block: fix use-after-free on gendisk

commit 2da7809 "block: Fix dev_t minor allocation lifetime"
specifically moved blk_free_devt(dev->devt) call to part_release()
to avoid reallocating device number before the device is fully
shutdown.

However, it can cause use-after-free on gendisk in get_gendisk().
We use md device as example to show the race scenes:

Process1		Worker			Process2
md_free
						blkdev_open
del_gendisk
  add delete_partition_work_fn() to wq
  						__blkdev_get
						get_gendisk
put_disk
  disk_release
    kfree(disk)
    						find part from ext_devt_idr
						get_disk_and_module(disk)
    					  	cause use after free

    			delete_partition_work_fn
			put_device(part)
    		  	part_release
		    	remove part from ext_devt_idr

Before <devt, hd_struct pointer> is removed from ext_devt_idr by
delete_partition_work_fn(), we can find the devt and then access
gendisk by hd_struct pointer. But, if we access the gendisk after
it have been freed, it can cause in use-after-freeon gendisk in
get_gendisk().

We fix this by adding a new helper blk_invalidate_devt() in
delete_partition() and del_gendisk(). It replaces hd_struct
pointer in idr with value 'NULL', and deletes the entry from
idr in part_release() as we do now.

Thanks to Jan Kara for providing the solution and more clear comments
for the code.

Fixes: 2da7809 ("block: Fix dev_t minor allocation lifetime")
Cc: Al Viro <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Reviewed-by: Keith Busch <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Suggested-by: Jan Kara <[email protected]>
Signed-off-by: Yufen Yu <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

block/genhd.c

@@ -531,6 +531,18 @@ void blk_free_devt(dev_t devt)
 	}
 }
 
+/**
+ *	We invalidate devt by assigning NULL pointer for devt in idr.
+ */
+void blk_invalidate_devt(dev_t devt)
+{
+	if (MAJOR(devt) == BLOCK_EXT_MAJOR) {
+		spin_lock_bh(&ext_devt_lock);
+		idr_replace(&ext_devt_idr, NULL, blk_mangle_minor(MINOR(devt)));
+		spin_unlock_bh(&ext_devt_lock);
+	}
+}
+
 static char *bdevt_str(dev_t devt, char *buf)
 {
 	if (MAJOR(devt) <= 0xff && MINOR(devt) <= 0xff) {
@@ -793,6 +805,13 @@ void del_gendisk(struct gendisk *disk)
 
 	if (!(disk->flags & GENHD_FL_HIDDEN))
 		blk_unregister_region(disk_devt(disk), disk->minors);
+	/*
+	 * Remove gendisk pointer from idr so that it cannot be looked up
+	 * while RCU period before freeing gendisk is running to prevent
+	 * use-after-free issues. Note that the device number stays
+	 * "in-use" until we really free the gendisk.
+	 */
+	blk_invalidate_devt(disk_devt(disk));
 
 	kobject_put(disk->part0.holder_dir);
 	kobject_put(disk->slave_dir);

block/partition-generic.c

@@ -285,6 +285,13 @@ void delete_partition(struct gendisk *disk, int partno)
 	kobject_put(part->holder_dir);
 	device_del(part_to_dev(part));
 
+	/*
+	 * Remove gendisk pointer from idr so that it cannot be looked up
+	 * while RCU period before freeing gendisk is running to prevent
+	 * use-after-free issues. Note that the device number stays
+	 * "in-use" until we really free the gendisk.
+	 */
+	blk_invalidate_devt(part_devt(part));
 	hd_struct_kill(part);
 }
 

include/linux/genhd.h

@@ -617,6 +617,7 @@ struct unixware_disklabel {
 
 extern int blk_alloc_devt(struct hd_struct *part, dev_t *devt);
 extern void blk_free_devt(dev_t devt);
+extern void blk_invalidate_devt(dev_t devt);
 extern dev_t blk_lookup_devt(const char *name, int partno);
 extern char *disk_name (struct gendisk *hd, int partno, char *buf);