linux-kernel - RE: [PATCH 1/2] x86/efi: Unmap efi boot services code/data regions from efi

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Date:   Mon, 22 Oct 2018 17:36:48 +0000
From:   "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...nel.org>
CC:     "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "Andy Lutomirski" <luto@...nel.org>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ard Biesheuvel" <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: RE: [PATCH 1/2] x86/efi: Unmap efi boot services code/data regions
 from efi_pgd

> On 10/21/2018 06:57 PM, Ingo Molnar wrote:
> > Does the CPU _ever_ look look at the PFN if the page is
> > !_PAGE_PRESENT, for example speculatively? If yes then what is the
> > recommended value for the pfn - zero perhaps?
> 
> I'll never say never. :)
> 
> For L1TF[1], we know the CPU did exactly this; it ignored the _PAGE_PRESENT
> bit when fetching data from the L1.  That's what is worked around with the gunk
> in arch/x86/include/asm/pgtable-invert.h.
> 
> I think Andi plugged the code in here at a low enough level in the page table
> manipulation that pageattr.c should inherit it without doing anything explicit.
> But, Sai, you might want to double-check this.
> 
> 1.
> https://www.intel.com/content/www/us/en/architecture-and-
> technology/l1tf.html

Sure! Dave. Thanks for the link. I will take a look at it.

Regards,
Sai