linux-kernel - Re: [PATCH v2] HID: wacom: Prevent potential null dereference after disconnect

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Message-ID: <20141009002800.GJ15198@dtor-ws>
Date:	Wed, 8 Oct 2014 17:28:00 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Jason Gerecke <killertofu@...il.com>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Benjamin Tissoires <benjamin.tissoires@...il.com>,
	Ping Cheng <pinglinux@...il.com>,
	Linux Input <linux-input@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] HID: wacom: Prevent potential null dereference after
 disconnect

On Wed, Oct 08, 2014 at 05:24:32PM -0700, Jason Gerecke wrote:
> On Wed, Oct 8, 2014 at 2:40 PM, Dmitry Torokhov
> <dmitry.torokhov@...il.com> wrote:
> > On Wed, Oct 08, 2014 at 11:25:42AM -0700, Jason Gerecke wrote:
> >> Repeated connect/disconnect cycles under GNOME can trigger an occasional
> >> OOPS from within e.g. wacom_led_select_store, presumably due to a timing
> >> issue where userspace begins setting a value immediately before the
> >> device disconnects and our shared data is whisked away.
> >>
> >> Signed-off-by: Jason Gerecke <killertofu@...il.com>
> >> ---
> >> Changes in v2:
> >>  * Added in missing escape character
> >>
> >>  drivers/hid/wacom_sys.c | 13 ++++++++++++-
> >>  1 file changed, 12 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
> >> index 8593047..265429b 100644
> >> --- a/drivers/hid/wacom_sys.c
> >> +++ b/drivers/hid/wacom_sys.c
> >> @@ -641,6 +641,9 @@ static ssize_t wacom_led_select_store(struct device *dev, int set_id,
> >>       unsigned int id;
> >>       int err;
> >>
> >> +     if (!wacom)
> >> +             return -ENODEV;
> >> +
> >
> > Strong NAK. If device could disappear before this check it could as well
> > disappear after your check.
> >
> > This patch does not solve anything.
> >
> 
> I assume I'll want to either disable interrupts or take a lock
> depending on if `wacom_remove` is called from within the interrupt
> context, but I haven't had to deal with concurrency in the kernel
> before so I'm not entirely sure which option (or which primitive if
> locking) would be appropriate...

Actually the sysfs core should not allow anyone descend into sysfs
show/store methods once you return from sysfs_remove*(). So you need to
make sure that pointer is valid until then.

Thanks.

-- 
Dmitry
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