ovl: fix use inode directly in rcu-walk mode

stable inclusion
from stable-v5.10.163
commit 740c537f52c1f54aff9094744483d1515c7c8b7b
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6GCCV

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=740c537f52c1f54aff9094744483d1515c7c8b7b

--------------------------------

commit 672e4268 upstream.

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: bccece1e ("ovl: allow remote upper")
Reported-and-tested-by: syzbot+a4055c78774bbf3498bb@syzkaller.appspotmail.com
Signed-off-by: NChen Zhongjin <chenzhongjin@huawei.com>
Cc: <stable@vger.kernel.org> # v5.7
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NZhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: NYang Erkun <yangerkun@huawei.com>
Reviewed-by: NZhang Yi <yi.zhang@huawei.com>
Signed-off-by: NJialin Zhang <zhangjialin11@huawei.com>

fs/overlayfs/super.c

@@ -138,11 +138,16 @@ 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);