loop: reduce the loop_ctl_mutex scope

syzbot is reporting circular locking problem at __loop_clr_fd() [1], for
commit a160c6159d ("block: add an optional probe callback to
major_names") is calling the module's probe function with major_names_lock
held.

Fortunately, since commit 990e78116d ("block: loop: fix deadlock
between open and remove") stopped holding loop_ctl_mutex in lo_open(),
current role of loop_ctl_mutex is to serialize access to loop_index_idr
and loop_add()/loop_remove(); in other words, management of id for IDR.
To avoid holding loop_ctl_mutex during whole add/remove operation, use
a bool flag to indicate whether the loop device is ready for use.

loop_unregister_transfer() which is called from cleanup_cryptoloop()
currently has possibility of use-after-free problem due to lack of
serialization between kfree() from loop_remove() from loop_control_remove()
and mutex_lock() from unregister_transfer_cb(). But since lo->lo_encryption
should be already NULL when this function is called due to module unload,
and commit 222013f9ac ("cryptoloop: add a deprecation warning")
indicates that we will remove this function shortly, this patch updates
this function to emit warning instead of checking lo->lo_encryption.

Holding loop_ctl_mutex in loop_exit() is pointless, for all users must
close /dev/loop-control and /dev/loop$num (in order to drop module's
refcount to 0) before loop_exit() starts, and nobody can open
/dev/loop-control or /dev/loop$num afterwards.

Link: https://syzkaller.appspot.com/bug?id=7bb10e8b62f83e4d445cdf4c13d69e407e629558 [1]
Reported-by: syzbot <syzbot+f61766d5763f9e7a118f@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/adb1e792-fc0e-ee81-7ea0-0906fc36419d@i-love.sakura.ne.jp
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -2111,18 +2111,6 @@ int loop_register_transfer(struct loop_func_table *funcs)
 	return 0;
 }
 
-static int unregister_transfer_cb(int id, void *ptr, void *data)
-{
-	struct loop_device *lo = ptr;
-	struct loop_func_table *xfer = data;
-
-	mutex_lock(&lo->lo_mutex);
-	if (lo->lo_encryption == xfer)
-		loop_release_xfer(lo);
-	mutex_unlock(&lo->lo_mutex);
-	return 0;
-}
-
 int loop_unregister_transfer(int number)
 {
 	unsigned int n = number;
@@ -2130,9 +2118,20 @@ int loop_unregister_transfer(int number)
 
 	if (n == 0 || n >= MAX_LO_CRYPT || (xfer = xfer_funcs[n]) == NULL)
 		return -EINVAL;
+	/*
+	 * This function is called from only cleanup_cryptoloop().
+	 * Given that each loop device that has a transfer enabled holds a
+	 * reference to the module implementing it we should never get here
+	 * with a transfer that is set (unless forced module unloading is
+	 * requested). Thus, check module's refcount and warn if this is
+	 * not a clean unloading.
+	 */
+#ifdef CONFIG_MODULE_UNLOAD
+	if (xfer->owner && module_refcount(xfer->owner) != -1)
+		pr_err("Danger! Unregistering an in use transfer function.\n");
+#endif
 
 	xfer_funcs[n] = NULL;
-	idr_for_each(&loop_index_idr, &unregister_transfer_cb, xfer);
 	return 0;
 }
 
@@ -2323,8 +2322,9 @@ static int loop_add(int i)
 	} else {
 		err = idr_alloc(&loop_index_idr, lo, 0, 0, GFP_KERNEL);
 	}
+	mutex_unlock(&loop_ctl_mutex);
 	if (err < 0)
-		goto out_unlock;
+		goto out_free_dev;
 	i = err;
 
 	err = -ENOMEM;
@@ -2393,15 +2393,19 @@ static int loop_add(int i)
 	disk->events		= DISK_EVENT_MEDIA_CHANGE;
 	disk->event_flags	= DISK_EVENT_FLAG_UEVENT;
 	sprintf(disk->disk_name, "loop%d", i);
+	/* Make this loop device reachable from pathname. */
 	add_disk(disk);
+	/* Show this loop device. */
+	mutex_lock(&loop_ctl_mutex);
+	lo->idr_visible = true;
 	mutex_unlock(&loop_ctl_mutex);
 	return i;
 
