ovl: fix use inode directly in rcu-walk mode

jira LE-2741
Rebuild_History Non-Buildable kernel-4.18.0-553.42.1.el8_10
commit-author Chen Zhongjin <chenzhongjin@huawei.com>
commit 672e426

ovl_dentry_revalidate_common() can be called in rcu-walk mode.  As document
said, "in rcu-walk mode, d_parent and d_inode should not be used without
care".

Check inode here to protect access under rcu-walk mode.

Fixes: bccece1 ("ovl: allow remote upper")
Reported-and-tested-by: syzbot+a4055c78774bbf3498bb@syzkaller.appspotmail.com
	Signed-off-by: Chen Zhongjin <chenzhongjin@huawei.com>
	Cc: <stable@vger.kernel.org> # v5.7
	Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
(cherry picked from commit 672e426)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

fs/overlayfs/super.c

@@ -141,11 +141,16 @@ static int ovl_dentry_revalidate_common(struct dentry *dentry,
 					unsigned int flags, bool weak)
 {
 	struct ovl_entry *oe = dentry->d_fsdata;
+	struct inode *inode = d_inode_rcu(dentry);
 	struct dentry *upper;
 	unsigned int i;
 	int ret = 1;
 
-	upper = ovl_dentry_upper(dentry);
+	/* Careful in RCU mode */
+	if (!inode)
+		return -ECHILD;
+
+	upper = ovl_i_dentry_upper(inode);
 	if (upper)
 		ret = ovl_revalidate_real(upper, flags, weak);