linux-kernel - Re: [PATCH v3 6/6] loop: be paranoid on exit and prevent new additions / removals

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Message-ID: <20200503090923.GP11244@42.do-not-panic.com>
Date:   Sun, 3 May 2020 09:09:23 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
        ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org,
        mhocko@...e.com, yukuai3@...wei.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/6] loop: be paranoid on exit and prevent new
 additions / removals

On Wed, Apr 29, 2020 at 11:50:34AM +0200, Greg KH wrote:
> On Wed, Apr 29, 2020 at 07:46:27AM +0000, Luis Chamberlain wrote:
> > Be pedantic on removal as well and hold the mutex.
> > This should prevent uses of addition while we exit.
> > 
> > Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> > ---
> >  drivers/block/loop.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index da693e6a834e..6dccba22c9b5 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -2333,6 +2333,8 @@ static void __exit loop_exit(void)
> >  
> >  	range = max_loop ? max_loop << part_shift : 1UL << MINORBITS;
> >  
> > +	mutex_lock(&loop_ctl_mutex);
> > +
> >  	idr_for_each(&loop_index_idr, &loop_exit_cb, NULL);
> >  	idr_destroy(&loop_index_idr);
> >  
> > @@ -2340,6 +2342,8 @@ static void __exit loop_exit(void)
> >  	unregister_blkdev(LOOP_MAJOR, "loop");
> >  
> >  	misc_deregister(&loop_misc);
> > +
> > +	mutex_unlock(&loop_ctl_mutex);
> >  }
> >  
> >  module_init(loop_init);
> 
> What type of issue is this helping with?  Can it be triggered today?  if
> so, shouldn't it be backported to stable kernels?

Just code inspection. I can't trigger a userspace test script to crash
the kernel yet, but suspect a race still does exist.

  Luis