[PATCH next V2] gfs2: Add sanity check for sd_jdesc

[Edward Adam Davis <eadavis@qq.com>]

Asynchronous withdraw, when sd_withdraw_work is scheduled later than
put_super, will set sd_jdesc to NULL when clearing all journal index
information, triggering the syz report null-ptr-deref.

Reported-by: syzbot+6b156e132970e550194c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6b156e132970e550194c
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
V1 -> V2: Updated the issue reported in the comments

 fs/gfs2/log.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/gfs2/log.c b/fs/gfs2/log.c
index 8312cd2cdae4..433a3a11a2f5 100644
--- a/fs/gfs2/log.c
+++ b/fs/gfs2/log.c
@@ -427,7 +427,8 @@ static void ail2_empty(struct gfs2_sbd *sdp, unsigned int new_tail)
  */
 
 bool gfs2_log_is_empty(struct gfs2_sbd *sdp) {
-	return atomic_read(&sdp->sd_log_blks_free) == sdp->sd_jdesc->jd_blocks;
+	return sdp->sd_jdesc &&
+	       atomic_read(&sdp->sd_log_blks_free) == sdp->sd_jdesc->jd_blocks;
 }
 
 static bool __gfs2_log_try_reserve_revokes(struct gfs2_sbd *sdp, unsigned int revokes)
-- 
2.43.0