lkml.org 
[lkml]   [2021]   [Jun]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2] mtd: break circular locks in register_mtd_blktrans
Hello,

Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> 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@syzkaller.appspotmail.com
> > Co-developed-by: Christoph Hellwig <hch@lst.de>
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> > ---
> >
> > 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

\
 
 \ /
  Last update: 2021-06-30 10:44    [W:0.056 / U:0.920 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site