linux-kernel - Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code

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Date:   Mon, 16 Nov 2020 21:24:26 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        thomas.lendacky@....com, jroedel@...e.de, konrad.wilk@...cle.com,
        jan.setjeeilers@...cle.com, junaids@...gle.com, oweisse@...gle.com,
        rppt@...ux.vnet.ibm.com, graf@...zon.de, mgross@...ux.intel.com,
        kuzuno@...il.com
Subject: Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code

On Mon, Nov 16, 2020 at 03:47:36PM +0100, Alexandre Chartre wrote:
> Deferring CR3 switch to C code means that we need to run more of the
> kernel entry code with the user page-table. To do so, we need to:
> 
>  - map more syscall, interrupt and exception entry code into the user
>    page-table (map all noinstr code);
> 
>  - map additional data used in the entry code (such as stack canary);
> 
>  - run more entry code on the trampoline stack (which is mapped both
>    in the kernel and in the user page-table) until we switch to the
>    kernel page-table and then switch to the kernel stack;

So PTI was added exactly to *not* have kernel memory mapped in the user
page table. You're partially reversing that...

>  - have a per-task trampoline stack instead of a per-cpu trampoline
>    stack, so the task can be scheduled out while it hasn't switched
>    to the kernel stack.

per-task? How much more memory is that per task?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette