linux-kernel - Re: [PATCH] usb: xhci: fix potential divide-by-zero in xhci_urb

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Message-ID: <1d014d1c-66f1-42cd-8890-0cce7e3963c2@rowland.harvard.edu>
Date: Sat, 10 Jan 2026 17:08:17 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: pip-izony <eeodqql09@...il.com>
Cc: Mathias Nyman <mathias.nyman@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kyungtae Kim <Kyungtae.Kim@...tmouth.edu>,
	Reyad Attiyat <reyad.attiyat@...il.com>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: xhci: fix potential divide-by-zero in
 xhci_urb_enqueue()

On Sat, Jan 10, 2026 at 01:34:21PM -0500, pip-izony wrote:
> From: Seungjin Bae <eeodqql09@...il.com>
> 
> The `xhci_urb_enqueue()` validates Bulk OUT transfers by checking if the
> buffer length is a multiple of the packet size. However, it doesn't check
> whether the endpoint's `wMaxPacketSize` is zero before using it as a
> divisor in a modulo operation.
> 
> If a malicious USB device sends a descriptor with `wMaxPacketSize` set to
> 0, it triggers a divide-by-zero exception (kernel panic). This allows an
> attacker with physical access to crash the system, leading to a Denial of
> Service.

How did you become aware of this problem?

> Fix this by adding a check to ensure `wMaxPacketSize` is greater than 0
> before performing the modulo operation.

Not necessary.  This can never happen, because transfers to or from 
endpoints with wMaxPacketSize set to 0 are rejected in usb_submit_urb() 
with error code -EMSGSIZE.

Alan Stern