gfs2: fix use-after-free in gfs2_qd_dealloc

gfs2: fix use-after-free in gfs2_qd_dealloc

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@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=42a37bf8045847d8f9d2
Tested-by: syzbot+42a37bf8045847d8f9d2@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>