From: Jan Kara <jack@suse.cz>
To: Kai-Heng Feng <kaihengfeng@me.com>
Cc: Jan Kara <jack@suse.cz>, Jens Axboe <axboe@kernel.dk>,
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
linux-block@vger.kernel.org,
John Lenton <john.lenton@canonical.com>,
jean-baptiste.lallement@canonical.com
Subject: Re: [PATCH] loop: Don't change loop device under exclusive opener
Date: Tue, 30 Jul 2019 11:29:39 +0200 [thread overview]
Message-ID: <20190730092939.GB28829@quack2.suse.cz> (raw)
In-Reply-To: <894DDAA8-2ADD-467C-8E4F-4DE6B9A50625@me.com>
On Thu 18-07-19 16:15:42, Kai-Heng Feng wrote:
> Hi Jan,
>
> at 21:34, Jens Axboe <axboe@kernel.dk> wrote:
>
> > On 5/27/19 6:29 AM, Jan Kara wrote:
> > > 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!
> >
> > Done!
>
> This patch introduced a regression [1].
> A reproducer can be found at [2].
>
> [1] https://bugs.launchpad.net/bugs/1836914
> [2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1836914/comments/4
Thanks for the notice and the references. What's your version of
util-linux? What your test script does is indeed racy. You have there:
echo Running:
for i in {a..z}{a..z}; do
mount $i.squash /mnt/$i &
done
So all mount(8) commands will run in parallel and race to setup loop
devices with LOOP_SET_FD and mount them. However util-linux (at least in
the current version) seems to handle EBUSY from LOOP_SET_FD just fine and
retries with the new loop device. So at this point I don't see why the patch
makes difference... I guess I'll need to reproduce and see what's going on
in detail.
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2019-07-30 9:29 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-16 14:01 [PATCH] loop: Don't change loop device under exclusive opener Jan Kara
2019-05-16 20:44 ` Jens Axboe
2019-05-27 12:29 ` Jan Kara
2019-05-27 13:34 ` Jens Axboe
2019-07-18 8:15 ` Kai-Heng Feng
2019-07-30 9:29 ` Jan Kara [this message]
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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