Re: [PATCH] gfs2: mark the VFS superblock read-only before quota cleanup

[Andreas Gruenbacher]

On Tue, Aug 4, 2026 at 4:34 AM Tao Yu <tao1.yu@xxxxxxxxx> wrote:
> During unmount, gfs2_put_super() calls gfs2_make_fs_ro(), which shuts
> down quota/statfs activity and then frees the quota bitmap via
> gfs2_quota_cleanup().
>
> However, gfs2_make_fs_ro() did not mark the VFS superblock read-only
> before that cleanup. As a result, later inode eviction could still
> enter the deallocation path through gfs2_evict_inode() ->
> gfs2_dinode_dealloc() -> gfs2_quota_hold() -> slot_get(), even though
> sd_quota_bitmap had already been freed and cleared. That leads to a
> NULL pointer dereference in find_first_zero_bit().
>
> Set SB_RDONLY after the final quota/statfs sync and log shutdown, but
> before gfs2_quota_cleanup(). This keeps subsequent inode eviction from
> re-entering write-side quota/deallocation paths once quota state has
> been torn down.
>
> This fixes a crash reported by syzbot:
>
>   Oops: general protection fault in _find_first_zero_bit
>   KASAN: null-ptr-deref in slot_get()
>
> Reported-by: syzbot+cb79de2cc8b76fbf474f@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=cb79de2cc8b76fbf474f
> Signed-off-by: Tao Yu <tao1.yu@xxxxxxxxx>
> ---
>  fs/gfs2/super.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
> index 4d854556b5299..6b5457f965673 100644
> --- a/fs/gfs2/super.c
> +++ b/fs/gfs2/super.c
> @@ -565,6 +565,14 @@ void gfs2_make_fs_ro(struct gfs2_sbd *sdp)
>                                    HZ * 5);
>                 gfs2_assert_warn(sdp, gfs2_log_is_empty(sdp));
>         }
> +
> +       /*
> +        * Once the final quota/statfs sync and log shutdown are complete, the VFS
> +        * must see the filesystem as read-only so later inode eviction cannot
> +        * re-enter deallocation paths that still expect live quota state.
> +        */
> +       sdp->sd_vfs->s_flags |= SB_RDONLY;
> +
>         gfs2_quota_cleanup(sdp);
>  }
>
> --
> 2.34.1
>

Thanks for the patch but no, that's not it. Instead, I've added this:

https://lore.kernel.org/gfs2/20260827090637.676820-1-agruenba@xxxxxxxxxx

Andreas