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

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Message-ID: <20251022094019.GAaPimg3VCgRu6eELd@fat_crate.local>
Date: Wed, 22 Oct 2025 11:40:19 +0200
From: Borislav Petkov <bp@...en8.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sohil Mehta <sohil.mehta@...el.com>, 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>,
	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>,
	David Laight <david.laight.linux@...il.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
Subject: Re: [PATCH v10 03/15] x86/alternatives: Disable LASS when patching
 kernel alternatives

On Wed, Oct 22, 2025 at 10:25:41AM +0200, Peter Zijlstra wrote:
> Initially the suggestion was to use stac/clac directly iirc; but that
> looses the information these are for LASS only. Hence the LASS specific
> ones.

Yap.

> (its an unfortunate arch detail that LASS and SMAP both use the AC flag
> and all that)

That is an implementation detail and users of the interface shouldn't care.

> But that's not the same, stac() and clac() are FEATURE_SMAP, these are
> FEATURE_LASS.

So?

Are you thinking of toggling features and then something else getting disabled
in the process?

> If you really want the _disable _enable naming that's fine with me, but
> then perhaps we should also s/clac/smap_disable/ and s/stac/smap_enable/
> for consistency.

So the enable/disable thing is I think what makes this a lot more
understandable when you read it this way: "disable linear address separation
around this code". And that is regardless of how the underlying machinery does
that toggling of LASS.

As to stac/clac - I wouldn't touch them. They've been there forever so it'll
only be unnecessary churn.

Btw, if you need an example which already does that:

arch/x86/include/asm/uaccess.h:37:#define __uaccess_begin() stac()
arch/x86/include/asm/uaccess.h:38:#define __uaccess_end()   clac()

So the lass_{enable,disable} will be yet another incarnation of this pattern.

Thx.

-- 
Regards/Gruss,
    Boris.

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