From 56230d956739b9cb1cbde439d76227d77979a04d 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: verify permissions in ovl_path_open() 
Git-commit: 56230d956739b9cb1cbde439d76227d77979a04d
References: bsc#1177470, CVE-2020-16120
Patch-mainline: v5.7

Check permission before opening a real file.

ovl_path_open() is used by readdir and copy-up routines.

ovl_permission() theoretically already checked copy up permissions, but it
doesn't hurt to re-do these checks during the actual copy-up.

For directory reading ovl_permission() only checks access to topmost
underlying layer.  Readdir on a merged directory accesses layers below the
topmost one as well.  Permission wasn't checked for these layers.

Note: modifying ovl_permission() to perform this check would be far more
complex and hence more bug prone.  The result is less precise permissions
returned in access(2).  If this turns out to be an issue, we can revisit
this bug.

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

---
 fs/overlayfs/util.c |   27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -271,7 +271,32 @@
 
 struct file *ovl_path_open(struct path *path, int flags)
 {
-	return dentry_open(path, flags | O_NOATIME, current_cred());
+	struct inode *inode = d_inode(path->dentry);
+	int err, acc_mode;
+
+	if (flags & ~(O_ACCMODE | O_LARGEFILE))
+		BUG();
+
+	switch (flags & O_ACCMODE) {
+	case O_RDONLY:
+		acc_mode = MAY_READ;
+		break;
+	case O_WRONLY:
+		acc_mode = MAY_WRITE;
+		break;
+	default:
+		BUG();
+	}
+
+	err = inode_permission(inode, acc_mode | MAY_OPEN);
+	if (err)
+		return ERR_PTR(err);
+
+	/* O_NOATIME is an optimization, don't fail if not permitted */
+	if (inode_owner_or_capable(inode))
+		flags |= O_NOATIME;
+
+	return dentry_open(path, flags, current_cred());
 }
 
 int ovl_copy_up_start(struct dentry *dentry)