURRENT_PACKET_FILTER:
/* these NDIS_PACKET_TYPE_* bitflags are shared with
* cdc_filter; it's not RNDIS-specific
* NDIS_PACKET_TYPE_x == USB_CDC_PACKET_TYPE_x for x in:
* PROMISCUOUS, DIRECTED,
* MULTICAST, ALL_MULTICAST, BROADCAST
*/
*params->filter = (u16)get_unaligned_le32(buf);
pr_debug("%s: RNDIS_OID_GEN_CURRENT_PACKET_FILTER %08x\n",
__func__, *params->filter);
Further step is to retrieve the packet filter value by utilizing a combination of USB_CDC_SEND_ENCAPSULATED_COMMAND with RNDIS_MSG_QUERY for the RNDIS_OID_GEN_CURRENT_PACKET_FILTER OID and USB_CDC_GET_ENCAPSULATED_RESPONSE control transfer requests.
Repeating the set/get packet filter with incremented InformationBufferOffset in the RNDIS request allows extraction of up to 0xffffffff bytes of kernel space memory by two bytes at a time. For large amounts of data the process is rather slow but still effective.
$ sudo python3 rndisco.py -v 0x1b67 -p 0x400c -l 0x3fffc > /tmp/rpi_rndis.dmp
strings /tmp/rpi_rndis.dmp -n8 | tail -n 6
stp_proto_unregister
<30>Jan 27 14:39:48 dhcpcd[486]: usb0: IAID be:53:70:24
<30>Jan 27 14:39:46 dhcpcd[486]: usb0: IAID be:53:70:24
<30>Jan 27 14:39:46 dhcpcd[486]: usb0: adding address fe80::6f70:c737:89e:697a
<30>Jan 27 14:39:40 dhcpcd[486]: usb0: carrier lost
<30>Jan 27 14:39:48 dhcpcd[486]: usb0: adding address fe80::6f70:c737:89e:697a
Linux devices exposing USB RNDIS gadgets may be exploited to extract sensitive information.