[PATCH net v5 1/7] usbnet: ipheth: fix possible overflow in DPE length check

[Foster Snowhill <forst@pen.gy>]

Originally, it was possible for the DPE length check to overflow if
wDatagramIndex + wDatagramLength > U16_MAX. This could lead to an OoB
read.

Move the wDatagramIndex term to the other side of the inequality.

An existing condition ensures that wDatagramIndex < urb->actual_length.

Fixes: a2d274c62e44 ("usbnet: ipheth: add CDC NCM support")
Cc: stable@vger.kernel.org
Signed-off-by: Foster Snowhill <forst@pen.gy>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
---
v5:
    No code changes. Added Cc to stable and Reviewed-by Jakub tags.
v4: https://lore.kernel.org/netdev/20250105010121.12546-3-forst@pen.gy/
    No change since v3.
v3: https://lore.kernel.org/netdev/20241123235432.821220-2-forst@pen.gy/
    Split out from a monolithic patch in v2 as an atomic change.
v2: https://lore.kernel.org/netdev/20240912211817.1707844-1-forst@pen.gy/
    No code changes. Update commit message to further clarify that
    `ipheth` is not and does not aim to be a complete or spec-compliant
    CDC NCM implementation.
v1: https://lore.kernel.org/netdev/20240907230108.978355-1-forst@pen.gy/
---
 drivers/net/usb/ipheth.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/ipheth.c b/drivers/net/usb/ipheth.c
index 46afb95ffabe..45daae234cb8 100644
--- a/drivers/net/usb/ipheth.c
+++ b/drivers/net/usb/ipheth.c
@@ -243,8 +243,8 @@ static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
 	while (le16_to_cpu(dpe->wDatagramIndex) != 0 &&
 	       le16_to_cpu(dpe->wDatagramLength) != 0) {
 		if (le16_to_cpu(dpe->wDatagramIndex) >= urb->actual_length ||
-		    le16_to_cpu(dpe->wDatagramIndex) +
-		    le16_to_cpu(dpe->wDatagramLength) > urb->actual_length) {
+		    le16_to_cpu(dpe->wDatagramLength) > urb->actual_length -
+		    le16_to_cpu(dpe->wDatagramIndex)) {
 			dev->net->stats.rx_length_errors++;
 			return retval;
 		}
-- 
2.45.1