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 <syzbot+4684a000d5abdade83fac55b1e7d1f935ef1936e@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index cce5d4e8e8632d649905314b7edf3564b8e1116e..b3f981ac8ef10069626b78a7f2ace5292011dea8 100644 (file)
--- 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) {
                /*