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

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Message-ID: <29f2d16f-361f-475c-957e-0ebcefcd1a8c@intel.com>
Date: Mon, 10 Nov 2025 10:15:23 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: <x86@...nel.org>, Borislav Petkov <bp@...en8.de>
CC: 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

Hi Boris,

On 10/29/2025 2:03 PM, Sohil Mehta wrote:
> +/*
> + * 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.
> + *
> + * Use lass_disable()/lass_enable() to toggle the AC bit for kernel data
> + * accesses (!_PAGE_USER) that are blocked by LASS, but not by SMAP.
> + *
> + * Even with the AC bit set, LASS will continue to block instruction
> + * fetches from the user half of the address space. To allow those,
> + * clear CR4.LASS to disable the LASS mechanism entirely.
> + *

Based on the EFI discussion, 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.

I am thinking of reverting this back to lass_clac()/lass_stac().

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?


> +
> +static __always_inline void lass_enable(void)
> +{
> +	alternative("", "clac", X86_FEATURE_LASS);
> +}
> +
> +static __always_inline void lass_disable(void)
> +{
> +	alternative("", "stac", X86_FEATURE_LASS);
> +}
> +