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 07:12:25 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.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