ovl: modify ovl_permission() to do checks on two inodes

BugLink: https://bugs.launchpad.net/bugs/1836668

commit c0ca3d70 upstream.

Right now ovl_permission() calls __inode_permission(realinode), to do
permission checks on real inode and no checks are done on overlay inode.

Modify it to do checks both on overlay inode as well as underlying inode.
Checks on overlay inode will be done with the creds of calling task while
checks on underlying inode will be done with the creds of mounter.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
[ Srivatsa: 4.4.y backport:
  - Skipped the hunk modifying non-existent function ovl_get_acl()
  - Adjusted the error path
  - Included linux/cred.h to get prototype for revert_creds() ]
Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/overlayfs/inode.c

@@ -9,6 +9,7 @@
 
 #include <linux/fs.h>
 #include <linux/slab.h>
+#include <linux/cred.h>
 #include <linux/xattr.h>
 #include "overlayfs.h"
 
@@ -91,6 +92,7 @@ int ovl_permission(struct inode *inode, int mask)
 	struct ovl_entry *oe;
 	struct dentry *alias = NULL;
 	struct inode *realinode;
+	const struct cred *old_cred;
 	struct dentry *realdentry;
 	bool is_upper;
 	int err;
@@ -143,7 +145,18 @@ int ovl_permission(struct inode *inode, int mask)
 			goto out_dput;
 	}
 
+	/*
+	 * Check overlay inode with the creds of task and underlying inode
+	 * with creds of mounter
+	 */
+	err = generic_permission(inode, mask);
+	if (err)
+		goto out_dput;
+
+	old_cred = ovl_override_creds(inode->i_sb);
 	err = __inode_permission(realinode, mask);
+	revert_creds(old_cred);
+
 out_dput:
 	dput(alias);
 	return err;