On Thu, Sep 22, 2022 at 12:20:50PM +0100, Daniel P. Berrangé wrote: > On Wed, Sep 21, 2022 at 03:44:24PM -0300, Jason Gunthorpe wrote: > > On Wed, Sep 21, 2022 at 12:06:49PM -0600, Alex Williamson wrote: > > > The issue is where we account these pinned pages, where accounting is > > > necessary such that a user cannot lock an arbitrary number of pages > > > into RAM to generate a DoS attack. > > > > It is worth pointing out that preventing a DOS attack doesn't actually > > work because a *task* limit is trivially bypassed by just spawning > > more tasks. So, as a security feature, this is already very > > questionable. > > The malicious party on host VM hosts is generally the QEMU process. > QEMU is normally prevented from spawning more tasks, both by SELinux > controls and be the seccomp sandbox blocking clone() (except for > thread creation). We need to constrain what any individual QEMU can > do to the host, and the per-task mem locking limits can do that. Even with syscall limits simple things like execve (enabled eg for qemu self-upgrade) can corrupt the kernel task-based accounting to the point that the limits don't work. Also, you are skipping the fact that since every subsystem does this differently and wrong a qemu can still go at least 3x over the allocation using just normal allowed functionalities. Again, as a security feature this fundamentally does not work. We cannot account for a FD owned resource inside the task based mm_struct. There are always going to be exploitable holes. What you really want is a cgroup based limit that is consistently applied in the kernel. Regardless, since this seems pretty well entrenched I continue to suggest my simpler alternative of making it fd based instead of user based. At least that doesn't have the unsolvable bugs related to task accounting. Jason
