Re: [PATCH] net: usb: cx82310_eth: stop parsing reboot marker as packet

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 30 Jun 2026 10:30:53 +0000 Tianchu Chen wrote:
> June 30, 2026 at 8:44 AM, "Jakub Kicinski" <kuba@kernel.org mailto:kuba@kernel.org?to=%22Jakub%20Kicinski%22%20%3Ckuba%40kernel.org%3E > wrote:
> > On Thu, 25 Jun 2026 15:32:04 +0000 Tianchu Chen wrote:
> > > From: Tianchu Chen <flynnnchen@tencent.com>
> > >  
> > >  Discovered by Atuin - Automated Vulnerability Discovery Engine.
> > >  
> > >  cx82310_rx_fixup() treats an RX length of 0xffff as a device reboot
> > >  marker and schedules work to re-enable ethernet mode, but then continues
> > >  processing the marker as a normal packet length. This is an out-of-bounds
> > >  heap write controlled by the usb device.
> > >   
> > Where? Can you be more specific in the commit message? At a glance 
> > the accesses seem to be bound-checked with skb->len.
> > -- 
> > pw-bot: cr
> >  
> 
> 
> The "len > skb->len" check bounds the source read, but the overflow is on the
> destination buffer.
> 
> The buggy path is:
> 
> 	if (len == 0xffff) {
> 		netdev_info(dev->net, "router was rebooted, re-enabling ethernet mode");
> 		schedule_work(&priv->reenable_work);
> 		/* <- BUG: missing return; 0xffff bypasses the oversized-length reject */
> 	} else if (len > CX82310_MTU) {
> 		netdev_err(dev->net, "RX packet too long: %d B\n", len);
> 		return 0;
> 	}
> 	if (len > skb->len) {
> 		dev->partial_len = skb->len; // skb->len is bounded by the USB transfer size (4K)
> 		dev->partial_rem = len - skb->len;
> 		memcpy((void *)dev->partial_data, skb->data,
> 		       dev->partial_len); /* <- TRIGGER: can copy 4K bytes into 1516-byte partial_data */

If skb->len (== dev->partial_len) is not bound-checked to the size
of dev->partial_data - aren't there more paths that could hit this
overflow? Are you fixing the right thing?

> ...
> ...
> 	}
> 
> For normal oversized lengths, the len > CX82310_MTU branch rejects
> before this copy. But 0xffff is special-cased and falls through. With a
> 4096-byte RX URB, after the 2-byte length header is pulled, skb->len can
> be 4094, while partial_data is allocated as dev->hard_mtu
> (CX82310_MTU + 2, 1516 bytes).
> 
> So the source read is bounded by skb->len; the destination write is not.
> 
> I am happy to send a v2 with this spelled out more clearly in the commit message
> if needed.
> 
> Best regards,
> Tianchu Chen
>