On Fri, 8 Dec 2017, Andy Lutomirski wrote:
> Can we take a step back here? I think there are four vaguely sane
> ways to make the LDT work:
>
> 1. The way it is right now -- in vmalloc space. The only real
> downside is that it requires exposing that part of vmalloc space in
> the user tables, which is a bit gross.
>
> 2. In some fixmap-like space, which is what my patch does, albeit
> buggily. This requires a PGD that we treat as per-mm, not per-cpu,
> but that's not so bad.
>
> 3. In one of the user PGDs but above TASK_SIZE_MAX. This is
> functionally almost identical to #2, except that there's more concern
> about exploits that write past TASK_SIZE_MAX.
>
> 4. In an actual vma. I don't see the benefit of doing this at all --
> it's just like #2 except way more error prone. Hell, you have to make
> sure that you can't munmap or mremap it, which isn't a consideration
> at all with the other choices.
Why? You can unmap vdso or uprobe or whatever VMAs and you will simply
die. You get what you asked for.
Thanks,
tglx
