[PATCH v2 3/5] bcache: fix refcount underflow in bcache_device_free()

Coly Li
2020-05-27T04:01:53Z

The problematic code piece in bcache_device_free() is,

785 static void bcache_device_free(struct bcache_device *d)
786 {
787 struct gendisk *disk = d->disk;
[snipped]
799 if (disk) {
800 if (disk->flags & GENHD_FL_UP)
801 del_gendisk(disk);
802
803 if (disk->queue)
804 blk_cleanup_queue(disk->queue);
805
806 ida_simple_remove(&bcache_device_idx,
807 first_minor_to_idx(disk->first_minor));
808 put_disk(disk);
809 }
[snipped]
816 }

At line 808, put_disk(disk) may encounter kobject refcount of 'disk'
being underflow.

Here is how to reproduce the issue,
- Attche the backing device to a cache device and do random write to
make the cache being dirty.
- Stop the bcache device while the cache device has dirty data of the
backing device.
- Only register the backing device back, NOT register cache device.
- The bcache device node /dev/bcache0 won't show up, because backing
device waits for the cache device shows up for the missing dirty
data.
- Now echo 1 into /sys/fs/bcache/pendings_cleanup, to stop the pending
backing device.
- After the pending backing device stopped, use 'dmesg' to check kernel
message, a use-after-free warning from KASA reported the refcount of
kobject linked to the 'disk' is underflow.

The dropping refcount at line 808 in the above code piece is added by
add_disk(d->disk) in bch_cached_dev_run(). But in the above condition
the cache device is not registered, bch_cached_dev_run() has no chance
to be called and the refcount is not added. The put_disk() for a non-
added refcount of gendisk kobject triggers a underflow warning.

This patch checks whether GENHD_FL_UP is set in disk->flags, if it is
not set then the bcache device was not added, don't call put_disk()
and the the underflow issue can be avoided.

Signed-off-by: Coly Li <colyli@suse.de>
---
Changelog:
v2: make the code to be more cleaner by suggestion from Jens Axboe.
v1: initial version.

drivers/md/bcache/super.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 467149f3bcc5..a10a3c78f4ff 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -797,7 +797,9 @@ static void bcache_device_free(struct bcache_device *d)
bcache_device_detach(d);

if (disk) {
- if (disk->flags & GENHD_FL_UP)
+ bool disk_added = (disk->flags & GENHD_FL_UP) != 0;
+
+ if (disk_added)
del_gendisk(disk);

if (disk->queue)
@@ -805,7 +807,8 @@ static void bcache_device_free(struct bcache_device *d)

ida_simple_remove(&bcache_device_idx,
first_minor_to_idx(disk->first_minor));
- put_disk(disk);
+ if (disk_added)
+ put_disk(disk);
}

bioset_exit(&d->bio_split);
--
2.25.0