linux-kernel - Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

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Message-ID: <20250626160707.GGaF1wK5tW37P6xt0O@fat_crate.local>
Date: Thu, 26 Jun 2025 18:07:07 +0200
From: Borislav Petkov <bp@...en8.de>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
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	Peter Zijlstra <peterz@...radead.org>,
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	Yian Chen <yian.chen@...el.com>
Subject: Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

On Thu, Jun 26, 2025 at 05:18:37PM +0200, Borislav Petkov wrote:
> On Mon, Jun 23, 2025 at 04:42:41PM +0300, Kirill A. Shutemov wrote:
> > Due to SLAM, we decided to postpone LAM enabling, until LASS is landed.
> > 
> > I am not sure if we want to add static
> > /sys/devices/system/cpu/vulnerabilities/slam with "Mitigation: LASS".
> > 
> > There might be other yet-to-be-discovered speculative attacks that LASS
> > mitigates. Security features have to visible to userspace independently of
> > known vulnerabilities.
> 
> ... and the fact that a vuln is being mitigated by stating that in
> /sys/devices/system/cpu/vulnerabilities/ needs to happen too.
> 
> I'm not talking about LAM enablement - I'm talking about adding a
> 
> SPECTRE_V1_MITIGATION_LASS
> 
> and setting that when X86_FEATURE_LASS is set so that luserspace gets told
> that
> 
> "Spectre V1 : Mitigation: LASS"
> 
> or so.
> 
> Makes more sense?

I meant this crap, ofc:

        switch (bug) {
        case X86_BUG_CPU_MELTDOWN:
                if (boot_cpu_has(X86_FEATURE_PTI))
                        return sysfs_emit(buf, "Mitigation: PTI\n");

This should say "Mitigation: LASS" if LASS is enabled...

Which begs the question: how do LASS and PTI interact now?

-- 
Regards/Gruss,
    Boris.

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