linux-kernel - Re: [PATCH v11 5/9] x86/efi: Disable LASS while mapping the EFI runtime services

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Message-ID: <27b6cafe-8cc9-4a01-af2b-3e847ff9aaa9@intel.com>
Date: Fri, 31 Oct 2025 12:04:03 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Andy Lutomirski <luto@...nel.org>, Dave Hansen <dave.hansen@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>, Dave Hansen
	<dave.hansen@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, "Ingo
 Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
CC: Jonathan Corbet <corbet@....net>, "H. Peter Anvin" <hpa@...or.com>, "Josh
 Poimboeuf" <jpoimboe@...nel.org>, "Peter Zijlstra (Intel)"
	<peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org>, "Kirill A .
 Shutemov" <kas@...nel.org>, Xin Li <xin@...or.com>, David Woodhouse
	<dwmw@...zon.co.uk>, Sean Christopherson <seanjc@...gle.com>, "Rick P
 Edgecombe" <rick.p.edgecombe@...el.com>, Vegard Nossum
	<vegard.nossum@...cle.com>, Andrew Cooper <andrew.cooper3@...rix.com>, "Randy
 Dunlap" <rdunlap@...radead.org>, Geert Uytterhoeven <geert@...ux-m68k.org>,
	Kees Cook <kees@...nel.org>, Tony Luck <tony.luck@...el.com>, "Alexander
 Shishkin" <alexander.shishkin@...ux.intel.com>, <linux-doc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<linux-efi@...r.kernel.org>
Subject: Re: [PATCH v11 5/9] x86/efi: Disable LASS while mapping the EFI
 runtime services

On 10/31/2025 10:38 AM, Andy Lutomirski wrote:

> I have another question: why is this one specific call a problem as opposed to something more general?  Wouldn’t any EFI call that touches the low EFI mapping be a problem?  Are there any odd code paths that touch low mapped EFI *data* that would fault?
> 

I assumed EFI is running in physical mode before this.
efi_sync_low_kernel_mappings() is called right before calling
set_virtual_address_map(). So, this is the only call that happens at the
low mapping while switching to virtual mode.

But, my EFI knowledge is fairly limited. I am realizing that there are
some assumptions built into this patch that I may not be aware of.

> Is there some way to be reasonably convinced that you haven’t missed another EFI code path?

We have been running the patches on internal test platforms for a couple
of years. But, that would only cover the common paths. I'll dig deeper
to get you a convincing answer.