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

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Date:   Tue, 17 Nov 2020 18:07:35 +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 Tue, Nov 17, 2020 at 09:19:01AM +0100, Alexandre Chartre wrote:
> We are not reversing PTI, we are extending it.

You're reversing it in the sense that you're mapping more kernel memory
into the user page table than what is mapped now.

> PTI removes all kernel mapping from the user page-table. However there's
> no issue with mapping some kernel data into the user page-table as long as
> these data have no sensitive information.

I hope that is the case.

> Actually, PTI is already doing that but with a very limited scope. PTI adds
> into the user page-table some kernel mappings which are needed for userland
> to enter the kernel (such as the kernel entry text, the ESPFIX, the
> CPU_ENTRY_AREA_BASE...).
> 
> So here, we are extending the PTI mapping so that we can execute more kernel
> code while using the user page-table; it's a kind of PTI on steroids.

And this is what bothers me - someone else might come after you and say,
but but, I need to map more stuff into the user pgt because I wanna do
X... and so on.

> The minimum size would be 1 page (4KB) as this is the minimum mapping size.
> It's certainly enough for now as the usage of the PTI stack is limited, but
> we will need larger stack if we won't to execute more kernel code with the
> user page-table.

So on a big machine with a million tasks, that's at least a million
pages more which is what, ~4 Gb?

There better be a very good justification for the additional memory
consumption...

-- 
Regards/Gruss,
    Boris.

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