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

[David Sterba <dsterba@suse.cz>]

On Thu, Feb 11, 2021 at 01:25:52PM +0200, Nikolay Borisov wrote:
> 
> 
> On 11.02.21 г. 0:50 ч., David Sterba wrote:
> > 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.
> > 
> 
> 
> Codewise I'm fine with it. However just had another read of the commit
> message and I think it could be rewritten to be somewhat simpler:
> 
> It's wrong calling btrfs_put_block_group in
> __btrfs_return_cluster_to_free_space if the block group passed is
> different than the block group the cluster represents. As this means the
> cluster doesn't have a reference to the passed block group. This results
> in double put and an UAF.
> 
> What prompted me is that the 2nd and 3rd sentences read somewhat awkward
> due to starting with 'However'

Ok, updated, thanks. I left the last paragraph "Fix that ...".