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
