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Date:   Mon, 4 Jul 2022 12:59:19 +0000
From:   Wedson Almeida Filho <wedsonaf@google.com>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
        arnd@arndb.de, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org
Subject: Re: [PATCH] char: misc: make misc_open() and misc_register() killable
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On Mon, Jul 04, 2022 at 09:34:04PM +0900, Tetsuo Handa wrote:
> On 2022/07/04 20:01, Greg KH wrote:
> > On Mon, Jul 04, 2022 at 07:25:44PM +0900, Tetsuo Handa wrote:
> >> On 2022/07/04 16:29, Greg KH wrote:
> >>> On Mon, Jul 04, 2022 at 03:44:07PM +0900, Tetsuo Handa wrote:
> >>>> syzbot is reporting hung task at misc_open() [1], for snapshot_open() from
> >>>> misc_open() might sleep for long with misc_mtx held whereas userspace can
> >>>> flood with concurrent misc_open() requests. Mitigate this problem by making
> >>>> misc_open() and misc_register() killable.
> >>>
> >>> I do not understand, why not just fix snapshot_open()?  Why add this
> >>> complexity to the misc core for a foolish individual misc device?  Why
> >>> not add the fix there where it is spinning instead?
> >>
> >> Quoting an example from [1]. Multiple processes are calling misc_open() and
> >> all but one processes are blocked at mutex_lock(&misc_mtx). The one which is
> >> not blocked at mutex_lock(&misc_mtx) is also holding system_transition_mutex.
> > 
> > And that is because of that one misc device, right?  Why not fix that
> > instead of papering over the issue in the misc core?
> 
> Since "struct file_operations"->open() is allowed to sleep, calling
> "struct file_operations"->open() via reassignment by "struct miscdevice"->fops
> with locks held can cause problems.
> 
> Assuming that this is not a deadlock hidden by device_initialize(), current
> mutex_lock(&misc_mtx) is as problematic as major_names_lock mentioned at
> https://lkml.kernel.org/r/b2af8a5b-3c1b-204e-7f56-bea0b15848d6@i-love.sakura.ne.jp .
> 
> >> If you don't like mutex_lock_killable(&misc_mtx), we will need to consider moving
> >> file->f_op->open() from misc_open() to after mutex_unlock(&misc_mtx).
> 
> Below is minimal changes for avoid calling "struct file_operations"->open() with
> misc_mtx held. This would be nothing but moving hung task warning from misc_open()
> to snapshot_open() (and therefore we would need to introduce killable version of
> lock_system_sleep()), but we can avoid making misc_mtx like major_names_lock above.
> 
> Greg, can you accept this minimal change?
> 
>  drivers/char/misc.c        | 4 ++++
>  include/linux/miscdevice.h | 1 +
>  kernel/power/user.c        | 1 +
>  3 files changed, 6 insertions(+)
> 
> diff --git a/drivers/char/misc.c b/drivers/char/misc.c
> index cba19bfdc44d..292c86c090b9 100644
> --- a/drivers/char/misc.c
> +++ b/drivers/char/misc.c
> @@ -139,6 +139,10 @@ static int misc_open(struct inode *inode, struct file *file)
>  
>  	err = 0;
>  	replace_fops(file, new_fops);
> +	if (iter->unlocked_open && file->f_op->open) {
> +		mutex_unlock(&misc_mtx);
> +		return file->f_op->open(inode, file);
> +	}

One of the invariants of miscdev is that once misc_deregister() returns,
no new calls to f_op->open() are made. (Although, of course, you can
still have open files but that's a whole different problem.)

This change breaks this invariant which I think is problematic because
drivers then can't know when new calls to open() are guaranteed to stop
coming.

>  	if (file->f_op->open)
>  		err = file->f_op->open(inode, file);
>  fail:
> diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
> index 0676f18093f9..e112ef9e3b7b 100644
> --- a/include/linux/miscdevice.h
> +++ b/include/linux/miscdevice.h
> @@ -86,6 +86,7 @@ struct miscdevice  {
>  	const struct attribute_group **groups;
>  	const char *nodename;
>  	umode_t mode;
> +	bool unlocked_open;
>  };
>  
>  extern int misc_register(struct miscdevice *misc);
> diff --git a/kernel/power/user.c b/kernel/power/user.c
> index ad241b4ff64c..69a269c4fb46 100644
> --- a/kernel/power/user.c
> +++ b/kernel/power/user.c
> @@ -441,6 +441,7 @@ static struct miscdevice snapshot_device = {
>  	.minor = SNAPSHOT_MINOR,
>  	.name = "snapshot",
>  	.fops = &snapshot_fops,
> +	.unlocked_open = true,
>  };
>  
>  static int __init snapshot_device_init(void)
> -- 
> 2.34.1
>