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

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Message-ID: <bc8a254e-deaa-388e-99ea-0291f5625b5b@oracle.com>
Date:   Mon, 16 Nov 2020 19:10:22 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        jan.setjeeilers@...cle.com, Junaid Shahid <junaids@...gle.com>,
        oweisse@...gle.com, Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Alexander Graf <graf@...zon.de>, mgross@...ux.intel.com,
        kuzuno@...il.com
Subject: Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of
 trampoline stack

On 11/16/20 5:57 PM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre
> <alexandre.chartre@...cle.com> 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.

alex.