From 0da03cab87e6323ff2e05b14bc7d5c6fcc531efd Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 8 Nov 2018 14:01:14 +0100 Subject: [PATCH] loop: Fix deadlock when calling blkdev_reread_part() 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 Signed-off-by: Jan Kara Signed-off-by: Jens Axboe --- 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) { /*