linux-kernel - Re: [PATCH] char: misc: make misc_open() and misc

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Date:   Tue, 5 Jul 2022 07:37:21 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org,
        Wedson Almeida Filho <wedsonaf@...gle.com>
Subject: Re: [PATCH] char: misc: make misc_open() and misc_register() killable

On Tue, Jul 05, 2022 at 02:21:17PM +0900, Tetsuo Handa wrote:
> On 2022/07/04 23:31, Greg KH wrote:
> > I don't understand what you are trying to "fix" here.  What is userspace
> > doing (as a normal user) that is causing a problem, and what problem is
> > it causing and for what device/hardware/driver is this a problem?
> 
> Currently the root cause is unknown.
> This might be another example of deadlock hidden by device_initialize().
> 
> We can see from https://syzkaller.appspot.com/text?tag=CrashReport&x=11feb7e0080000 that
> when khungtaskd reports that a process is blocked waiting for misc_mtx at misc_open(),
> there is a process which is holding system_transition_mutex from snapshot_open().

/dev/snapshot is not read/writable by anyone but root for obvious
reasons.

And perhaps it's something that syzbot shouldn't be fuzzing unless it
wants to take the system down easily :)

thanks,

greg k-h