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

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Message-ID: <CANRwn3TFS-jLwgjwautrofmuJ07PASBXMbTWocj3WQxQX3D6tg@mail.gmail.com>
Date:	Wed, 8 Oct 2014 17:24:32 -0700
From:	Jason Gerecke <killertofu@...il.com>
To:	Dmitry Torokhov <dmitry.torokhov@...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 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...

Jason
---
Now instead of four in the eights place /
you’ve got three, ‘Cause you added one  /
(That is to say, eight) to the two,     /
But you can’t take seven from three,    /
So you look at the sixty-fours....

>>       err = kstrtouint(buf, 10, &id);
>>       if (err)
>>               return err;
>
> Thanks.
>
> --
> Dmitry
> --
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