Re: [PATCH next] btrfs: Accessing head_ref within delayed_refs lock

[Filipe Manana]

On Tue, Oct 15, 2024 at 1:40 PM Edward Adam Davis <eadavis@xxxxxx> wrote:
>
> This is because the thread routine btrfs_work_helper released head_def after
> exiting delayed_refs->lock in add_delayed_ref.

This should be explained a lot better. Starting the changelog with
"This is because..." is odd. It should explain why the head reference
was freed (because delayed references were run).

> Causing add_delayed_ref to encounter uaf when accessing head_def->bytenr
> outside the delayed_refs->lock.
>
> Move head_ref->bytenr into the protection range of delayed_refs->lock
> to avoid uaf in add_delayed_ref.


This was already fixed yesterday, in a simpler way and with an
explanation of what's going on to trigger the use-after-free:

https://lore.kernel.org/linux-btrfs/02fc507b62b19be2348fc08de8b13bd7af1a440e.1728922973.git.fdmanana@xxxxxxxx/

Thanks.


>
> Fixes: a3aad8f4f5d9 ("btrfs: qgroups: remove bytenr field from struct btrfs_qgroup_extent_record")
> Reported-and-tested-by: syzbot+c3a3a153f0190dca5be9@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=c3a3a153f0190dca5be9
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> ---
>  fs/btrfs/delayed-ref.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
> index 13c2e00d1270..f50fc05847a1 100644
> --- a/fs/btrfs/delayed-ref.c
> +++ b/fs/btrfs/delayed-ref.c
> @@ -1012,6 +1012,7 @@ static int add_delayed_ref(struct btrfs_trans_handle *trans,
>         int action = generic_ref->action;
>         bool merged;
>         int ret;
> +       u64 bytenr;
>
>         node = kmem_cache_alloc(btrfs_delayed_ref_node_cachep, GFP_NOFS);
>         if (!node)
> @@ -1056,6 +1057,7 @@ static int add_delayed_ref(struct btrfs_trans_handle *trans,
>                 goto free_record;
>         }
>         head_ref = new_head_ref;
> +       bytenr = head_ref->bytenr;
>
>         merged = insert_delayed_ref(trans, head_ref, node);
>         spin_unlock(&delayed_refs->lock);
> @@ -1074,7 +1076,7 @@ static int add_delayed_ref(struct btrfs_trans_handle *trans,
>                 kmem_cache_free(btrfs_delayed_ref_node_cachep, node);
>
>         if (qrecord_inserted)
> -               return btrfs_qgroup_trace_extent_post(trans, record, head_ref->bytenr);
> +               return btrfs_qgroup_trace_extent_post(trans, record, bytenr);
>         return 0;
>
>  free_record:
> --
> 2.43.0
>
>