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

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Message-ID: <20251110192422.GBaRI75huOXoUSmoDt@fat_crate.local>
Date: Mon, 10 Nov 2025 20:24:22 +0100
From: Borislav Petkov <bp@...en8.de>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: x86@...nel.org, 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>,
	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 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 Mon, Nov 10, 2025 at 10:15:23AM -0800, Sohil Mehta wrote:
> lass_clac()/_stac():
> 	Disable enforcement for kernel data accesses similar to SMAP.
> 
> lass_enable()/_disable():
> 	Disable the entire LASS mechanism (Data and instruction fetch)
> 	by toggling CR4.LASS
> 
> Would that work? Any other suggestions?

Sure, as long as they're documented. And if we decide to change them later for
whatever reason, we can. More than enough bikeshedding we did here.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette