[PATCH v3 2/2] net: usb: cdc_ncm: add ndpoffset to NDP32 nframes bounds check

[tobgaertner <tob.gaertner@me.com>]

From: Tobi Gaertner <tob.gaertner@me.com>

The same bounds-check bug fixed for NDP16 in the previous patch also
exists in cdc_ncm_rx_verify_ndp32(). The DPE array size is validated
against the total skb length without accounting for ndpoffset, allowing
out-of-bounds reads when the NDP32 is placed near the end of the NTB.

Add ndpoffset to the nframes bounds check and use struct_size_t() to
express the NDP-plus-DPE-array size more clearly.

Compile-tested only.

Fixes: 0fa81b304a79 ("cdc_ncm: Implement the 32-bit version of NCM Transfer Block")
Signed-off-by: Tobi Gaertner <tob.gaertner@me.com>
---
 drivers/net/usb/cdc_ncm.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index a9d0162b5..81d7e99fc 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -1693,6 +1693,7 @@ int cdc_ncm_rx_verify_ndp32(struct sk_buff *skb_in, int ndpoffset)
 	struct usbnet *dev = netdev_priv(skb_in->dev);
 	struct usb_cdc_ncm_ndp32 *ndp32;
 	int ret = -EINVAL;
+	size_t ndp_len;
 
 	if ((ndpoffset + sizeof(struct usb_cdc_ncm_ndp32)) > skb_in->len) {
 		netif_dbg(dev, rx_err, dev->net, "invalid NDP offset  <%u>\n",
@@ -1712,8 +1713,8 @@ int cdc_ncm_rx_verify_ndp32(struct sk_buff *skb_in, int ndpoffset)
 					sizeof(struct usb_cdc_ncm_dpe32));
 	ret--; /* we process NDP entries except for the last one */
 
-	if ((sizeof(struct usb_cdc_ncm_ndp32) +
-	     ret * (sizeof(struct usb_cdc_ncm_dpe32))) > skb_in->len) {
+	ndp_len = struct_size_t(struct usb_cdc_ncm_ndp32, dpe32, ret);
+	if (ndpoffset + ndp_len > skb_in->len) {
 		netif_dbg(dev, rx_err, dev->net, "Invalid nframes = %d\n", ret);
 		ret = -EINVAL;
 	}
-- 
2.43.0