[PATCH 13/15] loop: Move loop_reread_partitions() out of loop_ctl_mutex

Jan Kara
2018-10-10T10:04:13Z

Calling loop_reread_partitions() under loop_ctl_mutex causes lockdep to
complain about circular lock dependency between bdev->bd_mutex and
lo->lo_ctl_mutex. The problem is that on loop device open or close
lo_open() and lo_release() get called with bdev->bd_mutex held and they
need to acquire loop_ctl_mutex. OTOH when loop_reread_partitions() is
called with loop_ctl_mutex held, it will call blkdev_reread_part() which
acquires bdev->bd_mutex. See syzbot report for details [1].

Move all calls of loop_rescan_partitions() out of loop_ctl_mutex to
avoid lockdep warning and fix deadlock possibility.

[1] https://syzkaller.appspot.com/bug?id=bf154052f0eea4bc7712499e4569505907d1588

Reported-by: syzbot <syzbot+4684a000d5abdade83fac55b1e7d1f935ef1936e@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
drivers/block/loop.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index db73fb5f16c7..0d54c3ee3a96 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -680,6 +680,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
{
struct file *file, *old_file;
int error;
+ bool partscan;

error = mutex_lock_killable_nested(&loop_ctl_mutex, 1);
if (error)
@@ -721,9 +722,10 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
blk_mq_unfreeze_queue(lo->lo_queue);

fput(old_file);
- if (lo->lo_flags & LO_FLAGS_PARTSCAN)
- loop_reread_partitions(lo, bdev);
+ partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;
mutex_unlock(&loop_ctl_mutex);
+ if (partscan)
+ loop_reread_partitions(lo, bdev);
return 0;

out_putf:
@@ -904,6 +906,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
int lo_flags = 0;
int error;
loff_t size;
+ bool partscan;

/* This is safe, since we have a reference from open(). */
__module_get(THIS_MODULE);
@@ -970,14 +973,15 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
lo->lo_state = Lo_bound;
if (part_shift)
lo->lo_flags |= LO_FLAGS_PARTSCAN;
- if (lo->lo_flags & LO_FLAGS_PARTSCAN)
- loop_reread_partitions(lo, bdev);
+ partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;

/* Grab the block_device to prevent its destruction after we
* put /dev/loopXX inode. Later in __loop_clr_fd() we bdput(bdev).
*/
bdgrab(bdev);
mutex_unlock(&loop_ctl_mutex);
+ if (partscan)
+ loop_reread_partitions(lo, bdev);
return 0;

out_unlock:
@@ -1158,6 +1162,8 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
int err;
struct loop_func_table *xfer;
kuid_t uid = current_uid();
+ struct block_device *bdev;
+ bool partscan = false;

err = mutex_lock_killable_nested(&loop_ctl_mutex, 1);
if (err)
@@ -1246,10 +1252,13 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
!(lo->lo_flags & LO_FLAGS_PARTSCAN)) {
lo->lo_flags |= LO_FLAGS_PARTSCAN;
lo->lo_disk->flags &= ~GENHD_FL_NO_PART_SCAN;
- loop_reread_partitions(lo, lo->lo_device);
+ bdev = lo->lo_device;
+ partscan = true;
}
out_unlock:
mutex_unlock(&loop_ctl_mutex);
+ if (partscan)
+ loop_reread_partitions(lo, bdev);

return err;
}
--
2.16.4