linux-kernel - Re: [PATCH] usb: core: Prevent null pointer dereference in update_port_device

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Message-ID: <1fafda18-8806-4036-bcc1-ac08e2d3b9cd@rowland.harvard.edu>
Date: Thu, 4 Jan 2024 09:56:11 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
  Udipto Goswami <quic_ugoswami@...cinc.com>
Cc: linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: core: Prevent null pointer dereference in
 update_port_device_state

On Thu, Jan 04, 2024 at 02:13:51PM +0100, Greg Kroah-Hartman wrote:
> On Thu, Jan 04, 2024 at 06:35:38PM +0530, Udipto Goswami wrote:
> > Hi Greg,
> > 
> > On 1/4/2024 4:14 PM, Greg Kroah-Hartman wrote:
> > > On Thu, Jan 04, 2024 at 03:56:16PM +0530, Udipto Goswami wrote:
> > > > Currently,the function update_port_device_state gets the usb_hub from
> > > > udev->parent by calling usb_hub_to_struct_hub.
> > > > However, in case the actconfig or the maxchild is 0, the usb_hub would
> > > > be NULL and upon further accessing to get port_dev would result in null
> > > > pointer dereference.
> > > 
> > > Is this true for any real (or fake) hardware?
> > 
> > We saw this in our QCOM hardwares where lvstest.c was calling
> > get_dev_desc_store:
> > 
> > 	usb_set_device_state+0x128/0x17c
> > 	create_lvs_device+0x60/0xf8 [lvstest]
> > 	get_dev_desc_store+0x94/0x18c [lvstest]
> > 	dev_attr_store+0x30/0x48
> > 
> > I think the part of the test procedure is to first unbind the hub driver
> > which calls hub_disconnect setting the maxchild = 0.
> 
> Are you sure lvstest is correct here?

This is what happens when people work behind the hub driver's back.  :-(

If you can't find another way to fix the problem, you should at least 
change the patch to include a comment before the "if (hub)" test, 
explaining why it is necessary.  Otherwise somebody in the future will 
remove the test, because under normal circumstances hub would never be 
NULL here.

Alan Stern