Re: [PATCH] btrfs: avoid double put of block group when emptying cluster

[David Sterba <dsterba@suse.cz>]

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.