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

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Message-ID: <a9aa4887-02ee-0e9a-a37e-63e9cb0ff27f@oracle.com>
Date:   Thu, 19 Nov 2020 20:55:33 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     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 11/19/20 8:10 PM, Thomas Gleixner wrote:
> On Mon, Nov 16 2020 at 19:10, Alexandre Chartre wrote:
>> 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 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.
> 
> Coming late, but this does not make any sense to me.
> 
> Unless you map most of the kernel into the user page-table sleeping with
> the user page-table _cannot_ work. And if you do that you broke KPTI.
> 
> You can neither pick arbitrary points in the C code of an exception
> handler to switch to the kernel mapping unless you mapped everything
> which might be touched before that into user space.
> 
> How is that supposed to work?
> 

Sorry I mixed up a few thing; I got confused with my own code which is not a
good sign...

It's not sleeping with the user page-table which, as you mentioned, doesn't
make sense, it's sleeping with the kernel page-table but with the PTI stack.

Basically, it is:
   - entering C code with (user page-table, PTI stack);
   - then it switches to the kernel page-table so we have (kernel page-table, PTI stack);
   - and then it switches to the kernel stack so we have (kernel page-table, kernel stack).

As this is all C code, some of which is executed with the PTI stack, we need the PTI stack
to be per-task so that the stack is preserved, in case that C code does a sleep/schedule
(no matter if this happens when using the PTI stack or the kernel stack).

alex.