linux-kernel - Re: INFO: task hung in wdm

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Message-ID: <87pnho85h7.fsf@miraculix.mork.no>
Date:   Tue, 19 Nov 2019 12:34:28 +0100
From:   Bjørn Mork <bjorn@...k.no>
To:     Oliver Neukum <oneukum@...e.de>
Cc:     syzbot <syzbot+854768b99f19e89d7f81@...kaller.appspotmail.com>,
        andreyknvl@...gle.com, baijiaju1990@...il.com,
        bigeasy@...utronix.de, colin.king@...onical.com,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        yuehaibing@...wei.com
Subject: Re: INFO: task hung in wdm_flush

Oliver Neukum <oneukum@...e.de> writes:
> Am Dienstag, den 19.11.2019, 10:14 +0100 schrieb Bjørn Mork:
>
>> Anyway, I believe this is not a bug.
>> 
>> wdm_flush will wait forever for the IN_USE flag to be cleared or the
>
> Damn. Too obvious. So you think we simply have pending output that does
> just not complete?

I do miss a lot of stuff so I might be wrong, but I can't see any other
way this can happen.  The out_callback will unconditionally clear the
IN_USE flag and wake up the wait_queue.

>> DISCONNECTING flag to be set. The only way you can avoid this is by
>> creating a device that works normally up to a point and then completely
>> ignores all messages,
>
> Devices may crash. I don't think we can ignore that case.

Sure, but I've never seen that happen without the device falling off the
bus.  Which is a disconnect.

But I am all for handling this *if* someone reproduces it with a real
device.  I just don't think it's worth the effort if it's only a
theoretical problem.

>>  but without resetting or disconnecting. It is
>> obviously possible to create such a device. But I think the current
>> error handling is more than sufficient, unless you show me some way to
>> abuse this or reproduce the issue with a real device.
>
> Malicious devices are real. Potentially at least.
> But you are right, we need not bend over to handle them well, but we
> ought to be able to handle them.

Sure, we need to handle malicious devices.  But only if they can be used
for real harm.

This warning requires physical acceess and is only slightly annoying.
Like a USB device making loud farting sounds.  You'd just disconnect the
device.  No need for Linux to detect the sound and handle it
automatically, I think.

Bjørn