linux-kernel - Re: [PATCH v11 4/9] x86/alternatives: Disable LASS when patching kernel code

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Message-ID: <7c26ae81-3495-457b-9f64-f5b2e169a63b@intel.com>
Date: Wed, 12 Nov 2025 06:51:45 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Ard Biesheuvel <ardb@...nel.org>, Sohil Mehta <sohil.mehta@...el.com>
Cc: x86@...nel.org, Borislav Petkov <bp@...en8.de>,
 Jonathan Corbet <corbet@....net>, "H . Peter Anvin" <hpa@...or.com>,
 Andy Lutomirski <luto@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, "Kirill A . Shutemov"
 <kas@...nel.org>, Xin Li <xin@...or.com>, David Woodhouse
 <dwmw@...zon.co.uk>, Sean Christopherson <seanjc@...gle.com>,
 Rick 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@...r.kernel.org,
 linux-efi@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v11 4/9] x86/alternatives: Disable LASS when patching
 kernel code

On 11/12/25 05:56, Ard Biesheuvel wrote:
...
>> it looks like we would now need to toggle
>> CR4.LASS every time we switch to efi_mm. The lass_enable()/_disable()
>> naming would be more suitable for those wrappers.
>>
> Note that Linux/x86 uses SetVirtualAddressMap() to remap all EFI
> runtime regions into the upper [kernel] half of the address space.
> 
> SetVirtualAddressMap() itself is a terrible idea, but given that we
> are already stuck with it, we should be able to rely on ordinary EFI
> runtime calls to only execute from the upper address range. The only
> exception is the call to SetVirtualAddressMap() itself, which occurs
> only once during early boot.

Gah, I had it in my head that we needed to use the lower mapping at
runtime. The efi_mm gets used for that SetVirtualAddressMap() and the
efi_mm continues to get used at runtime. So I think I just assumed that
the lower mappings needed to get used too.

Thanks for the education!

Let's say we simply delayed CR4.LASS=1 until later in boot. Could we
completely ignore LASS during EFI calls, since the calls only use the
upper address range?

Also, in practice, are there buggy EFI implementations that use the
lower address range even though they're not supposed to? *If* we just
keep LASS on for these calls is there a chance it will cause a
regression in some buggy EFI implementations?