On Mon, Dec 01, 2025, Sean Christopherson wrote:
> Fix a UAF due to leaving a dangling guest_memfd memslot binding by
> disallowing clearing KVM_MEM_GUEST_MEMFD on a memslot.  The intent was
> that guest_memfd memslots would be immutable (could only be deleted),
> but somewhat ironically we missed the case where KVM_MEM_GUEST_MEMFD
> itself is the only flag that's toggled.
> 
> This is an ABI change, but I can't imagine anyone was relying on
> disappearing a guest_memfd memslot.
> 
> Patch 2 hardens against the UAF, and prepares for allowing FLAGS_ONLY
> changes on guest_memfd memslots.  Sooner or later, we're going to allow
> dirty logging on guest_memfd, so I think it makes sense to guard against
> that so that whoever adds dirty logging support doesn't forget to unbind
> on a FLAGS_ONLY change.
> 
> I'll respond with the syzkaller reproducer (it's comically simple).

And almost forgot...