gfs2: Fix NULL pointer dereference in gfs2_log_flush

In gfs2_jindex_free(), set sdp->sd_jdesc to NULL under the log flush
lock to provide exclusion against gfs2_log_flush().

In gfs2_log_flush(), check if sdp->sd_jdesc is non-NULL before
dereferencing it.  Otherwise, we could run into a NULL pointer
dereference when outstanding glock work races with an unmount
(glock_work_func -> run_queue -> do_xmote -> inode_go_sync ->
gfs2_log_flush).
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/log.c

@@ -1108,7 +1108,8 @@ void gfs2_log_flush(struct gfs2_sbd *sdp, struct gfs2_glock *gl, u32 flags)
 	lops_before_commit(sdp, tr);
 	if (gfs2_withdrawing_or_withdrawn(sdp))
 		goto out_withdraw;
-	gfs2_log_submit_bio(&sdp->sd_jdesc->jd_log_bio, REQ_OP_WRITE);
+	if (sdp->sd_jdesc)
+		gfs2_log_submit_bio(&sdp->sd_jdesc->jd_log_bio, REQ_OP_WRITE);
 	if (gfs2_withdrawing_or_withdrawn(sdp))
 		goto out_withdraw;
 

fs/gfs2/super.c

@@ -67,9 +67,13 @@ void gfs2_jindex_free(struct gfs2_sbd *sdp)
 	sdp->sd_journals = 0;
 	spin_unlock(&sdp->sd_jindex_spin);
 
+	down_write(&sdp->sd_log_flush_lock);
 	sdp->sd_jdesc = NULL;
+	up_write(&sdp->sd_log_flush_lock);
+
 	while (!list_empty(&list)) {
 		jd = list_first_entry(&list, struct gfs2_jdesc, jd_list);
+		BUG_ON(jd->jd_log_bio);
 		gfs2_free_journal_extents(jd);
 		list_del(&jd->jd_list);
 		iput(jd->jd_inode);