linux-kernel - Re: [PATCH] network: wireless: p54u: Fix race between disconnect and firmware loading

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Date:   Fri, 24 May 2019 23:19:29 +0200
From:   Christian Lamparter <chunkeey@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     syzbot <syzbot+200d4bb11b23d929335f@...kaller.appspotmail.com>,
        kvalo@...eaurora.org, davem@...emloft.net, andreyknvl@...gle.com,
        syzkaller-bugs@...glegroups.com,
        Kernel development list <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] network: wireless: p54u: Fix race between disconnect and firmware loading

On Monday, May 20, 2019 4:44:21 PM CEST Alan Stern wrote:
> The syzbot fuzzer found a bug in the p54 USB wireless driver.  The
> issue involves a race between disconnect and the firmware-loader
> callback routine, and it has several aspects.
> 
> One big problem is that when the firmware can't be loaded, the
> callback routine tries to unbind the driver from the USB _device_ (by
> calling device_release_driver) instead of from the USB _interface_ to
> which it is actually bound (by calling usb_driver_release_interface).
> 
> The race involves access to the private data structure.  The driver's
> disconnect handler waits for a completion that is signalled by the
> firmware-loader callback routine.  As soon as the completion is
> signalled, you have to assume that the private data structure may have
> been deallocated by the disconnect handler -- even if the firmware was
> loaded without errors.  However, the callback routine does access the
> private data several times after that point.
> 
> Another problem is that, in order to ensure that the USB device
> structure hasn't been freed when the callback routine runs, the driver
> takes a reference to it.  This isn't good enough any more, because now
> that the callback routine calls usb_driver_release_interface, it has
> to ensure that the interface structure hasn't been freed.
> 
> Finally, the driver takes an unnecessary reference to the USB device
> structure in the probe function and drops the reference in the
> disconnect handler.  This extra reference doesn't accomplish anything,
> because the USB core already guarantees that a device structure won't
> be deallocated while a driver is still bound to any of its interfaces.
> 
> To fix these problems, this patch makes the following changes:
> 
> 	Call usb_driver_release_interface() rather than
> 	device_release_driver().
> 
> 	Don't signal the completion until after the important
> 	information has been copied out of the private data structure,
> 	and don't refer to the private data at all thereafter.
> 
> 	Lock udev (the interface's parent) before unbinding the driver
> 	instead of locking udev->parent.
> 
> 	During the firmware loading process, take a reference to the
> 	USB interface instead of the USB device.
> 
> 	Don't take an unnecessary reference to the device during probe
> 	(and then don't drop it during disconnect).
> 
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> Reported-and-tested-by: syzbot+200d4bb11b23d929335f@...kaller.appspotmail.com
> CC: <stable@...r.kernel.org>

Finally I'm at home where I have the device. Did some test with replugging
and module unloading, all seems fine. Thanks!

Acked-by: Christian Lamparter <chunkeey@...il.com>