Re: [PATCH 0/3] PTI for x86-32 Fixes and Updates

[Linus Torvalds]

On Mon, Jul 23, 2018 at 7:09 AM Pavel Machek <pavel@xxxxxx> wrote:
>
> Meanwhile... it looks like gcc is not slowed down significantly, but
> other stuff sees 30% .. 40% slowdowns... which is rather
> significant.

That is more or less expected.

Gcc spends about 90+% of its time in user space, and the system calls
it *does* do tend to be "real work" (open/read/etc). And modern gcc's
no longer have the pipe between cpp and cc1, so they don't have that
issue either (which would have sjhown the PTI slowdown a lot more)

Some other loads will do a lot more time traversing the user/kernel
boundary, and in 32-bit mode you won't be able to take advantage of
the address space ID's, so you really get the full effect.

> Would it be possible to have per-process control of kpti? I have
> some processes where trading of speed for security would make sense.

That was pretty extensively discussed, and no sane model for it was
ever agreed upon.  Some people wanted it per-thread, others per-mm,
and it wasn't clear how to set it either and how it should inherit
across fork/exec, and what the namespace rules etc should be.

You absolutely need to inherit it (so that you can say "I trust this
session" or whatever), but at the same time you *don't* want to
inherit if you have a server you trust that then spawns user processes
(think "I want systemd to not have the overhead, but the user
processes it spawns obviously do need protection").

It was just a morass. Nothing came out of it.  I guess people can
discuss it again, but it's not simple.

               Linus