usbip: validate number_of_packets in usbip_pack_ret_submit()

When a USB/IP client receives a RET_SUBMIT response,
usbip_pack_ret_submit() unconditionally overwrites
urb->number_of_packets from the network PDU. This value is
subsequently used as the loop bound in usbip_recv_iso() and
usbip_pad_iso() to iterate over urb->iso_frame_desc[], a flexible
array whose size was fixed at URB allocation time based on the
*original* number_of_packets from the CMD_SUBMIT.

A malicious USB/IP server can set number_of_packets in the response
to a value larger than what was originally submitted, causing a heap
out-of-bounds write when usbip_recv_iso() writes to
urb->iso_frame_desc[i] beyond the allocated region.

KASAN confirmed this with kernel 7.0.0-rc5:

  BUG: KASAN: slab-out-of-bounds in usbip_recv_iso+0x46a/0x640
  Write of size 4 at addr ffff888106351d40 by task vhci_rx/69

  The buggy address is located 0 bytes to the right of
   allocated 320-byte region [ffff888106351c00, ffff888106351d40)

The server side (stub_rx.c) and gadget side (vudc_rx.c) already
validate number_of_packets in the CMD_SUBMIT path since commits
c6688ef ("usbip: fix stub_rx: harden CMD_SUBMIT path to handle
malicious input") and b78d830 ("usbip: fix vudc_rx: harden
CMD_SUBMIT path to handle malicious input"). The server side validates
against USBIP_MAX_ISO_PACKETS because no URB exists yet at that point.
On the client side we have the original URB, so we can use the tighter
bound: the response must not exceed the original number_of_packets.

This mirrors the existing validation of actual_length against
transfer_buffer_length in usbip_recv_xbuff(), which checks the
response value against the original allocation size.

Kelvin Mbogo's series ("usb: usbip: fix integer overflow in
usbip_recv_iso()", v2) hardens the receive-side functions themselves;
this patch complements that work by catching the bad value at its
source -- in usbip_pack_ret_submit() before the overwrite -- and
using the tighter per-URB allocation bound rather than the global
USBIP_MAX_ISO_PACKETS limit.

Fix this by checking rpdu->number_of_packets against
urb->number_of_packets in usbip_pack_ret_submit() before the
overwrite. On violation, clamp to zero so that usbip_recv_iso() and
usbip_pad_iso() safely return early.

Fixes: 1325f85 ("staging: usbip: bugfix add number of packets for isochronous frames")
Cc: stable <stable@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Nathan Rebello <nathan.c.rebello@gmail.com>
Link: https://patch.msgid.link/20260402085259.234-1-nathan.c.rebello@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: nathancrebello

drivers/usb/usbip/usbip_common.c

@@ -470,6 +470,18 @@ static void usbip_pack_ret_submit(struct usbip_header *pdu, struct urb *urb,
 		urb->status		= rpdu->status;
 		urb->actual_length	= rpdu->actual_length;
 		urb->start_frame	= rpdu->start_frame;
+		/*
+		 * The number_of_packets field determines the length of
+		 * iso_frame_desc[], which is a flexible array allocated
+		 * at URB creation time. A response must never claim more
+		 * packets than originally submitted; doing so would cause
+		 * an out-of-bounds write in usbip_recv_iso() and
+		 * usbip_pad_iso(). Clamp to zero on violation so both
+		 * functions safely return early.
+		 */
+		if (rpdu->number_of_packets < 0 ||
+		    rpdu->number_of_packets > urb->number_of_packets)
+			rpdu->number_of_packets = 0;
 		urb->number_of_packets = rpdu->number_of_packets;
 		urb->error_count	= rpdu->error_count;
 	}