linux-kernel - Re: [PATCH] Input: pxrc

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Message-ID: <20180715101815.5derw7zfjm2d2ayn@penguin>
Date:   Sun, 15 Jul 2018 10:18:15 +0000
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Marcus Folkesson <marcus.folkesson@...il.com>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        "linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>, ldv-project@...uxtesting.org
Subject: Re: [PATCH] Input: pxrc - fix leak of usb_device

On Sun, Jul 15, 2018 at 12:12:44PM +0200, Greg Kroah-Hartman wrote:
> On Sun, Jul 15, 2018 at 01:06:32PM +0300, Dmitry Torokhov wrote:
> > On Sun, Jul 15, 2018 at 10:42 AM Marcus Folkesson
> > <marcus.folkesson@...il.com> wrote:
> > >
> > > On Sat, Jul 14, 2018 at 08:51:09AM +0000, Dmitry Torokhov wrote:
> > > > On Sat, Jul 14, 2018 at 10:09:20AM +0200, Marcus Folkesson wrote:
> > > > > Hi Alexey,
> > > > >
> > > > > Good catch!
> > > > >
> > > > > On Fri, Jul 13, 2018 at 11:07:57PM +0300, Alexey Khoroshilov wrote:
> > > > > > pxrc_probe() calls usb_get_dev(), but there is no usb_put_dev()
> > > > > > anywhere in the driver.
> > > > > >
> > > > > > The patch adds one to error handling code and to pxrc_disconnect().
> > > > > >
> > > > > > Found by Linux Driver Verification project (linuxtesting.org).
> > > > > >
> > > > > > Signed-off-by: Alexey Khoroshilov <khoroshilov@...ras.ru>
> > > > >
> > > > > Reviewed-by: Marcus Folkesson <marcus.folkesson@...il.com>
> > > >
> > > > Hmm, the biggest question however if we need to "take" the device, as I
> > > > do not think interface can outlive the device, and whether we actually
> > > > need to store it in pxrc, as we only need it during set up, as far as I
> > > > can see.
> > >
> > > Yep, the device is only used during setup.
> > > I interpret the comments for usb_get_dev() as you should take a
> > > reference count on the device even if you only use the interface, but I
> > > could be wrong.
> > >
> > > From usb_get_dev()::
> > >
> > >          * usb_get_dev - increments the reference count of the usb device structure
> > >          * @dev: the device being referenced
> > >          *
> > >          * Each live reference to a device should be refcounted.
> > >          *
> > >          * Drivers for USB interfaces should normally record such references in
> > >          * their probe() methods, when they bind to an interface, and release
> > >          * them by calling usb_put_dev(), in their disconnect() methods.
> > 
> > Hmm, usb device is a parent of usb interface so our driver model rules
> > ensure that usb device should not disappear while interface device is
> > still there. Greg, is this comment still valid?
> 
> Yes, that is true.  But remember that interface devices can go away
> while the parent is still present, so if you need the interface pointer,
> you have to grab a reference on it.

But not in a simple interface driver, as we'll unbind the driver before
destroying the interface... IOW we need to record the reference only if
we are doing something unusual.

Thanks.

-- 
Dmitry