Re: [PATCH] gfs2: fix use-after-free in gfs2_qd_dealloc

[Andreas Gruenbacher]

On Fri, May 1, 2026 at 1:02 PM Tristan Madani <tristmd@xxxxxxxxx> wrote:
> From: Tristan Madani <tristan@xxxxxxxxxxxxxxxxxxx>
>
> gfs2_qd_dealloc(), called as an RCU callback from gfs2_qd_dispose(),
> accesses the superblock object sdp through qd->qd_sbd after freeing qd.
> It does so to decrement sd_quota_count and wake up sd_kill_wait.
>
> However, by the time the RCU callback runs, gfs2_put_super() may have
> already freed sdp via free_sbd().  This can happen when
> gfs2_quota_cleanup() is called during unmount: it disposes of quota
> objects via call_rcu() and then waits on sd_kill_wait with a 60-second
> timeout.  If the timeout expires, or if gfs2_gl_hash_clear() triggers
> additional qd_put() calls that schedule more RCU callbacks after the
> wait completes, gfs2_put_super() will proceed to free the superblock
> while RCU callbacks referencing it are still pending.
>
> Add an rcu_barrier() before free_sbd() in gfs2_put_super() to ensure
> all pending RCU callbacks (including gfs2_qd_dealloc) have completed
> before the superblock is freed.
>
> Fixes: a475c5dd16e5 ("gfs2: Free quota data objects synchronously")
> Reported-by: syzbot+42a37bf8045847d8f9d2@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=42a37bf8045847d8f9d2
> Tested-by: syzbot+42a37bf8045847d8f9d2@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Tristan Madani <tristan@xxxxxxxxxxxxxxxxxxx>
> ---
>  fs/gfs2/super.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
> index a2ea121331f18..4d854556b5299 100644
> --- a/fs/gfs2/super.c
> +++ b/fs/gfs2/super.c
> @@ -643,6 +643,7 @@ static void gfs2_put_super(struct super_block *sb)
>         gfs2_delete_debugfs_file(sdp);
>
>         gfs2_sys_fs_del(sdp);
> +       rcu_barrier();
>         free_sbd(sdp);
>  }
>
> --
> 2.47.3
>

Applied, thanks.

Andreas