On Tue, Jan 26, 2021 at 09:30:45AM -0500, Josef Bacik wrote: > On 1/26/21 4:02 AM, Nikolay Borisov wrote: > > On 25.01.21 г. 23:42 ч., Josef Bacik wrote: > >> In __btrfs_return_cluster_to_free_space we will bail doing the cleanup > >> of the cluster if the block group we passed in doesn't match the block > >> group on the cluster. However we drop a reference to block_group, as > >> the cluster holds a reference to the block group while it's attached to > >> the cluster. If cluster->block_group != block_group however then this > >> is an extra put, which means we'll go negative and free this block group > >> down the line, leading to a UAF. > > > > Was this found by code inspection or did you hit in production. Also why > > in btrfs_remove_free_space_cache just before > > __btrfs_return_cluster_to_free_space there is: > > > > It was found in production sort of halfway. I was doing something for WhatsApp > and had to convert our block group reference counting to the refcount stuff so I > could find where I made a mistake. Turns out this was where the problem was, my > stuff had just made it way more likely to happen. I don't have the stack trace > because this was like 6 months ago, I'm going through all my WhatsApp magic and > getting them actually usable for upstream. > > > WARN_ON(cluster->block_group != block_group); > > > > IMO this patch should also remove the WARN_ON if it's a valid condition > > to have the passed bg be different than the one in the cluster. Also > > that WARN_ON is likely racy since it's done outside of cluster->lock. > > > > Yup that's in a follow up thing, I wanted to get the actual fix out before I got > distracted by my mountain of meetings this week. Thanks, Removing the WARN_ON in a separate patch sounds ok to me, this patch clearly fixes the refcounting bug, the warning condition is the same but would need a different reasoning. Nikolay, if you're ok with current patch version let me know if you want a rev-by added.
