Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack

[Andy Lutomirski]

On Mon, Nov 16, 2020 at 10:10 AM Alexandre Chartre
<alexandre.chartre@xxxxxxxxxx> wrote:
>
>
> On 11/16/20 5:57 PM, Andy Lutomirski wrote:
> > On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre
> > <alexandre.chartre@xxxxxxxxxx> wrote:
> >>
> >> When entering the kernel from userland, use the per-task PTI stack
> >> instead of the per-cpu trampoline stack. Like the trampoline stack,
> >> the PTI stack is mapped both in the kernel and in the user page-table.
> >> Using a per-task stack which is mapped into the kernel and the user
> >> page-table instead of a per-cpu stack will allow executing more code
> >> before switching to the kernel stack and to the kernel page-table.
> >
> > Why?
>
> When executing more code in the kernel, we are likely to reach a point
> where we need to sleep while we are using the user page-table, so we need
> to be using a per-thread stack.
>
> > I can't immediately evaluate how nasty the page table setup is because
> > it's not in this patch.
>
> The page-table is the regular page-table as introduced by PTI. It is just
> augmented with a few additional mapping which are in patch 11 (x86/pti:
> Extend PTI user mappings).
>
> >  But AFAICS the only thing that this enables is sleeping with user pagetables.
>
> That's precisely the point, it allows to sleep with the user page-table.
>
> > Do we really need to do that?
>
> Actually, probably not with this particular patchset, because I do the page-table
> switch at the very beginning and end of the C handler. I had some code where I
> moved the page-table switch deeper in the kernel handler where you definitively
> can sleep (for example, if you switch back to the user page-table before
> exit_to_user_mode_prepare()).
>
> So a first step should probably be to not introduce the per-task PTI trampoline stack,
> and stick with the existing trampoline stack. The per-task PTI trampoline stack can
> be introduced later when the page-table switch is moved deeper in the C handler and
> we can effectively sleep while using the user page-table.

Seems reasonable.

Where is the code that allocates and frees these stacks hiding?  I
think I should at least read it.