linux-kernel - Re: [PATCH 0/3] x86: Extend LASS support to EFI configurations

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Message-ID: <ce3a4c0c-1f18-459e-aba5-9d4582e45859@intel.com>
Date: Fri, 12 Dec 2025 16:17:06 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: Ard Biesheuvel <ardb@...nel.org>
CC: <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>, "H . Peter Anvin" <hpa@...or.com>, Andy Lutomirski
	<luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Kiryl Shutsemau
	<kas@...nel.org>, Rick Edgecombe <rick.p.edgecombe@...el.com>, Andrew Cooper
	<andrew.cooper3@...rix.com>, Tony Luck <tony.luck@...el.com>, "Alexander
 Shishkin" <alexander.shishkin@...ux.intel.com>,
	<linux-kernel@...r.kernel.org>, <linux-efi@...r.kernel.org>
Subject: Re: [PATCH 0/3] x86: Extend LASS support to EFI configurations

On 12/4/2025 11:03 AM, Ard Biesheuvel wrote:
> 
> efi_check_for_embedded_firmwares() maps EFI_BOOT_SERVICES_DATA regions
> in the kernel region, so bit 63 will be set.

Ah, I missed that efi_check_for_embedded_firmwares() remaps the regions.

> 
>> LASS wouldn't care whether there is an actual mapping behind the
>> address. It only relies on the MSB for enforcement. So, any code that
>> relied on accessing boot services memory before efi_free_boot_services()
>> could get affected by LASS.
>>
> 
> This only applies to code that accesses boot services memory via a
> mapping in the lower range.
> 

Yes, I was referring to usages which use the lower mapping. Though,
efi_check_for_embedded_firmwares() doesn't do that, enabling LASS after
freeing boot services memory is still the right thing to do. In theory,
someone could use the 1:1 mapping.