linux-kernel - Re: [tip:efi/core] x86/mm/pat: Use _PAGE_GLOBAL bit for EFI page table mappings

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Date:	Wed, 24 Feb 2016 08:47:07 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Matt Fleming <matt@...eblueprint.co.uk>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Toshi Kani <toshi.kani@...com>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Luis Rodriguez <mcgrof@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>, ricardo.neri@...el.com,
	Hugh Dickins <hughd@...gle.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [tip:efi/core] x86/mm/pat: Use _PAGE_GLOBAL bit for EFI page
 table mappings

On Wed, Feb 24, 2016 at 8:41 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Feb 24, 2016 at 05:20:02PM +0100, Borislav Petkov wrote:
>> That's mixed mode. I think you mean the FLUSH_TLB_ALL in efi_call.
>> That's EFI on 64-bit but that is mandated by the spec, AFAIR.
>
> Ok, so mfleming set me straight on IRC - that's tip/master I should be
> staring at.
>
> In any case, I think we should do __flush_tlb_all() in efi_call_cirt()
> just in case, for the simple reason that EFI could be installing some
> funky TLB entries which we don't want. I'm not saying it does and it
> probably won't but what's stopping it?
>
> Or am I being overly paranoid?

I think you may be overly paranoid here.  At least no working 32-bit
EFI does this because we run it in compat mode.  Any paging entries it
inserts would be misinterpreted and likely immediately cause a crash.
Also, the EFI code doesn't know a virtual address through which to
reference the paging structures in the first place -- it could read
CR3, but that gives a physical address, and it's not at all clear to
me what even the crazier firmware authors would do with a physical
address that doesn't live in EFI-defined ranges.

--Andy