CVE-2022-25375 : Linux RNDIS USB Gadget memory extraction via packet filter

[Szymon Heidrich <szymon.heidrich () gmail com>]

The RNDIS USB Gadget may be exploited to dump contents
of kernel memory space via packet filter update mechanism.

The RNDIS_MSG_SET 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

References
- https://github.com/torvalds/linux/commit/38ea1eac7d88072bbffb630e2b3db83ca649b826
- https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.16.10
- https://github.com/szymonh/rndis-co
- https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-25375


Best regards,
Szymon