 out_cleanup_tags:
 	blk_mq_free_tag_set(&lo->tag_set);
 out_free_idr:
+	mutex_lock(&loop_ctl_mutex);
 	idr_remove(&loop_index_idr, i);
-out_unlock:
 	mutex_unlock(&loop_ctl_mutex);
 out_free_dev:
 	kfree(lo);
@@ -2411,9 +2415,14 @@ out:
 
 static void loop_remove(struct loop_device *lo)
 {
+	/* Make this loop device unreachable from pathname. */
 	del_gendisk(lo->lo_disk);
 	blk_cleanup_disk(lo->lo_disk);
 	blk_mq_free_tag_set(&lo->tag_set);
+	mutex_lock(&loop_ctl_mutex);
+	idr_remove(&loop_index_idr, lo->lo_number);
+	mutex_unlock(&loop_ctl_mutex);
+	/* There is no route which can find this loop device. */
 	mutex_destroy(&lo->lo_mutex);
 	kfree(lo);
 }
@@ -2437,31 +2446,40 @@ static int loop_control_remove(int idx)
 		return -EINVAL;
 	}
 		
+	/* Hide this loop device for serialization. */
 	ret = mutex_lock_killable(&loop_ctl_mutex);
 	if (ret)
 		return ret;
-
 	lo = idr_find(&loop_index_idr, idx);
-	if (!lo) {
+	if (!lo || !lo->idr_visible)
 		ret = -ENODEV;
-		goto out_unlock_ctrl;
+	else
-	}
+		lo->idr_visible = false;
+	mutex_unlock(&loop_ctl_mutex);
+	if (ret)
+		return ret;
 
+	/* Check whether this loop device can be removed. */
 	ret = mutex_lock_killable(&lo->lo_mutex);
 	if (ret)
-		goto out_unlock_ctrl;
+		goto mark_visible;
 	if (lo->lo_state != Lo_unbound ||
 	    atomic_read(&lo->lo_refcnt) > 0) {
 		mutex_unlock(&lo->lo_mutex);
 		ret = -EBUSY;
-		goto out_unlock_ctrl;
+		goto mark_visible;
 	}
+	/* Mark this loop device no longer open()-able. */
 	lo->lo_state = Lo_deleting;
 	mutex_unlock(&lo->lo_mutex);
 
-	idr_remove(&loop_index_idr, lo->lo_number);
 	loop_remove(lo);
-out_unlock_ctrl:
+	return 0;
+
+mark_visible:
+	/* Show this loop device again. */
+	mutex_lock(&loop_ctl_mutex);
+	lo->idr_visible = true;
 	mutex_unlock(&loop_ctl_mutex);
 	return ret;
 }
@@ -2475,7 +2493,8 @@ static int loop_control_get_free(int idx)
 	if (ret)
 		return ret;
 	idr_for_each_entry(&loop_index_idr, lo, id) {
-		if (lo->lo_state == Lo_unbound)
+		/* Hitting a race results in creating a new loop device which is harmless. */
+		if (lo->idr_visible && data_race(lo->lo_state) == Lo_unbound)
 			goto found;
 	}
 	mutex_unlock(&loop_ctl_mutex);
@@ -2591,10 +2610,14 @@ static void __exit loop_exit(void)
 	unregister_blkdev(LOOP_MAJOR, "loop");
 	misc_deregister(&loop_misc);
 
-	mutex_lock(&loop_ctl_mutex);
+	/*
+	 * There is no need to use loop_ctl_mutex here, for nobody else can
+	 * access loop_index_idr when this module is unloading (unless forced
+	 * module unloading is requested). If this is not a clean unloading,
+	 * we have no means to avoid kernel crash.
+	 */
 	idr_for_each_entry(&loop_index_idr, lo, id)
 		loop_remove(lo);
-	mutex_unlock(&loop_ctl_mutex);
 
 	idr_destroy(&loop_index_idr);
 }

drivers/block/loop.h

@@ -68,6 +68,7 @@ struct loop_device {
 	struct blk_mq_tag_set	tag_set;
 	struct gendisk		*lo_disk;
 	struct mutex		lo_mutex;
+	bool			idr_visible;
 };
 
 struct loop_cmd {