From 05acefb4872dae89e772729efb194af754c877e8 Mon Sep 17 00:00:00 2001
From: Miklos Szeredi <mszeredi@redhat.com>
Date: Tue Jun  2 22:20:26 2020 +0200
Subject: [PATCH] ovl: check permission to open real file 
Git-commit: 05acefb4872dae89e772729efb194af754c877e8
References: bsc#1177470, CVE-2020-16120
Patch-mainline: v5.8-rc1

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: Miklos Szeredi <mszeredi@redhat.com>
Acked-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 1860e22..0f83c8d 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -40,10 +40,22 @@ static struct file *ovl_open_realfile(const struct file *file,
 	struct file *realfile;
 	const struct cred *old_cred;
 	int flags = file->f_flags | O_NOATIME | FMODE_NONOTIFY;
+	int acc_mode = ACC_MODE(flags);
+	int err;
+
+	if (flags & O_APPEND)
+		acc_mode |= MAY_APPEND;
 
 	old_cred = ovl_override_creds(inode->i_sb);
-	realfile = open_with_fake_path(&file->f_path, flags, realinode,
-				       current_cred());
+	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",