linux-kernel - RE: [PATCH v10 03/15] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-ID: <DS7PR11MB6077262111C66261BFB4D676FCF3A@DS7PR11MB6077.namprd11.prod.outlook.com>
Date: Wed, 22 Oct 2025 20:03:05 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: "Mehta, Sohil" <sohil.mehta@...el.com>, Borislav Petkov <bp@...en8.de>
CC: "x86@...nel.org" <x86@...nel.org>, Dave Hansen
	<dave.hansen@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, "Ingo
 Molnar" <mingo@...hat.com>, 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>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, Vegard
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	<andrew.cooper3@...rix.com>, David Laight <david.laight.linux@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>, Geert Uytterhoeven
	<geert@...ux-m68k.org>, Kees Cook <kees@...nel.org>, Alexander Shishkin
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	<linux-doc@...r.kernel.org>, "linux-kernel@...r.kernel.org"
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Subject: RE: [PATCH v10 03/15] x86/alternatives: Disable LASS when patching
 kernel alternatives

> +/*
> + * LASS enforcement is based on bit 63 of the virtual address. The
> + * kernel is not allowed to touch memory in the lower half of the
> + * virtual address space unless the AC bit is set.
> + *
> + * Use lass_disable()/lass_enable() when accessing kernel (!_PAGE_USER)
> + * mappings in the lower half of the address space that are blocked by
> + * LASS, but not by SMAP.

Maybe "when accessing kernel data ..."

Also add that for instruction fetch CR4.LASS must be cleared.

-Tony