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From: Jan Kara <jack@suse.cz>
To: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	linux-block@vger.kernel.org
Subject: Re: [PATCH] loop: Don't change loop device under exclusive opener
Date: Mon, 27 May 2019 14:29:15 +0200
Message-ID: <20190527122915.GB9998@quack2.suse.cz> (raw)
In-Reply-To: <50edd0fa-9cfa-38e1-8870-0fbc5c618522@kernel.dk>

On Thu 16-05-19 14:44:07, Jens Axboe wrote:
> On 5/16/19 8:01 AM, Jan Kara wrote:
> > Loop module allows calling LOOP_SET_FD while there are other openers of
> > the loop device. Even exclusive ones. This can lead to weird
> > consequences such as kernel deadlocks like:
> > 
> > mount_bdev()				lo_ioctl()
> >   udf_fill_super()
> >     udf_load_vrs()
> >       sb_set_blocksize() - sets desired block size B
> >       udf_tread()
> >         sb_bread()
> >           __bread_gfp(bdev, block, B)
> > 					  loop_set_fd()
> > 					    set_blocksize()
> >             - now __getblk_slow() indefinitely loops because B != bdev
> >               block size
> > 
> > Fix the problem by disallowing LOOP_SET_FD ioctl when there are
> > exclusive openers of a loop device.
> > 
> > [Deliberately chosen not to CC stable as a user with priviledges to
> > trigger this race has other means of taking the system down and this
> > has a potential of breaking some weird userspace setup]
> > 
> > Reported-and-tested-by: syzbot+10007d66ca02b08f0e60@syzkaller.appspotmail.com
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  drivers/block/loop.c | 18 +++++++++++++++++-
> >  1 file changed, 17 insertions(+), 1 deletion(-)
> > 
> > Hi Jens!
> > 
> > What do you think about this patch? It fixes the problem but it also
> > changes user visible behavior so there are chances it breaks some
> > existing setup (although I have hard time coming up with a realistic
> > scenario where it would matter).
> 
> I also have a hard time thinking about valid cases where this would be a
> problem. I think, in the end, that fixing the issue is more important
> than a potentially hypothetical use case.
> 
> > Alternatively we could change getblk() code handle changing block
> > size. That would fix the particular issue syzkaller found as well but
> > I'm not sure what else is broken when block device changes while fs
> > driver is working with it.
> 
> I think your solution here is saner.

Will you pick up the patch please? I cannot find it in your tree... Thanks!

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16 14:01 Jan Kara
2019-05-16 20:44 ` Jens Axboe
2019-05-27 12:29   ` Jan Kara [this message]
2019-05-27 13:34     ` Jens Axboe
2019-07-18  8:15       ` Kai-Heng Feng
2019-07-30  9:29         ` Jan Kara
2019-07-30  9:36           ` John Lenton
2019-07-30 10:16             ` Jan Kara
2019-07-30 13:36               ` Jan Kara
2019-07-30 17:59                 ` Kai-Heng Feng
2019-07-30 19:17                 ` Jens Axboe
2019-07-30 21:11                   ` John Lenton
2019-07-31  8:56                   ` Jan Kara
2019-08-05 16:41                 ` Bart Van Assche
2019-08-05 21:01                   ` Tetsuo Handa
2019-08-07  9:45                   ` Jan Kara
2019-08-07 18:45                     ` Bart Van Assche
2019-08-08 13:37                     ` Jens Axboe
2019-07-30 10:16           ` Tetsuo Handa

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