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

[Alexandre Chartre]

On 11/16/20 7:34 PM, Andy Lutomirski wrote:
> 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.

Stacks are allocated/freed with the task stack, this code is unchanged (see
alloc_thread_stack_node()). The trick is that I have doubled the THREAD_SIZE
(patch 8 "x86/pti: Introduce per-task PTI trampoline stack"). Half the stack
is a used as the kernel stack (mapped only in the kernel page-table), the
other half is used as the PTI stack (mapped in the kernel and user page-table).
The mapping to the user page-table is done in mm_map_task() in fork.c (patch 11
"x86/pti: Extend PTI user mappings").

alex.