Code: static struct mount *clone_mnt(struct mount *old, struct dentry *root,
					int flag)
{
	struct super_block *sb = old->mnt.mnt_sb;
	struct mount *mnt;
	int err;

	mnt = alloc_vfsmnt(old->mnt_devname);
	if (!mnt)
		return ERR_PTR(-ENOMEM);

	if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))
		mnt->mnt_group_id = 0; /* not a peer of original */
	else
		mnt->mnt_group_id = old->mnt_group_id;

	if ((flag & CL_MAKE_SHARED) && !mnt->mnt_group_id) {
		err = mnt_alloc_group_id(mnt);
		if (err)
			goto out_free;
 	}
 
 	mnt->mnt.mnt_flags = old->mnt.mnt_flags & ~MNT_WRITE_HOLD;
 	atomic_inc(&sb->s_active);
 	mnt->mnt.mnt_sb = sb;
 	mnt->mnt.mnt_root = dget(root);
	mnt->mnt_mountpoint = mnt->mnt.mnt_root;
	mnt->mnt_parent = mnt;
	br_write_lock(&vfsmount_lock);
	list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
	br_write_unlock(&vfsmount_lock);

	if ((flag & CL_SLAVE) ||
	    ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
		list_add(&mnt->mnt_slave, &old->mnt_slave_list);
		mnt->mnt_master = old;
		CLEAR_MNT_SHARED(mnt);
	} else if (!(flag & CL_PRIVATE)) {
		if ((flag & CL_MAKE_SHARED) || IS_MNT_SHARED(old))
			list_add(&mnt->mnt_share, &old->mnt_share);
		if (IS_MNT_SLAVE(old))
			list_add(&mnt->mnt_slave, &old->mnt_slave);
		mnt->mnt_master = old->mnt_master;
	}
	if (flag & CL_MAKE_SHARED)
		set_mnt_shared(mnt);

	/* stick the duplicate mount on the same expiry list
	 * as the original if that was on one */
	if (flag & CL_EXPIRE) {
		if (!list_empty(&old->mnt_expire))
			list_add(&mnt->mnt_expire, &old->mnt_expire);
	}

	return mnt;

 out_free:
	free_vfsmnt(mnt);
	return ERR_PTR(err);
}

Vulnerability Type: Bypass 
CWE ID: CWE-264
Summary: The clone_mnt function in fs/namespace.c in the Linux kernel before 3.8.6 does not properly restrict changes to the MNT_READONLY flag, which allows local users to bypass an intended read-only property of a filesystem by leveraging a separate mount namespace.
Commit Message: vfs: Carefully propogate mounts across user namespaces

As a matter of policy MNT_READONLY should not be changable if the
original mounter had more privileges than creator of the mount
namespace.

Add the flag CL_UNPRIVILEGED to note when we are copying a mount from
a mount namespace that requires more privileges to a mount namespace
that requires fewer privileges.

When the CL_UNPRIVILEGED flag is set cause clone_mnt to set MNT_NO_REMOUNT
if any of the mnt flags that should never be changed are set.

This protects both mount propagation and the initial creation of a less
privileged mount namespace.

Cc: stable@vger.kernel.org
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>