On Fri, Dec 8, 2017 at 9:37 AM, Thomas Gleixner <t...@linutronix.de> wrote: > 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.
But if you unmap ldt and then map something else at the same VA, you're root.
