Code: static int do_remount(struct path *path, int flags, int mnt_flags,
		      void *data)
{
	int err;
	struct super_block *sb = path->mnt->mnt_sb;
	struct mount *mnt = real_mount(path->mnt);

	if (!check_mnt(mnt))
		return -EINVAL;

	if (path->dentry != path->mnt->mnt_root)
		return -EINVAL;

	err = security_sb_remount(sb, data);
	if (err)
		return err;

	down_write(&sb->s_umount);
	if (flags & MS_BIND)
		err = change_mount_flags(path->mnt, flags);
	else if (!capable(CAP_SYS_ADMIN))
		err = -EPERM;
	else
 		err = do_remount_sb(sb, flags, data, 0);
 	if (!err) {
 		lock_mount_hash();
		mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
 		mnt->mnt.mnt_flags = mnt_flags;
 		touch_mnt_namespace(mnt->mnt_ns);
 		unlock_mount_hash();
	}
	up_write(&sb->s_umount);
	return err;
}

Vulnerability Type: Bypass 
CWE ID: CWE-264
Summary: The do_remount function in fs/namespace.c in the Linux kernel through 3.16.1 does not maintain the MNT_LOCK_READONLY bit across a remount of a bind mount, which allows local users to bypass an intended read-only restriction and defeat certain sandbox protection mechanisms via a *mount -o remount* command within a user namespace.
Commit Message: mnt: Only change user settable mount flags in remount

Kenton Varda <kenton@sandstorm.io> discovered that by remounting a
read-only bind mount read-only in a user namespace the
MNT_LOCK_READONLY bit would be cleared, allowing an unprivileged user
to the remount a read-only mount read-write.

Correct this by replacing the mask of mount flags to preserve
with a mask of mount flags that may be changed, and preserve
all others.   This ensures that any future bugs with this mask and
remount will fail in an easy to detect way where new mount flags
simply won't change.

Cc: stable@vger.kernel.org
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>