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

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Message-ID: <bf919e4b-d56f-711d-f7ae-b463b8fdadfd@oracle.com>
Date:   Tue, 17 Nov 2020 16:09:15 +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 7:34 PM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 10:10 AM Alexandre Chartre
> <alexandre.chartre@...cle.com> 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 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.
> 

I finally remember why I have introduced a per-task PTI trampoline stack right now:
that's to be able to move the CR3 switch anywhere in the C handler. To do so, we need
a per-task stack to enter (and return) from the C handler as the handler can potentially
go to sleep.

Without a per-task trampoline stack, we would be limited to call the switch CR3 functions
from the assembly entry code before and after calling the C function handler (also called
from assembly).

alex.