Re: [syzbot] KASAN: use-after-free Read in disk_part_iter_next (2)

From: Bart Van Assche
Date: Mon Mar 22 2021 - 18:21:59 EST

On 3/22/21 12:18 AM, Christoph Hellwig wrote:
I've been running the reproducer on a KASAN enable VM for about
15 minutes now, but haven't been able to reproduce it.

Is there a way to inject this proposed fix into the syzbot queue?

diff --git a/block/partitions/core.c b/block/partitions/core.c
index 1a7558917c47d6..f5d5872b89d57e 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -288,15 +288,12 @@ struct device_type part_type = {
void delete_partition(struct block_device *part)
xa_erase(&part->bd_disk->part_tbl, part->bd_partno);
- kobject_put(part->bd_holder_dir);
- device_del(&part->bd_device);
- /*
- * Remove the block device from the inode hash, so that it cannot be
- * looked up any more even when openers still hold references.
- */
+ synchronize_rcu();
+ kobject_put(part->bd_holder_dir);
+ device_del(&part->bd_device);

Hi Christoph,

disk_part_iter_next() calls bdgrab() before returning a pointer to a certain partition. 'part' is only freed if its reference count drops to zero. The function that frees the partition information, bdev_free_inode(), is invoked via call_rcu(). bdgrab() fails if the refcount of a partition is zero. Does that mean that it is not necessary to call synchronize_rcu() between xa_erase() and put_device()?