linux-kernel - Re: [PATCH v9] usb: gadget: u_serial: Add null pointer check in gs_read_complete & gs_write

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Message-ID: <2024090338-nutlike-oxymoron-90f0@gregkh>
Date: Tue, 3 Sep 2024 08:26:15 +0200
From: "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
To: 胡连勤 <hulianqin@...o.com>
Cc: Michael Nazzareno Trimarchi <michael@...rulasolutions.com>,
	Prashanth K <quic_prashk@...cinc.com>,
	"quic_jjohnson@...cinc.com" <quic_jjohnson@...cinc.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"opensource.kernel" <opensource.kernel@...o.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v9] usb: gadget: u_serial: Add null pointer check in
 gs_read_complete & gs_write_complete

On Thu, Aug 29, 2024 at 11:54:39AM +0000, 胡连勤 wrote:
> From: Lianqin Hu <hulianqin@...o.com>
> 
> Considering that in some extreme cases, when the unbind operation
> is being executed, gserial_disconnect has already cleared gser->ioport,
> triggering a gadget reconfiguration at this time and gs_read_complete
> gets called afterwards, which results in accessing null pointer,
> add a null pointer check to prevent this situation.
> 
> Added a static spinlock to prevent gser->ioport from becoming
> null after the newly added check.

In looking at this further, shouldn't we just be moving around where we
call usb_ep_disable() in gserial_disconnect()?

Can't we shut down the endpoints _BEFORE_ we set the port structures to
NULL?  After the endpoints are stopped, then we know that there is no
outstanding i/o possible and then we can clean up properly?

Not to say that your change doesn't fix the race condition here, but
cleaning up things in the proper order might be the better way as then
there can not be any race conditions at all?

Have you tried that?

thanks,

greg k-h