[PATCH 14/16] loop: Fix deadlock when calling blkdev_reread_part()

Jan Kara
2018-11-08T22:36:47Z

Calling blkdev_reread_part() 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 call to blkdev_reread_part() in __loop_clr_fd() from under
loop_ctl_mutex to finish fixing of the lockdep warning and the possible
deadlock.

[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 | 28 ++++++++++++++++------------
1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index cce5d4e8e863..b3f981ac8ef1 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1030,12 +1030,14 @@ loop_init_xfer(struct loop_device *lo, struct loop_func_table *xfer,
return err;
}

-static int __loop_clr_fd(struct loop_device *lo)
+static int __loop_clr_fd(struct loop_device *lo, bool release)
{
struct file *filp = NULL;
gfp_t gfp = lo->old_gfp_mask;
struct block_device *bdev = lo->lo_device;
int err = 0;
+ bool partscan = false;
+ int lo_number;

mutex_lock(&loop_ctl_mutex);
if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
@@ -1088,7 +1090,15 @@ static int __loop_clr_fd(struct loop_device *lo)
module_put(THIS_MODULE);
blk_mq_unfreeze_queue(lo->lo_queue);

- if (lo->lo_flags & LO_FLAGS_PARTSCAN && bdev) {
+ partscan = lo->lo_flags & LO_FLAGS_PARTSCAN && bdev;
+ lo_number = lo->lo_number;
+ lo->lo_flags = 0;
+ if (!part_shift)
+ lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN;
+ loop_unprepare_queue(lo);
+out_unlock:
+ mutex_unlock(&loop_ctl_mutex);
+ if (partscan) {
/*
* bd_mutex has been held already in release path, so don't
* acquire it if this function is called in such case.
@@ -1097,21 +1107,15 @@ static int __loop_clr_fd(struct loop_device *lo)
* must be at least one and it can only become zero when the
* current holder is released.
*/
- if (!atomic_read(&lo->lo_refcnt))
+ if (release)
err = __blkdev_reread_part(bdev);
else
err = blkdev_reread_part(bdev);
pr_warn("%s: partition scan of loop%d failed (rc=%d)\n",
- __func__, lo->lo_number, err);
+ __func__, lo_number, err);
/* Device is gone, no point in returning error */
err = 0;
}
- lo->lo_flags = 0;
- if (!part_shift)
- lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN;
- loop_unprepare_queue(lo);
-out_unlock:
- mutex_unlock(&loop_ctl_mutex);
/*
* Need not hold loop_ctl_mutex to fput backing file.
* Calling fput holding loop_ctl_mutex triggers a circular
@@ -1152,7 +1156,7 @@ static int loop_clr_fd(struct loop_device *lo)
lo->lo_state = Lo_rundown;
mutex_unlock(&loop_ctl_mutex);

- return __loop_clr_fd(lo);
+ return __loop_clr_fd(lo, false);
}

static int
@@ -1713,7 +1717,7 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
* In autoclear mode, stop the loop thread
* and remove configuration after last close.
*/
- __loop_clr_fd(lo);
+ __loop_clr_fd(lo, true);
return;
} else if (lo->lo_state == Lo_bound) {
/*
--
2.16.4