linux-kernel - Re: [PATCH RFC v2 0/2] Handle UEFI NX-restricted page tables

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Date:   Fri, 18 Mar 2022 12:37:39 -0400
From:   Peter Jones <pjones@...hat.com>
To:     Matthew Garrett <mjg59@...f.ucam.org>
Cc:     baskov@...ras.ru, Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 0/2] Handle UEFI NX-restricted page tables

On Thu, Mar 03, 2022 at 08:47:59PM +0000, Matthew Garrett wrote:
> On Thu, Mar 03, 2022 at 04:42:07PM +0300, baskov@...ras.ru wrote:
> > On 2022-02-28 21:30, Matthew Garrett wrote:
> > > On Mon, Feb 28, 2022 at 05:45:53PM +0100, Ard Biesheuvel wrote:
> > > 
> > > > Given that this is a workaround for a very specific issue arising on
> > > > PI based implementations of UEFI, I consider this a quirk, and so I
> > > > think this approach is reasonable. I'd still like to gate it on some
> > > > kind of identification, though - perhaps something related to DMI like
> > > > the x86 core kernel does as well.
> > > 
> > > When the V1 patches were reviewed, you suggested allocating
> > > EFI_LOADER_CODE rather than EFI_LOADER_DATA. The example given for a
> > > failure case is when NxMemoryProtectionPolicy is set to 0x7fd4, in which
> > > case EFI_LOADER_CODE, EFI_BOOT_SERVICES_CODE and
> > > EFI_RUNTIEM_SERVICES_CODE should not have the nx policy applied. So it
> > > seems like your initial suggestion (s/LOADER_DATA/LOADER_CODE/) should
> > > have worked, even if there was disagreement about whether the spec
> > > required it to. Is this firmware applying a stricter policy?
> > 
> > Yes, this firmware is being modified to enforce stricter policy.
> 
> Ok. I think this should really go through the UEFI spec process - I 
> agree that from a strict interpretation of the spec, what this firmware 
> is doing is legitimate, but I don't like having a situation where we 
> have to depend on the DXE spec.

It's in the process of getting into the UEFI spec now as
https://bugzilla.tianocore.org/show_bug.cgi?id=3519 .

> How does Windows handle this? Just update the page tables itself for any 
> regions it needs during boot?

Microsoft's bootloader sets up its own pagetables, though I believe
they're switching it to use the (soon to be) standardized API.

-- 
        Peter