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.