Re: [PATCH v2] mtd: break circular locks in register_mtd_blktrans

[Desmond Cheong Zhi Xi]

On 30/6/21 4:43 pm, Miquel Raynal wrote:
> Hello,
>
> Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx> wrote on Wed, 30 Jun
> 2021 16:04:17 +0800:
>
> > On 18/6/21 12:09 am, Desmond Cheong Zhi Xi wrote:
> > > Syzbot reported a circular locking dependency:
> > > https://syzkaller.appspot.com/bug?id=7bd106c28e846d1023d4ca915718b1a0905444cb
> > >
> > > This happens because of the following lock dependencies:
> > >
> > > 1. loop_ctl_mutex -> bdev->bd_mutex (when loop_control_ioctl calls
> > > loop_remove, which then calls del_gendisk; this also happens in
> > > loop_exit which eventually calls loop_remove)
> > >
> > > 2. bdev->bd_mutex -> mtd_table_mutex (when blkdev_get_by_dev calls
> > > __blkdev_get, which then calls blktrans_open)
> > >
> > > 3. mtd_table_mutex -> major_names_lock (when register_mtd_blktrans
> > > calls __register_blkdev)
> > >
> > > 4. major_names_lock -> loop_ctl_mutex (when blk_request_module calls
> > > loop_probe)
> > >
> > > Hence there's an overall dependency of:
> > >
> > > loop_ctl_mutex   ----------> bdev->bd_mutex
> > >         ^                            |
> > >         |                            |
> > >         |                            v
> > > major_names_lock <---------  mtd_table_mutex
> > >
> > > We can break this circular dependency by holding mtd_table_mutex only
> > > for the required critical section in register_mtd_blktrans. This
> > > avoids the mtd_table_mutex -> major_names_lock dependency.
> > >
> > > Reported-and-tested-by: syzbot+6a8a0d93c91e8fbf2e80@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Co-developed-by: Christoph Hellwig <hch@xxxxxx>
> > > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > > ---
> > >
> > > Changes in v1 -> v2:
> > >
> > > Break the circular dependency in register_mtd_blktrans instead of blk_request_module, as suggested by Christoph Hellwig.
> > >
> > >    drivers/mtd/mtd_blkdevs.c | 8 ++------
> > >    1 file changed, 2 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
> > > index fb8e12d590a1..7d26cfe24d05 100644
> > > --- a/drivers/mtd/mtd_blkdevs.c
> > > +++ b/drivers/mtd/mtd_blkdevs.c
> > > @@ -528,14 +528,10 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
> > >    	if (!blktrans_notifier.list.next)
> > >    		register_mtd_user(&blktrans_notifier);
> > >    > -
> > > -	mutex_lock(&mtd_table_mutex);
> > > -
> > >    	ret = register_blkdev(tr->major, tr->name);
> > >    	if (ret < 0) {
> > >    		printk(KERN_WARNING "Unable to register %s block device on major %d: %d\n",
> > >    		       tr->name, tr->major, ret);
> > > -		mutex_unlock(&mtd_table_mutex);
> > >    		return ret;
> > >    	}
> > >    > @@ -545,12 +541,12 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
> > >    	tr->blkshift = ffs(tr->blksize) - 1;
> > >    >   	INIT_LIST_HEAD(&tr->devs);
> > > -	list_add(&tr->list, &blktrans_majors);
> > >    > +	mutex_lock(&mtd_table_mutex);
> > > +	list_add(&tr->list, &blktrans_majors);
> > >    	mtd_for_each_device(mtd)
> > >    		if (mtd->type != MTD_ABSENT)
> > >    			tr->add_mtd(tr, mtd);
> > > -
> > >    	mutex_unlock(&mtd_table_mutex);
> > >    	return 0;
> > >    }
> >
> > Hi maintainers,
> >
> > Any chance to review this patch?
> >
> > For additional reference, the mtd_table_mutex --> major_names_lock hierarchy that can be removed by this patch also appears in a different lock chain:
> > https://syzkaller.appspot.com/bug?id=cbf5fe846f14a90f05e10df200b08c57941dc750
>
> I'm fine with the patch, but it came too late in the release cycle so
> now I'm waiting -rc1 to apply it.
>
> Thanks,
> Miquèl

Got it. Thanks for the review, Miquèl.