ovl: check permission to open real file

mainline inclusion
from mainline-v5.8-rc1
commit 05acefb4
category: bugfix
bugzilla: NA
CVE: CVE-2020-16120

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

Call inode_permission() on real inode before opening regular file on one of
the underlying layers.

In some cases ovl_permission() already checks access to an underlying file,
but it misses the metacopy case, and possibly other ones as well.

Removing the redundant permission check from ovl_permission() should be
considered later.
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Conflicts:
  fs/overlayfs/file.c
[yyl: adjust context]
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Nzhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>

fs/overlayfs/file.c

@@ -35,10 +35,22 @@ static struct file *ovl_open_realfile(const struct file *file,
 	struct file *realfile;
 	const struct cred *old_cred;
 	int flags = file->f_flags | OVL_OPEN_FLAGS;
+	int acc_mode = ACC_MODE(flags);
+	int err;
+
+	if (flags & O_APPEND)
+		acc_mode |= MAY_APPEND;
 
 	old_cred = ovl_override_creds(inode->i_sb);
+	err = inode_permission(realinode, MAY_OPEN | acc_mode);
+	if (err) {
+		realfile = ERR_PTR(err);
+	} else if (!inode_owner_or_capable(realinode)) {
+		realfile = ERR_PTR(-EPERM);
+	} else {
 		realfile = open_with_fake_path(&file->f_path, flags, realinode,
 					       current_cred());
+	}
 	revert_creds(old_cred);
 
 	pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",