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

[Josef Bacik <josef@toxicpanda.com>]

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,

Josef